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Author Topic:   Mercury 90-HP Water Pressure Reading Low
Whalen posted 04-07-2004 12:05 AM ET (US)   Profile for Whalen  
I have a 2004 [170 MONTAUK] with the 90 two-stroke engine, and last year when I got the boat the water pressure gauge would read up to 15 PSI. I have had the boat out at least 10 times. I have noticed that the pressure rarely exceeds 10-PSI and is now at 5 to 7 PSI at 3800-RPM speeds. I would like to think that this is because of the colder water temps since it was new. The water last September was 65 degrees. While I was out last week for two hours the water temp was 42. The water pressure after a long run would be about 7-PSI and after a hot restart would climb to 12-PSI and quickly fall back to the 5 to 7-PSI range. Is this normal? I would like to think it is due to the thermostat activation. But now I am wondering if I might of ingested silt in the impeller, and sanded it down. It runs great otherwise, but it will be several months till the water warms up enough to prove my theory. I do not want any problems in the meantime. Thanks-
Barney posted 04-07-2004 07:22 AM ET (US)     Profile for Barney  Send Email to Barney     
Whalen, Those recent figures look to low. The trend looks like you need to check the pump and also the gauge hose. Jim
Uncle Pug posted 04-07-2004 09:25 AM ET (US)     Profile for Uncle Pug    
This is a question I would be asking my dealer.
AQUANUT posted 04-07-2004 09:56 AM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
!> check to insure hose is not crimped..
2. check guage
3. check water pump impeller
cape_rover posted 04-07-2004 12:15 PM ET (US)     Profile for cape_rover  Send Email to cape_rover     
consider installing a temp guage
bobeson posted 04-07-2004 02:55 PM ET (US)     Profile for bobeson  Send Email to bobeson     

I've also been experiencing what I consider faulty gauge readings on my 2003 170. At first it worked fine, then I started getting intermittent low readings, but the tell-tale stream out the back of the motor looked healthy. Now I often get 0psi readings while the stream still looks perfect, and sometimes the guage reads a little higher (2-7 psi) but never full strength. I've been guessing a problem with the gauge hose also, but I've been unable to locate the problem exactly so far. That's on my to-do list for this weekend, actually, so I'll let you know if I figure it out.
tabasco posted 04-07-2004 03:05 PM ET (US)     Profile for tabasco  Send Email to tabasco     
Hey guys that's what a warrantee is for.......just bring it back to your dealer
Whalen posted 04-07-2004 04:05 PM ET (US)     Profile for Whalen    
Thanks for the input. The dealer says it is low and culd be the impeller or poppit valve. I live 85 miles from the dealer and he wants the boat for several days. It's fishing season now. I some how do not count on a warrantee fix.
Whalen posted 04-07-2004 08:21 PM ET (US)     Profile for Whalen    
I went out and checked the pressure gauge and found it to be reading 3 psi low. Also found it to be full of salt water even though the last four hours of running were in fresh water.
bobeson posted 04-07-2004 08:27 PM ET (US)     Profile for bobeson  Send Email to bobeson     

I would like to bring it back to my dealer, except that the last time I brought the boat to my dealer it was a disaster. They went from a great shop (Outboard Motor Shop) to something much less than great when they merged with SeaPower Marine. Last time they had my boat for over three weeks, after initially telling me it would be 4-7 days, and I missed a number of excellent fishing opportunities as a result. The resulting work also had some serious problems, and it cost me an arm and a leg. Unfortunately they are the only BW dealer in Northern California, and they are also a bit of a drive from my house (~90 minutes each way). I've decided to start doing the bulk of my maintenence myself.
whaler1234 posted 07-24-2005 05:21 AM ET (US)     Profile for whaler1234  Send Email to whaler1234     
I recently had a problem with my '04 170 pressure gauge as well. Water was coming out the telltale normally, but my gauge showed 0 pressure. I took it to a local shop where they diagnosed the problem as salt buildup in the line running up the tunnel. Even though I flush the motor after every outing for at least five minutes, it apparently wasn't enough. I'll now start using salt away with every flush.

Beaner posted 07-24-2005 09:34 AM ET (US)     Profile for Beaner  Send Email to Beaner     
My pressure gauge on my 2003 Montauk has never read over 10 psi. Usually it reads about 5-7 psi. I've mentioned it to my Whaler dealer and my local Mercury guy and nothing has been done to fix the problem (assuming there is one). I have always just checked to make sure I have a steady stream coming out of the tell tale hole. Anyway, I was under the impression that the pressure gauge measures that pressure, so if the stream is strong, all is well. I certainly wouldn't want to give my boat up now for a few weeks to fix a gauge problem that most 170 Montauk owners seem to be having.
whale tauk posted 07-24-2005 01:00 PM ET (US)     Profile for whale tauk  Send Email to whale tauk     
I had the same thing with my 2004 170 Montauk. The dealer said that as long as the telltale was strong there was nothing to worry about- from my perspective, I would like to use the gauge and not constantly look over my shoulder at the telltale stream. My dealer is 80 miles away as well.

One day it started giving readings again. I still check the telltale often as it's hard to have a lot of confidence in the gauge. I think the line needs an angioplasty

Whalen posted 07-24-2005 01:51 PM ET (US)     Profile for Whalen    
Well it has been quite some time since my first post of this problem. I had the dealer check it out last year and it made no difference. Since then I have replaced the gauge, replaced the impeller, and it is exactly the same reading. I have put 800 miles on it this way and it looks like a non issue. Just thought i would update you! By the way my water gauge is always fogged no matter what, the new one fogged right away!
bsmotril posted 07-25-2005 01:44 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Flushing the motor will not clear the gauge line of salt. There is little to no water movement through that line. The only way to flush salt out of the gauge line is to install a flushing tee near the gauge, and backflush it. Or, remove the pressure line from the gauge and backflush it from there. Use warm or hot water to get the salt out. BillS
tabasco posted 07-28-2005 09:33 AM ET (US)     Profile for tabasco  Send Email to tabasco     
Why not just clear the line by blowing through it with an air hose?
bsmotril posted 07-28-2005 10:13 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
It has been my experience that air pressure will not work on the small diameter hard plastic pressure lines. Maybe it would work on a rubber hose. The salt is packed in there too tight and you have to dissolve it. The other option is just replace the tubing, it's very inexpensive. BillS
Chuck Tribolet posted 07-28-2005 05:12 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
It seems to me that the pressure guage line could be flushed
by disconnecting it at the gauge and running the motor on
freshwater, either a lake/river or the hose.


bsmotril posted 07-28-2005 11:29 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Yup. If it had some kind of quick disconnect behind the gauge, you could just pull it off when you flush your motor normally, and flush the tube too in the process. Bills
whaler1234 posted 10-22-2005 03:36 AM ET (US)     Profile for whaler1234  Send Email to whaler1234     
Had my '04 170 in the shop again for round two of the water pressure gauge problem. It was the same problem as before... salt buildup. It completely clogged the small orifice on the engine block where the brass fitting screws into. The mechanic "chiseled" out all the salt after removing the water pressure gauge fitting and brass fitting. Its now reading just as it should.

The merc manual states to apply some loctite 567 on these threads during reinstall.

Barney posted 10-22-2005 08:25 AM ET (US)     Profile for Barney  Send Email to Barney     
Thanks for the update. My gauge stopped working the other day. The hose is clear so I will check the fitting. Barn
andygere posted 10-22-2005 12:22 PM ET (US)     Profile for andygere  Send Email to andygere     
Moore and Sons in Santa Cruz is a very good Mercury shop, and a lot closer to home for you.

1431 E Cliff Dr
Santa Cruz, CA  95062-4840

whaler1234 posted 10-22-2005 08:25 PM ET (US)     Profile for whaler1234  Send Email to whaler1234     
For reference, these measurements for Water pressure are from the 75/90HP model year 2000+ Merc service manual.

RPM Pump Pressure (PSI)
1000RPM ~6PSI
2000RPM ~8PSI
3000RPM ~12PSI
4000RPM ~15PSI
5000RPM ~17PSI
6000RPM ~20PSI

swist posted 10-22-2005 09:14 PM ET (US)     Profile for swist  Send Email to swist     
This does not sound like a problem that needs a visit to the dealer. Is it difficult to clean out this tube and engine fitting?
jimh posted 10-23-2005 06:51 PM ET (US)     Profile for jimh  Send Email to jimh     
Do the water pressure senders on all outboard motors which operate in saltwater require periodic removal and cleaning? Or is this something peculiar to the Mercury 90-HP two-stroke motor?
swist posted 10-23-2005 07:10 PM ET (US)     Profile for swist  Send Email to swist     
Oops, might be some confusion here - I had assumed there would be no difference in the way the pressure sender works between 2 and 4 storkes, but matbe not?
sosmerc posted 10-23-2005 08:00 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
One of the advantages of SmartCraft compatible engines such as the Optimax and Verado is that water pressure is monitored via an electronic sender back on the engine. No more problems with clogged or pinched lines from engine to guage (although if salt builds up between the block and where the transducer picks ups its pressure signal you could still have a problem).
On the mechanical water pressure guages Merc has gone from the old rubber hose, to the tiny black shiny plastic hose, to grey colored small diameter rubber hose..and now back to larger black rubber hose...I get the feeling that the larger diameter rubber hose is superior, if only because it is easier to blow out when it gets clogged!
I'll be happy when ALL Mercury engines finally become SmartCraft's a neat system that all they other manufacturers will soon have their own version of.
whaler1234 posted 10-24-2005 03:06 AM ET (US)     Profile for whaler1234  Send Email to whaler1234     
Sorry.. didn't catch that. Mine is a 4 stroke 90. My shop called Merc to inquire about this and Merc stated that they have not heard of this problem before. Don't know why this is happening just 3mo after my initial repair. I religiously flush the motor after every trip.
toolman posted 10-24-2009 05:52 PM ET (US)     Profile for toolman  Send Email to toolman     
Has anyone tried blowing thru the line and seeing what the gage reads? I put a new gage on my mercury 225 and it only reads 5 psi wot, It has a new water pump and lower unit. I replaced the water line and still only 5 psi. A friend told me you should be able to blow at least 10 psi on the gage by blowing thru the line with your mouth but I can only get 2 psi. Has anyone got any imput? Thanks
jimh posted 10-25-2009 08:39 PM ET (US)     Profile for jimh  Send Email to jimh     
sosmerc--Thank you for the updated information on the design of the hose being used by Mercury to connect their water pressure gauge to the engine. I had noticed that some Mercury motors I had seen used a very small diameter semi-rigid plastic-like hose to carry the water pressure from the engine to the gauge. With the many reports of clogging and loss of pressure, it sounds like Mercury changed their design and are now using a larger diameter hose.

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