Water Pressure

kordo

Anyone have an opinion as to what is causing low water pressure as shown on the below pics.  The first one shows water pressure at idle and the second shows water pressure at cruise.  I have already switched the water pressure senders, port engine still shows lower pressure.  I also replaced the impellor on the port engine, no change.  This problem just started a couple weeks ago.  Thanks.



The

skennelly

Did you collect all the bits and pieces of the old impeller?  Maybe clogged up.

Also maybe water inlet hose is pinched or collapsed?

GMSLITHO

Yep check inlet hose or blockage in drive

kordo

No pieces from old impellor.  Water inlet hose (the part that is visible without removing the drive) looks fine, not collapsed.  I guess I'll take it off and check inside for blockages.  The thing that puzzles me is that the engine temp is 163 degrees on the port engine, right in line with where it should be. 

Michael T

IMO - that may still indicate a sensor problem, In say this because a couple of years ago a friend had a similar problem - almost the exact engine temperatures with radically different water psi. His turned out to be a sensor after the original was replaced, then the supply hoses then the water pump. About 2K later it was determined the replacement sensor was faulty too!

IMO - I think your port engine would be much hotter if it were only receiving < 2 psi at almost 4K rpm.

On my port 2014 Merc 502 we had to replace  the psi water sensor THREE times to get a correct reading.

At first the Merc  doubted it and replaced - under warranty - several other expensive parts and a lot of labor, but in the end it was the friggin' sensor. Lucky for me it didn't cost me a cent!

Good luck with the trouble shooting!

reneechris14

Was there any scaring on the housing of the pump. This causes the pump to lose vacuum.

kordo

Michael T, I switched sensors between the port and starboard and found the same readings, so I don't think it is a sensor issue.  Thank you.

reneechris14, I'll check with the mechanic who replaced the impellor on whether or not he noticed any scoring in the pump.  Thanks.

Dream_Inn

reneechris14 said:

Was there any scaring on the housing of the pump. This causes the pump to lose vacuum.

Exactly what I was thinking as well.  If there is scaring, it may only be getting partial water pickup (not fully priming).  I've had to replace both of my water pumps over the last few years due to this.  Look in to the stainless Hardin pumps.

MarkB

Low pressure and it doesn't overheat?  Is your thermostat stuck open?

kordo

MarkB, not sure I understand your comment.  If thermostat was stuck open on one engine, that engine would run cold.  That is not the case, temps on both engines are normal at all rpms.  Thanks.

goalie59

I replaced my starboard pump with a rebuild kit from aftermarket marine and my pressure is 18. The port pump has a new impeller but has some scoring on the housings  , the pressure on the port side is 9. Both engines run at 154 degrees. I would check your pump housing. 

Dream_Inn

I'll add that installed a hardin pump on one engine beginning of last year, and a mercruiser on the engine the year before.  Both had new impellers last year (I'll replace this coming spring).  The engine with the Hardin pump gets about 23psi while the Merc one is around 20 psi (while on plane).  To me it shows the Hardin one is better built to give a bit more psi.

MarkB

kordo said:

MarkB, not sure I understand your comment.  If thermostat was stuck open on one engine, that engine would run cold.  That is not the case, temps on both engines are normal at all rpms.  Thanks.

My point is, if low pressure was caused by a block in the suction line of the pump, you wouldn't get flow, and the engine would overheat.  If you are maintaining temperature, then you have good flow. If that pressure is real (the sensor isn't messed up), then flow can only occur at high pressure with little to no resistance, which means the thermostat could be stuck open. If it's stuck open, then yes I'd be looking for 120 to 140F temperature ... which you don't have, so it's unusual. Probably another faulty sensor. Can you swap the one from the other engine?

MarkB

Dream_Inn said:

I'll add that installed a hardin pump on one engine beginning of last year, and a mercruiser on the engine the year before.  Both had new impellers last year (I'll replace this coming spring).  The engine with the Hardin pump gets about 23psi while the Merc one is around 20 psi (while on plane).  To me it shows the Hardin one is better built to give a bit more psi.

PSI is not about the pump ... it's about the resistance to flow which is everything downstream of the pump. We call that the system pressure.  As flow goes up, the system pressure goes up, and the pump and system come to balance (pump delivers flow at the system pressure).

Dream_Inn

I agree Mark, any way you look at it, the Hardin one is creating more flow, thus higher system pressure.  Therefore, with both engines running at the same RPMs, the Hardin one is better (if you want more flow).

raybo3

Dream_Inn said:

I agree Mark, any way you look at it, the Hardin one is creating more flow, thus higher system pressure.  Therefore, with both engines running at the same RPMs, the Hardin one is better (if you want more flow).

Not if it doesn't last as long as the Merc one........... Just say'n

kordo

Alswagg, I switched the water pressure sensors and no change in the water pressure readings, that is the port engine water pressure is still significantly lower.  See pics at beginning of thread.  Conclusion, the sensors are not the issue.  The water pressure sensor is located on the pipe (number 17) in the diagram below.  Thanks.

Dream_Inn

You could just switch the water pumps between engines.  Really doesn't take that long.

MarkB

Dream_Inn said:

I agree Mark, any way you look at it, the Hardin one is creating more flow, thus higher system pressure.  Therefore, with both engines running at the same RPMs, the Hardin one is better (if you want more flow).

That I agree with. Either it's creating more flow (which increases the pressure), or it's overcoming some restriction in the system, that the other one couldn't.

kordo

Some new information to add to the mystery.  At idle and when engine is cold, water pressure is the same for both engines.  Above 2,000 rpm, the water pressure on the port engine is half of what is on the starboard engine.  After running for half an hour, the water pressure on the port engine drops below that on the starboard at engine at idle.  Any ideas gentlemen?  Right now, I am leaning towards the port engine pump is scored or there is a blockage somewhere in the hoses on the port engine.  Thanks.

Dream_Inn

Did you swap the seawater pumps?

kordo

No, not yet.  Pretty sure the pump is the problem but thought the rpm observations might confirm it or provide another explanation.  Thanks.

reneechris14

I would thing when pump warms up the impeller gets as little softer thus the pressure may go down just a tad.

Dream_Inn

I do know that bad impellers will sometimes give ok pressure at idle, then get much worse at higher RPMs.  I'd definitely say swapping the pump will tell you something.  I'm not discounting that you could have blockage somewhere, but starting with the pump swap will be easiest.

Ian

So what is the expected pressure for a 5.0 MPI OM34xxxxera? Reason I ask is having just put on VesselView Mobile, I can see more than before and validated gauges are pretty close to ECM data on the SmartCraft bus. Except for water pressure - one reads pretty low, the other 0 so what does the ECM do with this data - if anything? I have not had temperature issues, installed new impellers over winter anyway, ran on land before launch and great flow out and all day today temps were 158/159F. I would think at least the 0 reading would raise an alarm unless this has been blocked in ECM programming due to no sensor - if that is possible. I have to see if I have a sensor in the PS cooler.

rasbury

There is a sensor on the PS cooler, be careful messing with as the mounting will break off very easily....

Ian

Thanks for the warning @rasbury, I saw where it was but can;t find anything documented as to the range you'd expect to see. And how I am not getting low flow alarms if in fact it's enabled to alarm.

Ian

So I found in another forum that water pressure for the 5.0 MPI at idle is in the 1-5 PSI range and at WOT 7-17 PSI range. That explains the "low" value I saw on the port and need to poke around and look at the starboard sensor giving me zero, maybe both are flakey. Doesn't explain why I get no alarms though.

LaRea

Those ranges are what I normally see on my boat.  It's usually around 2-3 psi at idle.  

Low pressure would throw a constant horn and put the engine in limp mode.  If a water pressure sensor is shorted or open, you'd get two beeps every minute.  

Check to make sure VesselView is displaying pressure in PSI and not some other units.

69fastback

I just put a new sea water pump on my boat and took it out for the first time Saturday. 4 psi at idle, and 16 at 3200 RPMS. 

Ian

Thanks @LaRea it is in PSI and that's what I'd figure too but no alarms which is what has me a bit puzzled. And @69fastback due to the flotsam in the water when we brought her home I kept things real slow and low all the way so could see the high numbers but at idle looked about right - for the port side anyway.

River is clearing up so a good run this weekend I hope to get a better look.