Engine Temperature Gauge v. Engine Water Pressure Gauge; Difficulty of Installation

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Author Topic:   Engine Temperature Gauge v. Engine Water Pressure Gauge; Difficulty of Installation

twista posted 04-15-2009 02:40 PM ET (US)
Would [you recommend] a temperature gauge or a water pressure gauge [for a Boston Whaler MONTAUK 17 boat]? [I] have a temperature beeper but no gauge. I have been reading that pressure gauges are good preventive devices. How difficult are they to install?
It is a 1995 Mercury 90HP.
efjr1 posted 04-15-2009 04:32 PM ET (US)
I have a temp gauge for each head on my 150. Easy to install. You can see what the temps are before the alarm sounds which is too late.
TransAm posted 04-15-2009 04:42 PM ET (US)
Correspondingly, if your engine is not getting appropriate water pressure, the engine cooling system will suffer and more likely to run hot. However, you can generally asses your engines water pressure by the tell tale stream. Unless your engine is mounted high on the transom, I would consider the temp gauge more appropriate. If you have room though, why not install both while you at it. Neither are terribly difficult to install and doing them both at the same time will not be much more trouble than just the one.
crabby posted 04-15-2009 10:46 PM ET (US)
Pay attention to Ed (above). I'm in the middle of a repower because I didn't listen to him last spring! On any "v" style motor if you want to monitor temps you need two gauges. Or just as well, go for the water pressure gauge.
jimh posted 04-15-2009 11:17 PM ET (US)
If forced to choose only one gauge and the choice was between an engine cylinder head temperature gauge or an engine cooling water pressure gauge, I would choose the cooling water pressure gauge. I have both on my boat. I have a V-block motor, and the engine cylinder head temperature gauge is installed on the cylinder bank that is the tallest and at the upper part of that bank. Typically that cylinder will be the hottest cylinder, so monitoring its temperature will be likely to give you the highest temperature reading between the two banks.
jimh posted 04-15-2009 11:22 PM ET (US)
A cooling system water pressure gauge does not prevent anything. It just gives you an indication of the pressure of the water in the cooling system. By careful observation of the gauge and reacting to its readings, you may prevent a problem, but the gauge itself does not.
Installation of a cooling water pressure gauge is straightforward. Most of the gauges are operated by connecting a hose between the gauge and a tap-in point in the engine cooling system. The hose is often a small plastic hose, typically 0.25-inch diameter or less. The precise nature of the hose and the location of the tap-in point in the engine cooling system will vary with the brand of the gauge and the brand and model of the engine.
L H G posted 04-15-2009 11:46 PM ET (US)
Based on experience with my Mercs of various sizes, I would disagree with Jim on first choice as a water pressure gauge.
Mercurys may have different flow patterns in the cooling systems than OMC, which may be part of this issue.
I have both temp and pressure on my twin 200 Mercs, and never use the pressure. I think it's only necessary for high engine installation where air can get sucked into the cooling system, and in jacked up high performance applications. Besides, it's a pain to install and rig. Mercury 2 strokes, 1989 and later, all come pre-wired for temp gauges right in the ignition harness and sender in the engine, making the installation simple plug and play. I guess that tells you something. In my style of boating, I monitor the temp gauges all the time, and ignore the pressure gauges, since they don't tell me much. The temperature horn is also a highly valuable warning, because when your running, you can't keep your eyes glued on gauges.
I have gotten rid of the pressure gauges on all of my other engine installations, and replaced them with temp gauges. I am installing new twin Merc 90's like you have, and using only temp gauges.
The only place I use the pressure gauges on my 200 EFI's is when flushing on the hose fitting. As I turn on the water, I can adjust the water pressure by reading the pressure of the water flow to show that the engine is circulating water for flushing out the salt.
twista posted 04-16-2009 12:28 PM ET (US)
[Since] my motor is a 1995 [Mercury] 90-HP, is there already a sending unit attached and a wire run up to the dash? And all [I] have to do is purchase a temperature gauge and mount it?
Buckda posted 04-16-2009 12:55 PM ET (US)
No. The powerhead is ready for the unit - it essentially just bolts on in a pre-prepared location and is an easy do-it-yourself job. I rate it three beers on the difficulty scale (on a scale of a six-pack).
You install the powerhead interface, run the hose and install the gauge.
Botta-bing, botta-bang, you're done.
Buckda posted 04-16-2009 12:59 PM ET (US)
Whoops. I should have put down that last beer.
The above is for the water pressure gauge. I disagree with Larry on the temperature gauge - the temperature gauge tells you that it overheated or that it is on the way to an overheat situation. The Water Pressure Gauge can indicate a problem before the temperature even gets out of control. No pressure - no cooling.
The water pressure gauge also serves as an important bellweather on the health of your water pump/impeller, as the system will lose pressure over time as that impeller wears out. You will notice this over the course of a few seasons, and it can help indicate to you when you need to have that system inspected/serviced/replaced.
Go with the water pressure gauge if you're only going to buy one gauge.
jimh posted 04-16-2009 01:12 PM ET (US)
When I bought my engine cylinder head temperature gauge, the mechanic--about 40 years of experience--laughed and said, "What do you want that for? To watch it go up and down?"
That is what a cylinder head temperature gauge does on most outboard motors: it goes up and down. It goes up when you are idling, and down when you are on plane.
jimh posted 04-16-2009 09:00 PM ET (US)
As far as I know, most OMC or BRP outboard motors have a threaded boss already machined into the cylinder head that is intended to receive a temperature sensor. Installation is very simple.
L H G posted 04-17-2009 12:50 AM ET (US)
I also like to watch the temperature gauge go up and down on my Caddy STS! I've never cared for watching pressure indicators go up and down. But Jim's mechanic must like to watch them, low when idling, and high when planing. Up and down, up and down.
Twista, you've been given some wrong information here. Yes, your 90 came factory equipped with a temp sender on the engine heads, and wiring already in the engine harness. Simply pick up a gauge for $25 and watch it go up and down! If you're concerned about water pressure, watch your engine telltale. It tells you a lot. Together you've got a pretty reliable system. I own seven Merc's on 4 Whalers and haven't blown an engine yet. Mercury clearly chose to provide temp gauges from the factory, instead of presure, for a reason. They probably know more about Merc's then Jim's OMC mechanic. Boston Whaler provided Temp gauges on all boats with factory rigged 2-stroke 90's.
Wonder what they know?. If you select a pressure gauge instead, then you will have 2 pressure indicators (gauge + telltale), and no temperature indicator. Doesn't sound right to me. Maybe three or four pressure indicators are needed, and then you can watch them all go up and down too!
On Mercs, unlike Evinrudes, the telltale does not even show until the water has been all the way through the engine, so you know you're getting water, and cooling at the top, if you see a telltale. Watch the temp gauge, and you can see how your thermostats and poppit valves are working also. On the Evinrudes that I have seen, the telltale shows up the second you start the engine. This tells you the impeller is working, and pumping it right out the telltale, but it doesn't tell you the top cylinders are getting water. There could be a blockage? Maybe that's why OMC mechanics recommend a pressure gauge on the top cylinder, to compensate for an inadequate telltale design.
You know, I just thought of something. I like to watch my tachs go up and down too.
It's your boat, hopefully my post will give you another point of view upon which to make your decision.
twista posted 04-17-2009 01:33 AM ET (US)
[I] [didn't] see a receptacle for a sensor, but [I] did see a small cover that had a wire coming out of it which [I] believed to be a sending unit. [I] cannot see where else one could be mounted without tapping and threading one. Since basically all [I] will have to do is find the correct wire under the dash to hook up a gauge for temperature, [I] will go that route. Maybe next season [I] will hook up the [cooling system water pressure gauge]. [I] thank all of you for your input!
L H G posted 04-17-2009 02:13 AM ET (US)
Yes, the temp sensor is as you described. It kind of slides into a pocket and reads the head temp. It is not a threaded sensor that probes the water passages.
The wire in your harness could terminate in a 5 pin plug, which inludes tach, trim, ignition and neg. Tach is grey, trim is brown and white, and temp is the "other" one. (forgot the color!) It's the same color as the wire on the engine.
jimh posted 04-17-2009 09:23 AM ET (US)
In a manner analogous to the suggestion that water pressure in the cooling system can be inferred by observing the flow from the aspirator, I would make the suggestion that engine cylinder head temperature can be inferred by placing your hand on the engine block. In both of these techniques, the operator uses his own senses to make an inference about a parameter that could be measured with a gauge. Checking cylinder head temperature in this manner eliminates the need for a gauge in precisely the same way that checking the cooling pressure by eyeing the aspirator flow volume eliminates the need for a water pressure gauge.
One thing to be mentioned, since we are discussing a Mercury engine, is that you will be getting very familiar with its water pump and impeller, as you will be changing them frequently.
ratherwhalering posted 04-17-2009 02:28 PM ET (US)
I'm going to vote for the engine temperature gauge for practical reasons. If there is a sudden total loss of water flow to the engine, your first indication will most likely be the overheat alarm, unless you are lucky enough to notice a low water pressure/high engine temperature reading. More likely, a gradual decrease in water flow will result in a noticeably higher engine temperature at operating speed rather than a lower water flow indication.
jimh posted 04-17-2009 03:08 PM ET (US)
Simple analogue needle-pointer gauges, which is what we are talking about, just display their readings. A water pressure gauge and a cylinder head temperature gauge will display their readings. As noted, they both vary with engine speed. The only time these gauges give you any diagnostic information is when the readings are observed and then compared to the anticipated or historical range of readings for the situation.
I need some clarification why a change in engine cylinder temperature shown on a gauge will be more noticeable to the operator than a change in water pressure shown on a gauge would be.
newt posted 04-19-2009 09:22 AM ET (US)
I would also install a temperature gauge before a water pressure gauge for two reasons. First, the temperature gauge is probably an easier install. Second, engine temperature is the end result you want to monitor. A pressure problem leads to a temperature problem not the other way around.
You can also learn more from a temperature gauge than the water pressure gauge. For example, a stuck thermostat could cause your engine to run cool and you would pick this up by reading the temperature gauge. Water pressure tells you less about the engine than temperature does.
jimh posted 04-20-2009 10:51 PM ET (US)
While getting out-voted here in my support for installing a water pressure gauge in preference to a temperature gauge, I am, however, in good company. At least one major boat builder I know--and they are strongly partnered with an engine building company-- is rigging their boats with a water pressure gauge, but no engine temperature gauge. The boat builder is Boston Whaler and the engine company they partner with is Mercury. I can't believe they're both getting this wrong, too.
L H G posted 04-21-2009 01:49 AM ET (US)
Jim, easy explanation, and one that was discussed here several years ago when Whaler first started putting Mercaha 4-stroke on the Montauks.
This thread is about a 2-stroke Mercury. As I said earlier, for these engines Mercury, and subsequently BW, recommended and installed Temp gauges. Any participant here who bought a Montauk with a 2-stroke will tell you the boat came with Temp gauge.
Regarding the 4-strokes, I was told that Mercury wanted those shipped with water pressure gauge instead of temp. Why the recommended difference between the two technologies, I can't answer.
jimh posted 04-21-2009 10:35 AM ET (US)
Larry--I was told--and I think it was by you--that Mercury provides a water pressure sensor for all of their SmartCraft motors as a standard set up. It seems odd that if there were some sort of preference from Mercury that water pressure was not an important engine parameter to monitor that they would provide a sensor for it.
All modern engines have electronic sensors for temperature because this parameter is monitored for use in calculating the engine operating parameters with fuel and air. Since the fuel and air are electronically controlled, the engine needs the temperature information, too. It goes without saying that modern engines will all provide electronic temperature readout.
As I mentioned, I have both a water pressure and a temperature gauge. I monitor both. The water pressure gauge is like an early-warning system. If there is debris fouled on the gearcase of the engine it usually shows up immediately as a reduction in water pressure. I stop and clear the debris off the gearcase.
Temperature may be more useful on Mercury engines since their cooling system seems to be more prone to problems with their regulating and control valves. The thermostats and poppet valves are often cited as having failed due to build-up of corrosion, and such a malfunction might not be reflected in a water pressure reading.
And, yes, I totally agree. If your engine cooling system is malfunctioning and is providing too much cooling, you will never detect this on a pressure gauge. You need a temperature gauge to see that there is too much cooling being provided.

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Author	Topic: Engine Temperature Gauge v. Engine Water Pressure Gauge; Difficulty of Installation
twista	posted 04-15-2009 02:40 PM ET (US) Would [you recommend] a temperature gauge or a water pressure gauge [for a Boston Whaler MONTAUK 17 boat]? [I] have a temperature beeper but no gauge. I have been reading that pressure gauges are good preventive devices. How difficult are they to install? It is a 1995 Mercury 90HP.
efjr1	posted 04-15-2009 04:32 PM ET (US) I have a temp gauge for each head on my 150. Easy to install. You can see what the temps are before the alarm sounds which is too late.
TransAm	posted 04-15-2009 04:42 PM ET (US) Correspondingly, if your engine is not getting appropriate water pressure, the engine cooling system will suffer and more likely to run hot. However, you can generally asses your engines water pressure by the tell tale stream. Unless your engine is mounted high on the transom, I would consider the temp gauge more appropriate. If you have room though, why not install both while you at it. Neither are terribly difficult to install and doing them both at the same time will not be much more trouble than just the one.
crabby	posted 04-15-2009 10:46 PM ET (US) Pay attention to Ed (above). I'm in the middle of a repower because I didn't listen to him last spring! On any "v" style motor if you want to monitor temps you need two gauges. Or just as well, go for the water pressure gauge.
jimh	posted 04-15-2009 11:17 PM ET (US) If forced to choose only one gauge and the choice was between an engine cylinder head temperature gauge or an engine cooling water pressure gauge, I would choose the cooling water pressure gauge. I have both on my boat. I have a V-block motor, and the engine cylinder head temperature gauge is installed on the cylinder bank that is the tallest and at the upper part of that bank. Typically that cylinder will be the hottest cylinder, so monitoring its temperature will be likely to give you the highest temperature reading between the two banks.
jimh	posted 04-15-2009 11:22 PM ET (US) A cooling system water pressure gauge does not prevent anything. It just gives you an indication of the pressure of the water in the cooling system. By careful observation of the gauge and reacting to its readings, you may prevent a problem, but the gauge itself does not. Installation of a cooling water pressure gauge is straightforward. Most of the gauges are operated by connecting a hose between the gauge and a tap-in point in the engine cooling system. The hose is often a small plastic hose, typically 0.25-inch diameter or less. The precise nature of the hose and the location of the tap-in point in the engine cooling system will vary with the brand of the gauge and the brand and model of the engine.
L H G	posted 04-15-2009 11:46 PM ET (US) Based on experience with my Mercs of various sizes, I would disagree with Jim on first choice as a water pressure gauge. Mercurys may have different flow patterns in the cooling systems than OMC, which may be part of this issue. I have both temp and pressure on my twin 200 Mercs, and never use the pressure. I think it's only necessary for high engine installation where air can get sucked into the cooling system, and in jacked up high performance applications. Besides, it's a pain to install and rig. Mercury 2 strokes, 1989 and later, all come pre-wired for temp gauges right in the ignition harness and sender in the engine, making the installation simple plug and play. I guess that tells you something. In my style of boating, I monitor the temp gauges all the time, and ignore the pressure gauges, since they don't tell me much. The temperature horn is also a highly valuable warning, because when your running, you can't keep your eyes glued on gauges. I have gotten rid of the pressure gauges on all of my other engine installations, and replaced them with temp gauges. I am installing new twin Merc 90's like you have, and using only temp gauges. The only place I use the pressure gauges on my 200 EFI's is when flushing on the hose fitting. As I turn on the water, I can adjust the water pressure by reading the pressure of the water flow to show that the engine is circulating water for flushing out the salt.
twista	posted 04-16-2009 12:28 PM ET (US) [Since] my motor is a 1995 [Mercury] 90-HP, is there already a sending unit attached and a wire run up to the dash? And all [I] have to do is purchase a temperature gauge and mount it?
Buckda	posted 04-16-2009 12:55 PM ET (US) No. The powerhead is ready for the unit - it essentially just bolts on in a pre-prepared location and is an easy do-it-yourself job. I rate it three beers on the difficulty scale (on a scale of a six-pack). You install the powerhead interface, run the hose and install the gauge. Botta-bing, botta-bang, you're done.
Buckda	posted 04-16-2009 12:59 PM ET (US) Whoops. I should have put down that last beer. The above is for the water pressure gauge. I disagree with Larry on the temperature gauge - the temperature gauge tells you that it overheated or that it is on the way to an overheat situation. The Water Pressure Gauge can indicate a problem before the temperature even gets out of control. No pressure - no cooling. The water pressure gauge also serves as an important bellweather on the health of your water pump/impeller, as the system will lose pressure over time as that impeller wears out. You will notice this over the course of a few seasons, and it can help indicate to you when you need to have that system inspected/serviced/replaced. Go with the water pressure gauge if you're only going to buy one gauge.
jimh	posted 04-16-2009 01:12 PM ET (US) When I bought my engine cylinder head temperature gauge, the mechanic--about 40 years of experience--laughed and said, "What do you want that for? To watch it go up and down?" That is what a cylinder head temperature gauge does on most outboard motors: it goes up and down. It goes up when you are idling, and down when you are on plane.
jimh	posted 04-16-2009 09:00 PM ET (US) As far as I know, most OMC or BRP outboard motors have a threaded boss already machined into the cylinder head that is intended to receive a temperature sensor. Installation is very simple.
L H G	posted 04-17-2009 12:50 AM ET (US) I also like to watch the temperature gauge go up and down on my Caddy STS! I've never cared for watching pressure indicators go up and down. But Jim's mechanic must like to watch them, low when idling, and high when planing. Up and down, up and down. Twista, you've been given some wrong information here. Yes, your 90 came factory equipped with a temp sender on the engine heads, and wiring already in the engine harness. Simply pick up a gauge for $25 and watch it go up and down! If you're concerned about water pressure, watch your engine telltale. It tells you a lot. Together you've got a pretty reliable system. I own seven Merc's on 4 Whalers and haven't blown an engine yet. Mercury clearly chose to provide temp gauges from the factory, instead of presure, for a reason. They probably know more about Merc's then Jim's OMC mechanic. Boston Whaler provided Temp gauges on all boats with factory rigged 2-stroke 90's. Wonder what they know?. If you select a pressure gauge instead, then you will have 2 pressure indicators (gauge + telltale), and no temperature indicator. Doesn't sound right to me. Maybe three or four pressure indicators are needed, and then you can watch them all go up and down too! On Mercs, unlike Evinrudes, the telltale does not even show until the water has been all the way through the engine, so you know you're getting water, and cooling at the top, if you see a telltale. Watch the temp gauge, and you can see how your thermostats and poppit valves are working also. On the Evinrudes that I have seen, the telltale shows up the second you start the engine. This tells you the impeller is working, and pumping it right out the telltale, but it doesn't tell you the top cylinders are getting water. There could be a blockage? Maybe that's why OMC mechanics recommend a pressure gauge on the top cylinder, to compensate for an inadequate telltale design. You know, I just thought of something. I like to watch my tachs go up and down too. It's your boat, hopefully my post will give you another point of view upon which to make your decision.
twista	posted 04-17-2009 01:33 AM ET (US) [I] [didn't] see a receptacle for a sensor, but [I] did see a small cover that had a wire coming out of it which [I] believed to be a sending unit. [I] cannot see where else one could be mounted without tapping and threading one. Since basically all [I] will have to do is find the correct wire under the dash to hook up a gauge for temperature, [I] will go that route. Maybe next season [I] will hook up the [cooling system water pressure gauge]. [I] thank all of you for your input!
L H G	posted 04-17-2009 02:13 AM ET (US) Yes, the temp sensor is as you described. It kind of slides into a pocket and reads the head temp. It is not a threaded sensor that probes the water passages. The wire in your harness could terminate in a 5 pin plug, which inludes tach, trim, ignition and neg. Tach is grey, trim is brown and white, and temp is the "other" one. (forgot the color!) It's the same color as the wire on the engine.
jimh	posted 04-17-2009 09:23 AM ET (US) In a manner analogous to the suggestion that water pressure in the cooling system can be inferred by observing the flow from the aspirator, I would make the suggestion that engine cylinder head temperature can be inferred by placing your hand on the engine block. In both of these techniques, the operator uses his own senses to make an inference about a parameter that could be measured with a gauge. Checking cylinder head temperature in this manner eliminates the need for a gauge in precisely the same way that checking the cooling pressure by eyeing the aspirator flow volume eliminates the need for a water pressure gauge. One thing to be mentioned, since we are discussing a Mercury engine, is that you will be getting very familiar with its water pump and impeller, as you will be changing them frequently.
ratherwhalering	posted 04-17-2009 02:28 PM ET (US) I'm going to vote for the engine temperature gauge for practical reasons. If there is a sudden total loss of water flow to the engine, your first indication will most likely be the overheat alarm, unless you are lucky enough to notice a low water pressure/high engine temperature reading. More likely, a gradual decrease in water flow will result in a noticeably higher engine temperature at operating speed rather than a lower water flow indication.
jimh	posted 04-17-2009 03:08 PM ET (US) Simple analogue needle-pointer gauges, which is what we are talking about, just display their readings. A water pressure gauge and a cylinder head temperature gauge will display their readings. As noted, they both vary with engine speed. The only time these gauges give you any diagnostic information is when the readings are observed and then compared to the anticipated or historical range of readings for the situation. I need some clarification why a change in engine cylinder temperature shown on a gauge will be more noticeable to the operator than a change in water pressure shown on a gauge would be.
newt	posted 04-19-2009 09:22 AM ET (US) I would also install a temperature gauge before a water pressure gauge for two reasons. First, the temperature gauge is probably an easier install. Second, engine temperature is the end result you want to monitor. A pressure problem leads to a temperature problem not the other way around. You can also learn more from a temperature gauge than the water pressure gauge. For example, a stuck thermostat could cause your engine to run cool and you would pick this up by reading the temperature gauge. Water pressure tells you less about the engine than temperature does.
jimh	posted 04-20-2009 10:51 PM ET (US) While getting out-voted here in my support for installing a water pressure gauge in preference to a temperature gauge, I am, however, in good company. At least one major boat builder I know--and they are strongly partnered with an engine building company-- is rigging their boats with a water pressure gauge, but no engine temperature gauge. The boat builder is Boston Whaler and the engine company they partner with is Mercury. I can't believe they're both getting this wrong, too.
L H G	posted 04-21-2009 01:49 AM ET (US) Jim, easy explanation, and one that was discussed here several years ago when Whaler first started putting Mercaha 4-stroke on the Montauks. This thread is about a 2-stroke Mercury. As I said earlier, for these engines Mercury, and subsequently BW, recommended and installed Temp gauges. Any participant here who bought a Montauk with a 2-stroke will tell you the boat came with Temp gauge. Regarding the 4-strokes, I was told that Mercury wanted those shipped with water pressure gauge instead of temp. Why the recommended difference between the two technologies, I can't answer.
jimh	posted 04-21-2009 10:35 AM ET (US) Larry--I was told--and I think it was by you--that Mercury provides a water pressure sensor for all of their SmartCraft motors as a standard set up. It seems odd that if there were some sort of preference from Mercury that water pressure was not an important engine parameter to monitor that they would provide a sensor for it. All modern engines have electronic sensors for temperature because this parameter is monitored for use in calculating the engine operating parameters with fuel and air. Since the fuel and air are electronically controlled, the engine needs the temperature information, too. It goes without saying that modern engines will all provide electronic temperature readout. As I mentioned, I have both a water pressure and a temperature gauge. I monitor both. The water pressure gauge is like an early-warning system. If there is debris fouled on the gearcase of the engine it usually shows up immediately as a reduction in water pressure. I stop and clear the debris off the gearcase. Temperature may be more useful on Mercury engines since their cooling system seems to be more prone to problems with their regulating and control valves. The thermostats and poppet valves are often cited as having failed due to build-up of corrosion, and such a malfunction might not be reflected in a water pressure reading. And, yes, I totally agree. If your engine cooling system is malfunctioning and is providing too much cooling, you will never detect this on a pressure gauge. You need a temperature gauge to see that there is too much cooling being provided.