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Author Topic:   PSI Gauge on Montauk 170
Jim D posted 05-09-2003 01:35 PM ET (US)   Profile for Jim D   Send Email to Jim D  
My 2003 90hp Mercury outboard has a psi gauge instead of a temperature gauge. The gauge reading varies from 2psi, usually at slow speed,to 10psi at WOT. It seems to vary from 2 to 10 with no apparent reason however. The owners manual does not mention this gauge or how to interpret the information. If it's trying to tell me that the water pump is working...well, the stream from the outboard tells me that. The engine does have a high temp warning alarm.
Can someone tell what the "normal" reading is for this gauge and how I interpret the information?
jaccoserv posted 05-09-2003 01:41 PM ET (US)     Profile for jaccoserv  Send Email to jaccoserv     
My advise, ignore the gauge and just keep your eye on the stream. It's pretty silly to put a water pressure gauge on these boats.
Jarhead posted 05-09-2003 02:06 PM ET (US)     Profile for Jarhead  Send Email to Jarhead     
Thanks for asking the question Jim D. I was wondering the same thing.....
triblet posted 05-09-2003 04:24 PM ET (US)     Profile for triblet  Send Email to triblet     
I think it's going to vary based on engine
speed and how far the thermostat is open.


osillator posted 05-11-2003 05:15 PM ET (US)     Profile for osillator  Send Email to osillator     
On my 170 the gage runs about the same, I beleive that mercury says there is pressure control on the pump. I have noticed that as you drop RPMs the pressure will drop to a point and when you get to idle it will jump back up in pressure a bit.

Give me a water temp gage anyday!!

BMR posted 05-11-2003 06:18 PM ET (US)     Profile for BMR    
I know from a well informed source that Mercury ahs recommended that builders install water pressuer gauges rather than temp guages if only one or the other is used. The reasoning is that when a temp gauge reacts to the the change in the operating temperature, it is most likely too late.

I run a Merc 90 4 stroke and at idle in gear I see about 5PSI. At WOT, I see between 15 and 20 PSI. The more you use your boat, the more familure you will become as to the normal opeating range of your boat at different RPMs. Remeber that the PSI gauge will show a higher reading at a higher RPM.

Jerry Townsend posted 05-11-2003 07:09 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
I am not running a Mercury engine - but as an engineer, BMR's statement caught my attention. From an engineering, safety and accident analyses viewpoint and a system control viewpoint, temperature is a much preferable indicator.

That is, pressure will tell you that there is pressure from the pump - but nothing else. There may be pressure, but the passage may be blocked or restricted, a thermostat stuck closed, Whereas, a temperature indicator will tell you that heat, the principal parameter of interest, is being removed or is not being removed. And, as everyone knows, if heat is not being removed, the lack of coolant flow, for what-ever reason, is normally the problem.

If the alarm systems are working properly, the problem is identified sufficiently early to prevent major damage. ------ Jerry/Idaho

lhg posted 05-11-2003 09:37 PM ET (US)     Profile for lhg    
I have always understood that with outboards, at least, the water pressure gauge is a more reliable, and immediate, source of proper cooling system operation. The reason for this being, at least based on my on my rigging experience with Mercurys, is that the water pressure take-off fitting is on the very top cylinder, so if adequate water is not getting all the way up there, you have a potential burn out of this cylinder, or others. As an impeller goes, it's always the top cylinders that get damaged first, hence the take-off at the top of the cylinder bank.
Jerry Townsend posted 05-11-2003 10:30 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Larry - while it is true that if you are not getting water to the top cylinder, that cylinder may be damaged - but, as I mentioned above, having pressure doesn't say anything about cooling. Certainly, you can have pressure at the top cylinder and the flow blocked or restricted which would not provide much, if any cooling and that cylinder would be damaged.

Personally, I want to see the result of some action or result rather than what goes into it and hoping that everything in the system works as designed. A business doesn't buy equipment without seeing a need. Of course, a business can buy equipment based on an expected or anticipated need - but the real proof of the pudding is the end result. An investor doesn't keep buying a stock without seeing the result of previous purchases. Essentially in every case, we look at a result, an ending result (feedback) for an indication of correction.

As such in the case of the cooling system performance, the temperature is a much more direct and true indication of cooling performance. But, know what - Mercury can build their engines any way they see fit - they are running that show. ---- Jerry/Idaho

Jim D posted 05-12-2003 12:01 AM ET (US)     Profile for Jim D  Send Email to Jim D     
Thanks to all for the great advice.
I can't talk to the mechanics of how or why a psi gauge operates. My concern is more basic.
As the operator of my boat, what is this gauge trying to tell me? What should I be seeing on the gauge that should concern me and cause me to seek repair service? If I just ignore it, then why did Mercury put it on my dash?
Thanks again.
Jerry Townsend posted 05-12-2003 01:32 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Jim D - with a pressure gauge - 1) with pressure indicated, you know your pump is working AND the pump inlet is open, but that is it: 2) with no pressure, you know your pump is not working OR the inlet flow to the pump is blocked. Obviously, with no pressure, you need to correct the problem.

As to why Mercury put a pressure gauge on your dash - you'll have to ask them. ---- Jerry/Idaho

Clark Roberts posted 05-12-2003 06:07 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
JimD, different engines have different "normal" jacket water PSI at different speeds. Mercury should be able to supply you with the normal PSI range so that you can make some sense out of the gauge. Press gauges are useful when a jack plate is used or engine mounted very high on transom. The water intake ports can be so high that air is sucked in and water press goes to nothing... not good...etc. I have a power jack plate on a 21 Revenge with 115 Merc 4S and use a press gauge primarily to make sure engine is not raised too high (guess I'm a gauge junkie in that I run water press, water temp and oil press gauges). Bottom line, I suppose, is that you really don't NEED any gauges (including a tach) and can rely solely on the engines alarm audible system! It's a matter of choice and for me the more information the better! Happy Whalin'... Clark... Spruce Creek Navy
PS> I'm no fan of the "check engine" light phylosophy... don't like guessing games... What?... just the facts, Mam, and let ME decide if the engine needs "checking"!
lhg posted 11-08-2003 03:19 PM ET (US)     Profile for lhg    
I just experienced a situation regarding a water pressure gauge, and urge you to refer to BMR's first post above. I now can say with authority that what he said was correct, in that a temp gauge does not repsond fast enough to save an engine. Mercury knows what they're talking about after all.

My Merc 200 EFI's, equipped with both Temp and Water Pressure gauges, needed their 2 year impellers service, so I had my dealer do the replacements. For some reason, a different mechanic did the work on each engine.

Taking out the boat last weekend, after launching, I idled it over to a dock to pick up friends. Both telltales working perfectly, but one of the water pressure gauges was dead, the other showing the usual 2-3 lbs at idle. Seemed strange after new work just being done. On Merc V-6's at least, the water pressure take off is on the top of the uppermost cylinder, which certainly makes sense, as this is the farthest that water needs to be pushed up. Checked the engine fitting, no problem, checked the gauge, no problem. Still no water pressure, and top of engine getting very hot. Cylinder head temp gauge only showing a medium reading. So I shut it down immediately. My suspicion was correct, (but I didn't know it at the time) absolutely NO WATER getting up to the top cylinders, but still enough to cool the lower cylinders and produce a telltale.

So I let it cool off for a while, then started them up and planed the boat off. One gauge showing 10 lbs, other one DEAD, no water at all even at 3000RPM. So I back down fast, and shut off the bad engine, now engine temp gauge showing a 1/2 reading, which is not unusal at all, and no overheat alarm. It was obviously hot in there somewhere, as some steam was evident. Enjoyed the rest of our day on one engine. With no pressure reading at all, I knew the impeller replacement work was bad.

So I took it to the dealer, and they confirmed bad workmanship, and thanked me for being so attentive to the water pressure (and saving them from buying me a new 200 EFI!) Some gasket or bushing had gone in bad, and was partially blocking water flow. He said I would have fried the top cylinders in a matter of minutes at running speed, if I hadn't noticed the absence of water pressure, and long before the temp and alarms would have gone off.

So get yourselves water pressure gauges if you don't already have them. Nor am I saying the temp gauge is not needed. You should have both, because between them, the telltale and the overheat alarm, you get all of the information needed to prevent damage and/or failure.

Knot at Work posted 11-09-2003 09:06 AM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     
I use my gauge as a baseline, I know that for example at idle I can expect 4 to 6 psi at idle and I should see a coresponding rise in psi as I increase speed and rpm. I use it to scan for abnormalities instead of a accurate tick mark by tick mark analysis of performance.

I agree If NO rise in guage while running check for the blockage, if you notice a wild fluncuation in your guage you also need to have that checked.

my 2 cents

Jerry Townsend posted 11-09-2003 05:02 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Though I see the words - the premise that a pressure guage saved an engine is wrong. As I have mentioned before, pressure is not an indication of proper engine cooling! That is fact - there are no ands-ifs-or-buts about it. A pressure indication simply shows the pump/impeller is working - nothing else. Whereas a temperature indication tells one several things - that the cooling system is working (or not working) - which requires an adequate flow of coolant and the heat removal process being effective.

Further, relying on pressure alone is not a viable control parameter. That is, pressure is a feed-forward control parameter - indicative of only the input conditions. Temperature is a feed-back control parameter - telling one the process is working - or not working.

Relying on pressure is akin to using throttle position for speed control on your vehicle - wouldn't work too well going up or down a long steep hill would it? In this case, throttle position is a feed-forward parameter.

Therefore, proper engine performance indications requires including the temperature. One can have the pressure, the GPS location, the geophysical position of the moon and the temperature of the operator - but it is the temperature of the engine that tells how the engine is being cooled.

Now Larry (lhg), your scenario leads me to several questions. One, if you had tell-tale flow, you had pressure - by definition. Now, if your pressure was low, the tell-tale flow would be noticeably slow. The tell-tale flow is a very good indication of pump/impeller operation. Now, I don't know where the tell-tale flow is taken from - i.e. from the impeller discharge or from near the exit as it is on my Johnson. In my mind, it should be from a high point on the engine. Two, if you had steam in your engine - and your temperature was just normal, something is wrong with your temperature indication - the instrumentation is defective, the thermocouple wasn't seeing the steam, or That is, a thermocouple doesn't care if the media is water, steam, Canadian Club or pepsi - but 212F is not normal . And again, I don't know the design details on the temperature measurements on your engine. Here, I would think/hope/ that the temperature indication would be the highest temperature of all cylinders and the exit temperature of the fluid.

Now, why would a manufacturer use only pressure? There are a multitude of possible reasons including - the selection/decision may not have been made by one familiar with the technical aspects of the choice. Also, using a single pressure guage is a lot cheaper than installing several thermocouples and the electronics to average or select the maximum. And there are undoubtedly other possibilities - and only the responsible engine manufacturering people know the answer. --- Jerry/Idaho

lhg posted 11-09-2003 07:01 PM ET (US)     Profile for lhg    
Jerry, as I mentioned, my 200's have all four of the standard cooling indicators - C-H temp gauge with 1/4 increments and electric sender on top cylinder head, 30 psi Water pressure gauge, with mechanical water tubing take-off on top of top cylinder, Overheat warning horn, with separate sender from temp gauge, and telltale.

I've got to say, I also was very surprised to see the strong telltale with a zero water pressure gauge reading, expecially at 3000 rpm, which is about 32 MPH cruising speed on my boat. I was convinced the gauge was wrong, but this was not the case. The temp gauge was clearly, very slowly, moving up relative to the gauge on the other engine, but by the time it would have taken to reach full hot, which also sets off the alarm horn & kills starting circuit, my mechanic told me the engine would have been severely damaged, if not frozen up. Only the -0- water pressure gauge reading gave me the immediate signal to shut down, resulting in no damage at all.

The problem, and partial water tube block, was down in the lower unit, and according to the dealer, when the mechanic slid it back up after installing a whole new impeller kit, some bushing misaligned on the water tube, unknown to him. They tested the engine with boat on the trailer, since they are not in the water, and the telltale was OK. They didn't run it long enough to see it overheat. Maybe this was a unique situation, and unique to a Mercury V-6? Who knows, but I am surprised to see you doubt the judgement of Mercury's engineers, with the company's long time outboard racing history.

SuburbanBoy posted 11-09-2003 09:20 PM ET (US)     Profile for SuburbanBoy  Send Email to SuburbanBoy     
Lost my Black Max to the demons of improper water cooling. It nerver lost it's telltail. It was too far out of the water while on plane (it had a hydraulic jacking plate) and it was drawing air. It did not have a gauge.


Jerry Townsend posted 11-10-2003 12:52 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Larry - from your explanation - your event was most obviously caused by the blocked flow as you point out and evident from your earlier words. Now there were a few contributing factors including - the full tell-tale stream and the low temperature.

Your explanation tells me several things - 1) the tell-tale stream is taken from a point between the impeller and the blocked bushing and 2) there was no water in the block at all and the cylinders were dissapating heat to basically air. Had there been water around the cylinders - even stagnant water, there would have been steam which would have run the temperature gauge up-scale and triggered the
alarm system.

Now, taking the tell-tale stream from a point high on the engine eliminates a lot of problems - indicates proper/adequate flow and also is indicative of the heat removal process. Further, it also is indicative of any blocked flow. Eliminates a lot of problems - agreed?

I was thinking, but am not sure, that the tell-tale stream from my engine is warm/hot. Johnson only refers to the tell-tale stream as a pump pressure indicator.

The blocked flow is simply a design engineering failure - that is, in my experience, it would be an engineering failure. One question that should be asked during the design is - "Is there any way the flow can be blocked or fail?" Of course, an impeller failure would prevent flow. A blocked flow path would also prevent flow. Then, another question - is there any way the flow can be blocked? The answer should reveal a problem if the bushing is improperly installed. The design must then devise the method for preventing that bushing from being improperly installed or alter the design to preclude that possibility. All of this stuff comes out of fault-tree safety/accident analyses - which would reveal all similar potential problems.

Now, if the design cannot guarantee the flow cannot be blocked, the alarm/shut-down system should include no flow capability.

But, engineers/management from every outboard engine manufacturer have their reasons for designing the engines as they do. And in a commercial business, cost is often the controlling parameter. Only they can answer these questions.

Larry - I am not an outboard engine design engineer - just an engineer with many, many years of experience in fluid flow, heat transfer, dynamics, safety and accident analyses, - and applicable instrumentation. If this thing were to eliminate all potential problems, the outboard might look like a nuclear system or a missile - heaven forbid. ---- Jerry/Idaho

lhg posted 11-10-2003 03:03 AM ET (US)     Profile for lhg    
Jerry - From what the mechanic told me, the partial blockage was down in the lower unit, below the powerhead. A reduced amount of water was being pumped up to the engine, but not enough to get all the way up to the top cylinders, at least not enough to cool them properly or register on the water pressure gauge, even at speed. I don't know exactly where Mercury takes off the telltale, but it was definitely after the blockage. I suppose the telltale could have been slightly less volume, but it was not evident visually.

I have noticed that on idle, with normal functioning, the telltale is always extremely hot, enough that you can't hold your hand under it at all. And when the engine is started, it takes a couple of minutes for the telltale to even show up. This indicates the water goes all the way through the engine first. And I did see some abnormal steam. Remember also that these engines have poppit valves, which intentionally heat the engine up for a smooth idle, usually to where the temp gauge registers 1/2, then allow it to cool down at higher RPM. In 80 degree water, idling these engines through a no-wake zone at about 1000 RPM in gear, will bring the temp gauge up to a 7/8 reading, but no higher.

Maybe Clark Roberts, who is familiar with the Mercury V-6, will help us out here? Clark?

cape_rover posted 11-10-2003 06:44 AM ET (US)     Profile for cape_rover  Send Email to cape_rover     
The problem with temp guages and the overtemp sounder is that they do not provide realtime information. You can have an engine fry before the temp alarm sounds and then once it is fried the alarm sounds. This can occur if you pickup heavy weeds or trim the engine too high. Under both these conditions, the PSI guage would give you early warning.
NoviceWhaler posted 11-10-2003 10:47 AM ET (US)     Profile for NoviceWhaler  Send Email to NoviceWhaler     
And the moral of the story is...

Know your engine instrumentation and use it appropriately to diagnose your problem before you fry your engine.

Knot at Work posted 11-10-2003 11:00 AM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     
Once bitten, Twice Fried....
Tom W Clark posted 11-10-2003 01:28 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
If I could only have one gauge, water temperature or water pressure, it would definitely be the water pressure gauge.

As pointed out several times above, a water temperature gauge will only tell you the engine is hot after it's been that way for a LONG time. A water pressure gauge will tell you there's a problem NOW.

I have always installed both types of gauges. Being a bit of a gear head who likes to have as much information as possible, I like both, but the pressure gauge is the more sensible.

A water pressure gauge does not cost much at all. It is very simple device and operates just like a pitot tube powered speedometer. c. $30 for the whole kit. Why not install one if dash space allows?

Tom2697 posted 11-10-2003 02:53 PM ET (US)     Profile for Tom2697  Send Email to Tom2697     
Ahhhh...the infamous "idiot light" issue...

Warning alarms (sound, lights, etc) tell you when there is a problem. Always after some predetermined mark has been reached (low oil, high temp, low battery, etc). They do not tell you when a problem will occur or is likely to occur. These devices only state that there is a problem. The tell-tale is basically the idiot light for water pressure. But, you can have perfect cooling even without water flowing from this tube. It is nothing more than an indicator. The over-heat alarm will tell you that the engine is hotter than it should be. Hopefully, the sending unit will still be working as the designers intended it to. Here are your two "idiot lights" on your outboard. Now, many people have run their motors for years using only these two devices.

Gauges give you real-time data on the existing conditions. A good gauge, if designed and working properly, will tell you how things are right now, and if you check them frequently, that something has changed. So, if you were to install only one gauge, which would it be? A water temp gauge could be installed in a location that does get bypassed by a coolant flow blockage as LHG has seen. I've seen water temp gauges showing normal readings because the coolant is cool but, the engine was passing the coolant too fast for an effective amount of heat to be dissipated. A water pressure gauge provides better information than a water temperature gauge. But, it still can show strange readings. I've seen water pressure gauges to read normal when the engine is turned off because dirt (salt, silt, sand, mud, etc) got stuck in the pick-up. A third gauge that you can use is an engine temp gauge. The engine temp gauge should tell you the temp of the hottest part of the motor. This gauge reads the temperature in the engine block and is not affected by coolant flowing around the sensor. The engine temp gauge has only failed me when it got physically damaged during an engine repair.

So, the best gauges are the engine temp, the water pressure, and the water temp; in that order.

BTW - the water pressure should be between 3 psi (at idle) and 15-20 psi (at WOT) or thereabouts. Anything vastly different at these speeds indicates a problem. For instance, a reading of 20 psi at idle means that the water passage is blocked after the sending unit. A reading of 3 psi at WOT means that not enough water is flowing thru the motor.

lhg posted 11-10-2003 03:27 PM ET (US)     Profile for lhg    
Tom - It has been my experience that different engine brands, and Hp ratings within a brand, often operate normally at differing water pressures. My in-line 6 Mercurys are specified to use a 15 psi gauge, while the V-6 Mercury's are specified to use a 30 PSI gauge. I think I have seen some OMC engines that specifiy 60 psi gauges.

With outboards, the so-called temperature gauge is always a cylinder head temperature reading, as opposed to the block.
The electric sender is always on the uppermost cylinder head.

In my case, the -0- water pressure reading gave an immediate warning that no water was present, while the temp sender on the SAME top cylinder takes at least 5 minutes to show full hot, and set off the alarm. That's your possible engine damage window.

And the other thing I have noticed, that when you're out running at speed, enjoying the ride, etc., one does not often check the gauges continually. On one such occasion, my son was driving the boatat about 35mph, and all of a sudden that obnoxious overheat horn went off. The gauges were not being continually observed, as is usually the case quite frankly. One engine had picked up a plastic bag, totally blocking water flow. We probably went along like this for 3-5 minutes before the horn went off. The engine was screaming hot, and would not even start for 10 miutes while it cooled down to a 7/8 temp reading. So in this situation, the horn was absolutely the engine saver. But here too, HAD the WP gauge been observed, it would have given the FIRST indication that something was wrong. The engine survived, but the impeller was cooked and had to be replaced. I really believe all 4 overheat prevention devices are necessary, working together.

Tom2697 posted 11-11-2003 10:07 AM ET (US)     Profile for Tom2697  Send Email to Tom2697     
That is definitely the must watch the gauges. How many people read the gauges in their car? Many only know about the speedo and the fuel gauge. They have no idea what the other gauges do. Personally, I like having more info than needed. I have the standard tell-tale and overheat horn on my motor. I also have am engine temp gauge and a water pressure gauge. I look at these more than I look at the fuel gauge.

Since you said that your temp gauge was not reading properly (it was slow), check the installation. I had a mechanic work on my boat and he told me that the sensing unit is not supposed to touch the block. I told him to pull his head out of some orifice. When I adjusted the sensing unit to the correct installation level, I immediately got readings indicating a partial blockage of water flow (higher than normal temp). I snaked all the lines and found crud in the external lines that was preventing good water movement.

I'm glad you were able to save your engine! As you stated, I'm certain the repair shop was even more glad!

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