Mercruiser 8.1 raw water pressure - typical?

paulswagelock

paulswagelock

Well-Known Member
Oct 25, 2010
2,196
pa
Boat Info
2018 SDX 270 OB 300 Verado
Engines
Verado 300
2008 Merc 496, fresh water cooled engine.
What is the typical water pressure developed by the impeller at idle and elevated Rpm? I see .7 psi at idle and around 6-7 psi at 2800 rpm.
 
I get about 1.7 on the starboard in neutral and about 1.1 on the port engine. I have 8.1 HOs

Low pressure can be a sign of your impellers hitting the end of their life.
 
WDCboater said:
Low pressure can be a sign of your impellers hitting the end of their life.
Click to expand...

Thanks, Agreed. That is why I am trying to see what the specs are so I can determine if my pressure is lower than the recommended pressures.
 
I called Merc tech support today and asked if my pressures were within spec.
They said that they do not publish specs for pressures at specific RPMs, but then said "less than one Psi at idle and 7 psi at 2800 rpm is typical".
 
paulswagelock said:
I called Merc tech support today and asked if my pressures were within spec.
They said that they do not publish specs for pressures at specific RPMs, but then said "less than one Psi at idle and 7 psi at 2800 rpm is typical".
Click to expand...

This is good to know.
 
I take most of my engine stats around 1050RPM ... water pressure is 5.6psi port and 6.3psi on starboard. Out of curiosity, I'll check at 2800 RPM this weekend but I'm sure I'll be above 7psi.
 
Based on what many are seeing, I plan to have the impeller changed even though I am within acceptable pressures, but on the low side. I want to look at access to see if I want to tackle the job or pay the marina to do it.
 
Decided access was too hard. Had marina mechanic change it. Now water pressure is lower. .1 at idle and 6 psi at running rpm. He said the housing was ok. Appears to have sufficient water running out of exhaust, and temp was ok running at the dock. Not sure what to do now. Couldn't go run it to see if temps stay in range because of weather.
 
Its your gauge.....more appropriately, the sender. They suck - take it out and clean (or replace) it, your reading will change again.....I have one motor that reads 7 at 3000rpm and one that reads 36!
 
Had a chance to run it this afternoon. Twice it overheated coming off of speed back to idle, and went into Guardian mode. So the sender could be wrong, but I have less water flow now then before the impeller change because it didn't overheat previously. Either the new impeller is bad, or installed with the vanes the wrong direction ( I sure hope not, mechanic has 30 yrs experience).
 
What year boat? What pump? How many hours on the pump body? If it's the newer model brass pump it self destructs rather quickly by getting grooved up.

The old impellor would get bedded into those grooves. They make a bandaid repair kit for that pump
 
Mechanic decided the scoring must be the issue and plans to swap out the entire pump for a new one. Ordered the replacement and will be installed next week. $680.
i plan to pay for the pump and the labor to install it, but not the original labor and impeller - that will be on him for mis-diagnosing the problem.

my plan is to R&R the old pump by either resurfacing the face, or installing the SS repair plate that is aftermarket. I will either keep it as my spare, or put it up for sale for $250 or so.
 
Your replacing a poorly designed pump with the same. The grooving is only 1 of its problems, the seal and bearing also crap out with low hours.
SS Harden is the way to go
 
Anyone have any actual experience with the Hardin. Several people recommend it, but I haven't found anyone that actually has one.
 
paulswagelock said:
Mechanic decided the scoring must be the issue and plans to swap out the entire pump for a new one. Ordered the replacement and will be installed next week. $680.
i plan to pay for the pump and the labor to install it, but not the original labor and impeller - that will be on him for mis-diagnosing the problem.

my plan is to R&R the old pump by either resurfacing the face, or installing the SS repair plate that is aftermarket. I will either keep it as my spare, or put it up for sale for $250 or so.
Click to expand...

That sounds kind of brutal. Reading the entire thread sounds more like you did the diagnosis but didn't want to do the crappy job. Guessing he was only doing what was asked of him. Hopefully you let him know your intensions ahead of time so he can tell you to go pound some sand.
I can bet his first choice would have been to bolt a new one in.
 
Last edited:
Actually, I had a specific conversation with him to pull it and evaluate. He called me when it was apart and said the housing had some wear but was ok. He recommended just the impeller, not me. So yes, I diagnosed low flow, he picked the solution.
 
Although not in the pleasure craft industry, I have been a licensed hd mech for the past 25 yrs so perhaps this hit a soft spot when someone is quick to blame the monkey. I am well respected in my industry and occasionally get a call from a jippo guy (not implying that's you) that has it all figured out and does not want to spend the $200/hr to properly diagnose things. It can add up real quick and sometimes it is just more economical to throw a cheaper part in quick if it is an obvious problem. Obviously don't know all the info in your situation but can say if it were me having to eat time and parts, and didn't feel it was my mistake alone, then don't come back for help cause you won't be getting it.
I do stand behind my work, and if I do make a mistake I will make it right for the customer. Think any reputable person would no matter what their field. You should probably discuss your concerns with him and work something out together before openly rendering your decision on a forum.
Sorry if I am way off here. Just one guys 2cents.
Good luck with your heating problem!
 
Since when do harley mechanics get $200 an hour ? I think you're full of BS and this used to be a friendly Forum. Just my rant as some of the posts lately have been on the mean side.
 
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