Mercruiser engine overheats when at idle or low running speeds

[RookieAlphaOne]

Boat is a 1990 Bayliner Ciera 2651 with Mercruiser 5.0 LX/V8 Alpha One engine. Engine runs at 150 to 160 temp when crusing above 10 miles/hr. But when at idle or at low "no wake" cruising speeds the boat starts to overheat. A Bayliner mechanic instructed me to change water impeller. I did this, but problem is still there. I am a backyard mechanic and am confident that the water impeller was installed correctly. Any suggestions?

 

[Bondo]

Re: Mercruiser engine overheats when at idle or low running speeds

What was the condition of the Old one ????<br />Did you find All the Pieces of it ????<br />You've got a Restriction SomeWhere.........

 

[RookieAlphaOne]

Re: Mercruiser engine overheats when at idle or low running speeds

the condition of the old one was intact, but worn

 

[Bondo]

Re: Mercruiser engine overheats when at idle or low running speeds

How about the Hoses between the Impeller,+ the motor ????<br />Also, Check the P/S Cooler, located under the motor, in the incoming line......

 

[RookieAlphaOne]

Re: Mercruiser engine overheats when at idle or low running speeds

I have not checked that, but did notice that the hose which would connect to the outdrive water pump...that flows up through to the engine seems to be "stuffed" in pretty tight with other hoses and lines. I do not know all the technical terms for marine parts, but it is in there with what appears to be the exhaust.<br /><br />The hose is not pinched shut, but is in there tight. Do you feel that this may be enough to cause the engine to overheat at low speeds and at idle?<br /><br />As it relates to p/s...are you referring to a power steering cooler? Is that hard to get to?<br /><br />Thanks for you help

 

[Bondo]

Re: Mercruiser engine overheats when at idle or low running speeds

The hose is not pinched shut, but is in there tight. Do you feel that this may be enough to cause the engine to overheat at low speeds and at idle?

Possibly.........<br />

As it relates to p/s...are you referring to a power steering cooler? Is that hard to get to?

Yes.......<br />It's located Under the Engine, In the Incoming Water Line............<br /><br /> "Is that hard to get to? " <br />Depends on Your Boat.........

 

[MFG197]

Re: Mercruiser engine overheats when at idle or low running speeds

did you check the hose that comes from the OD plastic fitting, my hose was crimped around/in the engine coupler. pulled free and re-attached. had a very tight bend in it.

 

[RookieAlphaOne]

Re: Mercruiser engine overheats when at idle or low running speeds

WWYC na7, is this the hose that starts from the sterndrive and worms it's way through the wall (transom if that is correct) and then makes it's way to the engine? <br /><br />If so, this hose does seem to be in there tight, but not tight enough to stop the flow but could hamper the flow. I will check again. <br /><br />Have you had this type of problem, also?<br /><br />thanks for the help

 

[MFG197]

Re: Mercruiser engine overheats when at idle or low running speeds

on my A1 Gen2 i was doing routine spring check out and found the hose kinked behind the engine coupler. I pulled it free and un twisted it then reattached it to the plastic 45 degree elbow that is on the drive housing (bell housing?) was not overheating last year, just stumbled on it.

 

[RookieAlphaOne]

Re: Mercruiser engine overheats when at idle or low running speeds

The latest: I removed the connection of the hoses from the sterndrive to the p/s cooler and checked for blockage. I attached a rigged hose to the line and ran it overboard, started the engine, place in drive and watched the water pump up from the (stern drive. It was not a high rate of flow, but water was being pumped. I followed the same type of logic for all hoses, all the way to the distribution junction on top of the engine with all hoses related to incoming water. Then I took a garden hose and backflushed it and watched as water came out the drive.<br /><br />I called Bayliner mechanic and he recommend replacing the water circulating pump. I did this, and $150 later the problem is still there. I did not see any pinched or collapsed hoses.<br /><br />The mechanic did say something that caught my attention. "Maybe we are not getting the water out of the engine quickly enough, instead of worrying if we are getting enough water in"<br /><br />ANy other suggestions wold be appreciated.<br /><br />So far, water impeller, water circulating pump have been replaced. Lines for intake from stern check and free on debree and hoses are not pinched.

 

[Destin]

Re: Mercruiser engine overheats when at idle or low running speeds

If you cruise at higher speeds and quickly slow to idle it is normal for the temp to rise 5 or 10 deg for a few minutes, but then cool back to normal. I don't think this is your problem though.<br />At low speed the outdrive water pump operates in positive displacement mode. The top and bottom of the impeller have to fit snugly, as well as the vanes, to properly pump. If you only replaced the impeller, I would install a good quality water pump kit, which includes a good quality rubber impeller. Carefully torque the nuts to specs so that the gaskets are compressed the proper amount to seal the top and bottom of the impeller with the body.

 

[BMDODGE]

Re: Mercruiser engine overheats when at idle or low running speeds

thermostat ???

 

[Bondo]

Re: Mercruiser engine overheats when at idle or low running speeds

Well Rookie...........<br />It Sounds like you're going at it the Right Way......................<br />

"Maybe we are not getting the water out of the engine quickly enough,

That would be the Risers.......<br />They're the Elbows bolted to the top of the Exhaust Manifolds.......<br />If there's Rust, they can clog......<br />And,<br />BMDODGE mentions the T-stat.......<br />That's a Cheap Part,+ If you don't Remember changing it, It's Probably time........<br />It's located in the "distribution junction on top of the engine"....................<br />Otherwise known as the T-Stat Housing................. <br />Good Luck......

 

[MrBill]

Re: Mercruiser engine overheats when at idle or low running speeds

Rookie...what you describe (overheating at idle and low speed) is the opposite of what you might experience with a worm lower unit impeller. I think Bondo set you on the right path...definitely a restriction in cooling flow. Unless something is lodged int eh the cooling passages, rust is the likely culprit...head for the risers. If running normal or near normal at higher speed, the impeller and water pump are getting enough flow through the restricted cooling passages and risers, but can't at lower engine RPM.

 

[rodbolt]

Re: Mercruiser engine overheats when at idle or low running speeds

hello<br /> on the alpha units overheat at low speeds/idle is almost always a seawater pump problem. if the manifolds and risers or block or tstat or anything else was so clogged it caused a problem at low speeds think how bad it will get at high speeds.<br />I would have another look at the seawater pump and the water pocket area of the upper unit as well as the lower pump base for any signs of melting. carefully inspect the upper as well. any signs of melting and the housings are trash.<br /> good luck and think before ya wrench. makes for more avalible funds for cool refreshing beverages

 

[RickCarman]

Re: Mercruiser engine overheats when at idle or low running speeds

Just fixed this problem myself. Assuming you have a small block Chevy when you pulled the water circulation pump (on engine) you should have made sure the small hole below the lower left (facing engine) water pump bolt was clear. Check both the block and the pump and be sure not to block it with sealant when replacing the pump. When this hole is blocked only the lower part of the pump will move water and can only provide sufficient cooling at high rpm.<br /><br />Sorry you have to pull the pump again but at least it's easy to check.

 

[RickCarman]

Re: Mercruiser engine overheats when at idle or low running speeds

One last Thing ... The hole in the block does not go straight back. IT goes in about a half inch and is met by another hole that travels up to the left toward the cylinder head deck. DON'T TRY TO DRILL IT OUT. First use a small screw driver to go straight back and then use a pick with a hooked end to clean up the second hole reaching as far as you can. This second hole usually doesn't plug too far up and you can test it with compressed air to see if it's clear.

 

[RookieAlphaOne]

Re: Mercruiser engine overheats when at idle or low running speeds

thanks to everyone for all your responses and suggestions.<br /><br />unfortunately due to time constraints I have had to take the boat to a Bayliner service shop to find the answer..<br /><br />I will let you know what they find as the issue and the resolution<br /><br />Again thanks

 

[RookieAlphaOne]

Re: Mercruiser engine overheats when at idle or low running speeds

The resolution: I took the boat to the shop and the boat was fixed for under $200. They found a cracked gromit (not sure if correct spelling)between the lower drive where the pipe from the water impeller sits into the upper part of the drive. The cracked gromit was hampering the correct flow up into the engine. Water was getting there, just not enough at idle speeds.<br /><br />Thanks for all the replies and suggestions.

 

[john1947]

Great resolution I tried fixing mine to
Overhead as I sped up
Only 1 year on impeller
Turned out bad bad sucking exhaust after pulled bpat found water in bellows
I took boat to a reliable shop
Got it all done bellows new lower case allow good as new $1700.
No cheap fgn parts
I'm glad I got a pro
And it's fixed and no worries
Lol