Mercruiser 5.7 1990 Alpha I overheats at High speed

[kdmiller8251]

Ok folks need your help on this one..<br />I have a 1990 Alpha I 5.7 Mercruiser that was formally a saltwater boat. I have replaced impeller,waterpump, and Thermostat. The boat runs great until I get up to full throttle. Then it gets hot real fast. I currently have a Glenwood Aluminum exhaust manifold without risers but an elbow with water cooling on it. Is it possible that the problem is the manifold, thermostat or what else?? When I pulled the thermostat there was a lot of rust and crap in the housing. I think it may be plugging stuff up. One mechanic said it may be head gasket but the oil is clean and it does not feel like its missing. Has anyone heard of using Muratic ACid to eat the rust and build up away??? I dont really want to spend a lot of money right now to fix it I am planning on putting new engine off season.Any ideas would be great... is there supposed to be a seawater pump on there also??<br /><br />Thanks<br /><br />tallman

 

[Walter]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

Have you tried running it without the thermostat in place? If it still gets hot then you've got some type of water circulation problem...whether it's a restriction or lack of pump boost. I'm not clear if yours is a fresh water cooled or closed closed cooling system. How you'd go about troubleshooting depends on the type system you have.<br /><br />One thought...are you sure your engine is really getting hot? Could your gauge be giving you false readings? That's always something to consider...especially after you've replaced so many components. Of course, there's the possiblity one of those components is faulty or was installed incorrectly... No offense, but that happens! Trust me...I KNOW! Hee, hee...<br /><br />Good luck...<br /><br />Walt

 

[daveswaves]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

If you are overheating and the gauges are ok there is only one cause generally.....not enough cooling water. You ask about a sea water pump, not sure what your set up is, but you either need to have water passing straight through your block and out again or passing through a heat exchanger to cool a recirculating system. You have changed the impeller etc however there may be blockages in the passages to the engine through the lower end. Remove the covers on the water inlet(s) and make sure there is no crud, zebra mussels etc blocking the inlet. Make sure your prop edges are not too badly dinged up, sometimes it can create air pockets over the inlets at speed and reduce the water flow. Other things that can cause poor water flow at higher speeds are collapsed hoses. Check the simple stuff first, I personally would not use muriatic acid to de scale the system. If you feel there is scale build up in the engine remove the hoses and try to backflush with clean water ...any crap you get out is a bonus. Others may have more specific ideas on your particular set up, good luck.

 

[Trent]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

Does this engine have a belt driven pump? If not do this.<br /><br />Replace the hose (looks like heater hose) from the transom plate to the engine with a clear hose. This hose supplies the engine its water from the impeller in the outdrive. Run the boat at cruise speed and look at the hose IF THEIR ARE ANY BUBBLES your problem is in the outdrive.

 

[kdmiller8251]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

Engine does not have a belt driven pump and it is an open cooling system..<br /><br />I will try all these suggestions and see what happens...<br /><br />Thanks<br /><br />Tallman

 

[scott boswell]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

I have used muratic acid several times without a problem. Just be sure to flush it with clean water after and don't run it through an engine take the parts off then dunk or even soak in the acid. There is also a salt remover sold by West marine and others that is good to clean the salt deposits out.<br />Scott

 

[daveswaves]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

Just to clarify, I agree with Mr. Boswell, muriatic acid is ok if you are cleaning a part. I was concerned that you may try to "run" it through the engine.

 

[kdmiller8251]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

Well to be honest with ya all, I plan on running it thru the cooling system or trying to.... The rust is pretty bad and I figure it cant hurt worse than it is..... How bad do saltwater cooling systems really get? My mechanic who is a freshwater mech says it looks like my engine was underwater due to all the rust... Is this possible??<br /><br />Tallman

 

[flashback]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

listen to what Trent said and then' if all is OK, then check the water pump on the engine itself. is the belt tight enough? you can have plenty of fresh water being brought into your engine, but if the circulation pump is bad, all the fresh water in the world won't mean a thing........

 

[kdmiller8251]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

Waterpump is brand new, Impeller is brand new... Thermostat brand new & removed... Thinking it could be exhaust manfiold possibly...

 

[kdmiller8251]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

Ok just got the exhaust manifolds off and what a mess they are... No wonder it overheats....<br />Gonna spend some Coin now.....<br />Ohh yeah Galaxy I am going to beat you...... All I got to do is get the manifolds and I am ready to roll! HA HA<br /><br />Thanks<br />for the help<br /><br />Tallman

 

[24Albie]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

One more thing that no one has mentioned. Get a new set of freeze plugs. While you have the manifolds off, pull the water pump also and then remove all freeze plugs and take a pressure washer and blow out the inside of the block. All of the sand and crap that has been brought up from the water settles in the bottom of the block between the cylinders and will cause high RPM overheating. I had the same problem with mine. Brass freeze plugs are only $20 a set, water pump gasket($2) and this will definitely help your cooling problems.

 

[kdmiller8251]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

In case anyone wants to see what the manifolds look like....<br /><br />

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[galaxy225]

Re: Mercruiser 5.7 1990 Alpha I overheats at High speed

hey tallman that is a good deal......i will be doing impeller tomorrow...hope to have boat on water by weeekend......the guy i keep waiting on never showed up .....so i am going to give it try and replace impeller my self.....<i am the little engine that could think i can ..i think i can> i hope after i put the impeller on it dont do any of that stuff that happened to you......well any ways good luck....happy boating...let me know how it goes...............seeeee yaaaaa galaxy