mountainstone

My closed-cooling 5.7 EFI runs about 160 degrees at idle, as I accelerate the water temp rises and seems to hold steady with speed/rpm. At 3500 rpm the temp will rise to almost 200and hold there; about 185-190 at 3000 rpm and 175 at 2000 rpm.

The boat is new to me, so I suspected the impeller. Replaced with a new merc unit but the old one looked like it had very low hours. There is a visible but very shallow groove worn into the plastic one-piece impeller housing, not around the circumference but worn into the end of the housing opposite the steel wear plate. It seemed very slight to me so I didn't replace the housing. I also removed the end caps on the heat exchanger and cleaned out all the tubes with a coat hanger and wire brush (some were blocked with pieces of old disintegrated zinc anode and I thought that was my problem).

With new impeller and cleaned heat exchanger the boat exhibits the same symptoms.

Would the slight groove in the back of the plastic impeller housing cause this or do I need to be looking for something else? What is the logical progression of troubleshooting? Thermostat next, or circulation pump or exhaust manifold risers or what? Engine is bone stock with 500 hours, 2000 year model.

Thanx for any advice!

steveh1209

I dont think the groove would cause this. i would steer towards thermostat. Then move onto checking passages even though it is closed cooled.

ThirdBird

Agreed.

jeff32

I would not say housing can't be the problem. of course depending on depth of groove... but when I see a groove, I change it, cause it was my problem once...

glassdave

iTrader: (1)

i would replace the housing anyway, i had a similar problem and it ended up being a small piece of the housing that was missing. In one of the holes for either intake or pressure there is a little rib that the impeller rides on to clear the opening that was gone. You'd never know it unless you held them side by side. After a season of fluctuating temps and several impellers on my port motor i just put a whole new assembly on it. While i had everything apart i compared them and made the discovery. Been perfect ever since.

Had a similar temp problem on another boat that would be nice and cool but as i got higher in the rpm range temp would fluctuate and set my temp alarm off. Turned out to be a slightly loose clamp on the hose on the intake side of the pump allowing air into the system.

mountainstone

Thanks for all the advice, everybody. I count two votes for and two votes against, but glassdave's post makes me think maybe I should go ahead and replace the housing.

How on earth did you determine you were sucking air? I can't begin to imagine how to troubleshoot that, especially since I can't get down into my engine compartment while under way.

bowtie

Check where the water inlet goes throught the gimbal housing. The plastic fitting gets crushed with corrosion and restricts the water flow. This happens alot if the boat is kept on a lift or out of the water.

speedreeder

I would say you have rocks or trash in the heat exchanger. There will be small holes in the cooler and take a rod and go down through each of them, Remove the end caps on the heat exchanger,And clean out the ends. Check if you have another cooler and look inside it. You probably will find some impeller pieces blocking it.

mountainstone

I read of this phenomenon in another thread. This is my first sterndrive so I am just learning my way around it; how do I check this plastic fitting? Pull off the hose from inside the boat, or do I have to pull the drive off the transom? Thanx for the tip!

mountainstone

During my initial troubleshooting I did remove the end plates from the heat exchanger to inspect its condition. I found several tubes blocked with bits of the zinc anode which had been left too long by the previous owner and had disintegrated. I cleaned all the tubes but the overheating symptoms remained.

Your suggestion to check other coolers seems a good one. I will check and clean the power steering cooler and the fuel cooler next; hopefully there is some debris blocking water flow in one of those coolers. The impeller I replaced was in perfect condition but who knows what may have happened to previous units.