1998 5.7 high rpm overheat

[MikeM40]

New waterpump in the spring & bellows kit. New t-stat now all hoses & coolers clear. Water pumps great at under 3000 rpm I found a Merc service bulletin about overheating at over 3000 RPM's. I ran a clear hose from t-stat to the starboard manifold & went for a ride all was well until about 3500 RPM's & then suddenly no water. This happened twice. The merc fix is to use perfect seal on the first gasket against the housing. I'm going to check the impeller & use perfect seal . So I'll try that & post back. Also my port shutter was almost completley gone found part of it by the prop. anything else to check maybe I'll find more when i do the impeller

 

[Brentathon]

Re: 1998 5.7 high rpm overheat

I'm having a similar high-rpm overheat problem, where did you find the service bulletin?

 

[RichRab]

Re: 1998 5.7 high rpm overheat

(This is my post-June 13,2910 Running hot- Everything looks good and the mechanic says he thinks it is the Sierra pump I used. I put a post up today for other opinions. Hopefully we will find out tonite. Will keep you updated.The boat has not been used due to the oil pan rotting out. All hoses flushed clear and motor is now back in.) I have a 2002 Merc alpha 1 gen 2 5.0 sea water cooled engine not multi port draining system. Where we boat low tide is difficult so we have hit mud a few times over the years. The boat runs fine but when going at higher rpm (not completley opened up) the temp goes up to around 200 deg. At idle or as soon as you slow down the temp drops to about 170. The boat was kept in the water and the first thing I tried was therm. (I do not remember any flow restrictors of any kind in the housing) Second pulled the boat and water pump in outdrive. Third tried flushing/ backwashing everything I could find including the power steering cooler. Beginning of this year man. risers elbows. Still the same. The Barr manifolds came wit 2 sets of gaskets foe the 3 inch risers. I put the ones that were orig in with the full f;ow big 4 holes and had the same prob. I called Barr and was told the reccomend the ons with 2 big and 2 small holes to restrict water flow and carry more heat off. I put them in (4) and no diff. I think the gauge is accurate because if it gets too hot he buzzer comes on and it consistently reads the same results. I am looking at the Merc book that came with the boat and it after the steering cooler it shows a check valve and then a cool fuel box. ( I wish I looked closer with the man. off). I don't remember seeing them and did not specificully look for them. I am thinking one of them might be a prob. Also have heard but never saw the water pump shaft on the motor breaking. What do you think? Thank You RR

 

[Brentathon]

Re: 1998 5.7 high rpm overheat

I had an engine circulating pump impeller complete fail; when I removed the pump, I could hear the impeller rattling around inside. That problem was REALLY tricky to solve, since it actually made the temp gauge go to FULL COLD, even though no water was circulating through the block.

 

[Philster]

Re: 1998 5.7 high rpm overheat

My overheat issue was resolved by replacing the water tubes coming in from the drive at the transom (removal of drive required).


Corrosion at transom will squeeze them down to almost nothing. Won't show until water demand at higher rpm is higher. I was skeptical, but my merc tech was on the money. I had a new pump, t-stat and the whole cooling system gone over. It was the tubes coming in that were squeezed down to nothing.

 

[RichRab]

Re: 1998 5.7 high rpm overheat

According to mech. the temp gauge is off by 25 deg-- he said it is a common problem that my boat is equipped with a factory device that cuts rpm to prevent damage. I will not be confident until I correct the prob. The next step will be to change the sending unit.