Super-Puzzling Overheating Merc 5.7 - Need Help

[Brentathon]

My engine temp has been creeping up when cruising, while the steady-state temp at low RPM's is fine. At one time, temp even crept up to the "red zone" (15 sec), before coming back down when I backed off throttle. I noticed this temp creeping toward end of last season, too, but this heat wave probably exacerbated the issue.

Ideas????

Background info: 1991 Mercruiser 5.7 Alpha One saltwater cooled, O/D impeller new, elbows new, T-stat new, T-stat housing is a few years old (new style w/ brass tee), manifolds have 3 seasons on them, engine circulating pump new, temp sender replaced a few years ago and gauge seems to be fine, no noticeable restriction on O/D intake holes.

Also, there seems to be decent flow coming from the O/D, as i pulled the hose off the power steering cooler, and ran the boat breifly to check flow through cooler.

 

[ddclapp]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

I had a similar problem last year - it turned out to bits and pieces of an impeller that where stuck in parts of the cooling system. I ended up back flushing all parts I could get to, and bits of rubber came trickling out. Exhaust elbows, exchanger and power steering cooler are catch points. Did you shred any impellers lately?

 

[Brentathon]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Thanks for reply.
I replaced the impeller over this past winter myself, and it was perfect. My mechanic replaced them every other season, and he never said any of them had broken vanes.
The flow coming from the O/D seems OK.....I checked it at 1500 rpm...it's a fat steady stream (not pulsating), but it's tough to gauge water flow without any pressure or flowrate requirements.

 

[Mischief Managed]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Might be a head gasket issue. Sometimes they fail in such a way that combustion gas gets into the cooling system making the cooling system less effective. I's excacerbated while the engine is under load because it makes more combustion gas at a higher pressure AND needs the cooling system more than it does when not under load. A leakdown test may find this particular problem.

 

[Brentathon]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

I'm kinda doubting a head gasket issue......the engine runs perfectly (except for temp issue), and is a remanufactured long block I got in late 2007.
I would think head gasket issue would result in more deficiencies than temp slowly creeping up at high rpm's no?

 

[numpux2020]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

what is your t-stat gasket shaped like if u have brass t style housing is there a 1/2 inch square block in front of the t-stat must have correct gasket to cover passage to allow block water (heated) to go straight out exhaust and fresh water to enter circ pump also a hidden passage in housing prone to obstructions...

 

[adeneo]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

I'm kinda doubting a head gasket issue......the engine runs perfectly (except for temp issue), and is a remanufactured long block I got in late 2007.
I would think head gasket issue would result in more deficiencies than temp slowly creeping up at high rpm's no?

I've seen a lot of these engines run perfectly after freezing during the cold winters where I live, and when taking the inlet of, the block has cracked the entire length on both sides right behind the pushrods. Still runs great, and heating problems only presents when being pushed over 1500-2000rpm. Of course salt water will run into the oil pan eventually filling it with water.
This amazes my customers everytime, seeing their engine running just great, but at the same time being completely ruined. The point being that a head gasket failure can cause overheating at higher rpm's even if everything else seems just fine, depends on where the head gasket blew. Have you checked the oil for water or is oil disappearing.
Could be a flow problem as well, or any number of other things, but you've already ruled out the usual suspects for most flow problems.

 

[full stringer]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

sounds like hot exhaust gases could be recirculating back through the cooling system u need to check for air bubbles in the system this usally happens when impeller housing has a crack or a seal is bad nearby

 

[Brentathon]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

I have the new T-stat housing #87290A23 (which is actually the original design) from aro 2003 with brass Tee, and top gasket has always had a filled-in square section (27-41812), and the other day I tried an open square section gasket (27-48818-1) for the heck of it, and it seemed to make low rpm steady-state temp just a little bit higher.
They also have a gasket that just has a small 1/4" hole in it (27-41812-2)......which is the correct one for housings with check balls???

This temp problem seemed to have slowly come on, as I was seeing the temp creep last season, too. Since oil had no water in it in the fall when changed, and O/D was a brand new Sterndrive Engineering unit from aro 2005, this is a real doozy....

 

[Brentathon]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Ok, I'm still stumped with this. I just installed a brand new T-stat housing, to eliminate that as a culprit.
I installed clear vinyl hoses going from t-stat housing to elbows, to see water flow. It appears that high rpm's, as the temp is climbing higher than the typical stead-state position on the gauge, the flow to the elbows seems to die-off. I would expect that under all operating temps, there should always be some excess flow that the O/D impeller is pumping in, which is supposed to exit via the ball-check valve tee to the elbows.

 

[Don S]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Pull the exhaust risers and check them for corrosion and blockage. Manifolds and risers in salt water have a life span of 6 to 10 years before they destroy your engine.
It sounds like the risers are clogging up with rust and scale and blocking the flow of water out of the engine. You can pump in all you want, but if it can't get out, it won't cool.

Oh, and one note, NO, you can not clean them out and keep going. The overheat is an early warning system telling you any longer and you will loose your engine.

 

[Brentathon]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

On my first post, you can see that risers are new, and manifolds have only 3 1/2 seasons, if that. The ports looked fine in the manifolds, when I put the risers on.
I have clear vinyl now on the 2 hoses going to elbows, and 2 hoses going to manifolds; it looks like at high rpm's the "excess" water out of ball check tee seems to trickle-off, but low rpm's even more flow comes out of tee. Boat needs to get hauled, I guess.

 

[Blk-n-Blu]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Here is a link that covers the basics for that condition: Hope it helps:
http://www.sterndrives.com/overheating_highspeed.html

 

[fossill]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Do a google search and check out Merc service bulletin 95-13 titled: Alpha 1 Gen II Over Heat

I'd attach but dont know how. Hope that helps.

 

[Brentathon]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Interesting service bulletin........there's a chance that I have that issue, although I have a 1st Gen Alpha, not Gen II.
I do see quite a lot of small air bubbles in the hoses that go to the manifolds, and it seem as the "extra flow" of water going to the elbows slows down almost completely, even though the temp is creeping up.
This is seriously a puzzling problem. The oldest component on the boat are the manifolds, and 3 1/2 seasons is definitely not alot of time on manifolds, in any saltwater condition/use.

 

[Brentathon]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Just to update/correct this.............after sending back the impeller to Sierra, they determined that it was not theirs, and likely a Mercury impeller.
Therefore, someone put a mercury impeller into the Sierra pump kit that I bought from my marina, and the mercury impeller is from some unknown origin, which had a harder rubber, and the vanes took a considerable set (more than my old impeller did), and therefore, the pumping efficiency (flow/head) was reduced.

So.........it looks like I'm screwed.
At least I didn't toast the engine.

 

[Don S]

Re: Super-Puzzling Overheating Merc 5.7 - Need Help

Thread closed.
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http://forums.iboats.com/showthread.php?t=433560