Engine Overheating

[woodl]

Hi everyone,
I recently replaced the impeller on my 1991 OMC 5.7l, I noticed it was running hot, so I thought it may be the thermostat, so I replaced that as well. It still runs really hot, and I cannot figure out why. The temp is OK, until I engage the outdrive, then it gets hot so I disengage and let it idle, or rev the engine out of gear and it cools down. Any ideas on what may be causing this would be really helpful, thank you.

 

[QBhoy]

How did the impeller look when you took the old one out ?
Not too familiar with these, but thought I’d ask if you have a oil cooler in line on the intake cooling pipe between the transom and and the water pump ? I was on a VP engine from a similar age that was drawing air at this cooler and not priming or lifting water to the impeller. Or have you proven that the impeller has a free flowing supply of water to it and not blocked ?
Also ask if you are running her on the hose and seeing her overheat ? Some engine driven water pumps can struggle to take water up from muffs on the outdrive occasionally.

 

[southkogs]

Welcome aboard: are you sure the impeller is installed properly? (Start with the basics, right?) - I'm assuming this is a boat you've replaced the the impeller before?

 

[Scott Danforth]

You are feeding with muffs or in the water, correct? And not trying to suck water from a bucket?

Is this a new problem with an existing boat, or a new to you boat?

Overheating is a lack of water flow

 

[woodl]

Thank you for all the replies.
This is the first time I have replaced the impeller, when I took the old one out, it was trashed and the housing was somewhat melted. So I replaced all the seals, gaskets, impeller and housing. I did end up going with an aftermarket one (housing and all) which was shaped a bit differently, Is it possible that the aftermarket one may not be pulling enough water?
It sounds like my first order of business should be to check if the new impeller is installed correctly and then go from there.

This is a boat my family has had since it was new (30 years ago), it all started when I was trying to find a leak in the power steering pump... that problem ended up being easy, just 2 hose clamps had snapped off and I just needed to replace them.

Also, I have been putting the boat in the water to test it each time.

 

[Scott Danforth]

when you pulled the old impeller from the back of the drive, did you find all the blade pieces that were missing?

on a Cobra drive, replacing the impeller is a very quick job.

 

[southkogs]

If the boat had been running normally up until the impeller change (as in it hadn't sat for years), then that would be my first suspect - either something was done wrong, or the aftermarket kit didn't work right.

Can you test water flow up by the thermostat and make sure that you're pulling water up?

 

[Lou C]

Well most of these Cobras will have a P/S cooler on the back of the engine, bits from the old impeller can be in there, or lodged in the thermostat housing.
I have always used OE housings and impellers and usually do not have overheating problems, normal temps are 160-175 with the 160 stat. Hopefully you can fairly easily get at the P/S cooler, on some boats it can be a bear to reach if there is no access above the rear of the engine.
I have a couple of Sierra kits for spares but always use an OE OMC/BRP kit when doing mine. I replace them every 3 seasons, that's when I see the rubber start to have small stress marks on it if you look closely.