5.7 Mercruiser overheating

[Si Rey]

Got an 89 Searay 5.7 Mercruiser with Alpha 1 outdrive. At idle for extended periods of time (5 min or more)engine temp will redline and temp alarm sounds. Rev engine to 1,000 or more - engine rapidly cools to 145 degrees. Under power engine temp is normal only at idle does it get hot, is this normal? Oil pressure doesn't drop with high engine temp. Boat is in fresh water and is raw water cooling. Relaced engine circulating pump, t-stat, both exhaust manifolds due to corrosion, all hoses. Would the impeller replacement be next on the list or don't worry about it? Thanks in advance for any advice.

 

[Bondo]

Re: 5.7 Mercruiser overheating

Would the impeller replacement be next on the list or don't worry about it?

Ayuh,......... That's where you Start, when there's an Overheat Issue in a Boat...........

As for the Worry,............. I'd either be Ordering the Parts,.......
Or,........
Scheduling the Service Visit...........

 

[Si Rey]

Re: 5.7 Mercruiser overheating

The water pump replacement, t-stat and hoses were replaced due to condition/age when I replaced the leaking exhaust manifolds it wasn't specifically to solve the overheating problem. The book calls for a 142 degree t-stat which is what I took out and also what I replaced-sooooo looks like I'll be pulling the outdrive and inspect/replace the impeller. Thanks

 

[Don S]

Re: 5.7 Mercruiser overheating

The water pump replacement, t-stat and hoses were replaced due to condition/age when I replaced the leaking exhaust manifolds

Which water pump are you talking about, the "Circulation" pump on the front of the engine (called a water pump by auto mechanices) or the raw water pump that is in the outdrive on Alpha drives?????

 

[Si Rey]

Re: 5.7 Mercruiser overheating

I replaced the engine circulating pump. As I write this I removed the lower end and found scoring on the water pump housing and the vanes look good - some minor scratches on them and no vanes missing. I'll replace anyways as well as the housing as it is already removed. Anything else I should be looking for? Outdrive oil looked good with no h2o in it. Thank you for your reply

 

[badbowtie]

Re: 5.7 Mercruiser overheating

my mercruiser 5.7 is doing the same thing. But mine is only when I am at idle 800 rpm it starts to rise bump it to 1000 rpm and it will drop right back down to 150 please post back if you figure out the problme I have replaced the impeller pump already in the lower unit that did nothing different. Good luck

 

[Si Rey]

Re: 5.7 Mercruiser overheating

Thst is exactly what I have going on. At idle after about 5 min it will climb to 200 degrees, but bump it to 1000 and drops back to 145. I already pulled the lower unit and will replace impeller. I won't know anything until next weekend and I'll let you know.

 

[Don S]

Re: 5.7 Mercruiser overheating

Replace the impeller if the vanes don't straighten out in about 5 minutes. And if the housing is scored replace it.
Bet it will work then.
PS: The impeller is something that should be replaced every other year.

 

[Si Rey]

Re: 5.7 Mercruiser overheating

Thank you Don for your help-again. You've been right on the couple of times in the past that I've needed advise. Will be replacing impeller now and more frequently.
How do you ever get time to go boating with all the time you spend helping others.
Thank you again!

 

[buldogboats]

Re: 5.7 Mercruiser overheating

Hi I have a problem with my 5.7 mercruiser overheating i replaced the raw water pump, risers, exughst manifolds, and the pump on the front of the engine and i still have an overheating problem i can run around 2500 rpms and be at about 165 as soon as i go to 3200 it shoots way up and yes i also replaced the thermostat then i removed it completly and it still does the same thing this is a raw water system any sugjestions would be greatly apreciated

 

[badbowtie]

Re: 5.7 Mercruiser overheating

Si Rey
Have to figured anything out with the problem of over heating only at idle. I am still having the problem just have not been idling But I am going to try to get it fixed this week.