Overheating Mercruiser conundrum

[pete78]

Hi,

First time here on this really useful forum, so be gentle!

Apologies for a long first post, I’m wondering if anybody can help with a few challenges with the running temperatures on my 1990 Searay 200. I’ve done a lot of reading about the well-trodden path when these engines run warm, so wanted a some advice specific to my next steps.

A bit of background; 1990 Searay 200, 4.3 Mercruiser (carb), with Alpha 1 leg. Serviced annually, last impellor change March 2014, flushed after every use. Importantly, Trailer stored about 1 hr from any decent marine engineers, who all also seem to have a long wait time, and the season is fast drawing to a close if the weather has its way(!).

Last week on holiday in Plymouth I launched and very quickly (within 5 minutes) overheated. The engine was running at 220F for probably a minute. I’ve now checked and changed the impeller which had shredded all the blades (I’ve only recovered 5 of them). I’ve pulled the hose that goes to the circulating pump, and there is now plenty of water is now coming from the leg into the engine. When I ran on muffs, the engine seemed to stabilise around the 150F mark, risers were only warm to the touch and plenty of water is being expelled through the transom.

I then took her for a sea trial on Saturday and although the temperature climbed much slower, it still climbed to 220f. at this point I removed the thermostat, and the engine would then run between 160f (with some revs in natural) and 190/200f when under load so still too warm, but there was some leaking from the thermostat housing as the gasket was difficult to re-seal. The risers never became untouchably hot.

I’ve subsequently;

•Removed ALL rubber houses to check for impeller debris and blockage, there is none I can see
•Removed and cleaned the Thermostat housing, which was quite corroded inside, and there looked to be a flake of rust (or stone?) floating around that could have blocked the flow through the bottom of the thermostat housing
•Checked the oil, which has no water in it, and the engine starts fine and sounds and performs exactly as it ever did – hopefully ruling out cracked head, and possibly blocked risers?
•I plan on checking the exhaust flappers / shutters next and refitting the thermostat and cleaned up housing
•I know at least one of the risers / manifolds was changed 6 years ago, but they are now proving very difficult to shift and it is likely a job for an engineer to check / change. Additionally, if you suspect a blockage from the impellor blades elsewhere, I think that is beyond my ability.

So my questions are really;

1.Is a slight leak or blockage of the thermostat housing enough to produce the symptoms shown above from the sea-trial?
2.What else could this be? I’m thinking it’s either a blockage in flow of water through or out of the engine due to impeller pieces, shutters or blocked risers / manifolds? Or Could it be the temperature sender?
3.Other than checking the above, and presuming normal operation again on the muffs, would you risk another sea trial? where I launch is sheltered and I have an auxillary so safety wouldn't be a concern. I could ‘invest’ in an infrared temperature gun to test the sender and actual engine temps?

Any thoughts or advice gratefully received.

Thanks, Pete

 

[gordmac]

Welcome. Don't know much about raw water cooling. You can't run under load on muffs so not a lot of heat generated, if you want to check you will have to put it in the water. My understanding of these engines is that the manifolds only last a few years and need changing.
You will get a better response on the motorboat forum though.

 

[pete78]

HI,

thanks for the reply. I'll pop the post over there.

In terms of raw water wooling and a proper test, that's what I thought. I thought the manifolds might be the next thing to look at, It just seems strange that this has only now (post impeller issues) come to light.

thanks again

 

[TheOrs]

Unfortunately, it could be many things. I had a similar problem after rebuilding my drive & it turned out that I had left out a seal above the impeller housing (diagnosed by my friendly local mechanic just by looking at it running on the muffs - too many bubbles in the water apparently!)

It was no. 34 in the diagram attached; part no. 38970. I said there was no seal in my rebuild kit. They checked about 6 kits and only one contained a seal.

 

[pete78]

Thanks TheOrs.

I agree it could be many thngs, I suppose once I have got it back together with the cleaned up thermostat and housing then I'll try it again. If no joy then I'll have to bite the bullet and get somebody to look at it.

Number 34 looks like a seal that beds into the Impellar housing, so the copper pipe is snug? if so I think I did have one. When you say bubbles, were these coming out of the transom or spotted at the water intake into the thermostat housing?

thanks again

 

[TheOrs]

Yes, number 34 is what you describe. My original seal had lost it's flex & was covered in grot, so I assumed it was plastic/part of the housing & didn't notice it was missing from the kit. I thought I'd mention it as so many of the other kits also had it missing & the symptoms were what you describe.

The bubbles were in the water ejected from the transom assy/leg unit. Looked normal to me!