Mercruiser 5.0 v8 1995 overheats above 2500rpm

Hi Guys,

I saw few post for smiler topics however still don't get a good vibe about my issue. I see some people with very impressive knowledge therefore I though of seeking some help, any comments would be greatly appreciated.

Specs

• 1995 Mercruiser 5.0L V8 with Alpha I Gen II.
• Raw water cooled.
• Engine starts right up with one turn of the key, no noise or vibration at all.
• Engine seems to have a fresh paint (don't know if this is important).
• All pipes look rather new.

History
Just bought this 1995 Bayliner 2452 classic last week, according to seller it was sitting for two years.

Drive oil bottle was empty and there was some water in the drive (about two table spoons) when I changed the oil, its leaks out in a day and bottle was empty again, have it serviced by mechanic, he replaced new water pump, new gaskets and one seal (don't know which one) new engine oil, new fuel filter etc. Drive oil bottle is now holding the oil.


Problem
Took it out to sea for a test drive, getting out to open sea from my bay (Stratford, CT) is about 3 mile so I drove slow but when I put it above 2500 rpm it goes from 175f to above 250 very fast, bring throttle back to less then 2500 rpm cool it down fast. I also head some squeaking noise of a belt that goes around generator and tighten it little more upon return. (They are almost new)

Total drive time was about 1 hr and going out and coming back in was fine other then the fact that I cannot drive it above 2500 rpm.

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

Engine seems to have a fresh paint (don't know if this is important).
All pipes look rather new.

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I would think that is rather important. The only thing that new paint would be covering up is rust and or corrosion. The PO is hiding something...

"pipes" looking good on the outside mean nothing. Your risers and manifolds might be trashed on the inside and could be causing your overheating issues...

I put it above 2500 rpm it goes from 175f to above 250 very fast,

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How fast? Seconds or minutes???

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

Running up to 250 = warped heads or blown head gasket. While it's not guaranteed, the odds are not pretty. Small cooling problem could lead to major engine damage.

200 should frighten you.

225 = pee your pants

250 = "Hmmm... I really want to break something major.... like my wallet."

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

temperature it goes up very fast like in few seconds but it is proportion to the rpm ie if I give more throttle it goes up fast....

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

You need to confirm that your gauge and temp sensor are working properly. I would pull off your hoses and check for obstructions as well as your thermostat housing. Just make sure pieces of old impeller, some other foreign material or rust and or corrosion is clogging up the system...

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

does water circulate around engine on muffs.your mechanic should have tested basic things so may be time to ask him why theres a problem now if it wasnt there before he touched it.he may have missed/forgot to connect pipe into tube whilst assembling the lower.

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

I put the hose on that and if I start water does come out from back of OD, I think pipe connections are good, don't know the condition of the T-sate, will open it tomorrow.

gauge and temp sensor are working properly as I open the cover when heat alarm sound and I can tell that engine was very hot, I can even hear the knocking sound and it took few sec for engine to complete shut down when I turned off the switch however its starts right away when cooled down a bit.

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

thermostat is stuck wide open
doesnt give the water a chance to disapate the heat
bought a boat that did that once and there was no thermostat at all in in
put one in problem solved
at least in my case good luck

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

The first thing you check on any overheat on a marine engine is the raw water pump impeller. There are just too many things that can happen to an impeller not to check it first.
A stuck open thermsostat will give you low temps, not high. NO thermostat at all can cause an overheat of part of the engine, but not that would be seen on the temp gauge.

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

Don S said:

The first thing you check on any overheat on a marine engine is the raw water pump impeller. There are just too many things that can happen to an impeller not to check it first.

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I apologize if I don't understand the terms, by impeller mean water pump in the lower unit then it is new, and if you mean water circulation pump in engine then I have to check it.

does any one has a water circulation diagram of this engine?

Re: Mercruiser 5.0 v8 1995 overheats above 2500rpm

Update: Open both Riser and they were choked, put new one and cleaned the Manifolds (below) with a long screwdriver, No more Overhearing...

Thank you all for your help.