Mercruiser 470 overheating at idle

[salty3rd]

470 overheated the end of last season due to loss of coolant from a hose that broke. I fixed the hose and ran the boat. It was fine for a 20 min run. It cooled a couple hours and when I ran it to go home it started overheating again, I shut it down and did some troubleshooting and couldn't find anything, and the engine wouldn't start again. After some troubleshooting this spring I pulled the head and found a crack. I replaced it with a refurbished head and an OEM gasket. It is running in the driveway but overheats at an idle. If I increase the Rpms it cools it down. The Raw water pump is pumping plenty, and the heat exchanger is clean. The riser elbow is also clear. I have eliminated the raw water side problems and the issue still happens with the thermostat removed. The best I can tell The engine circ pump seems to be pumping. I'd appreciate any thoughts.. Thanks Bill

 

[philbullet]

I once had trouble like that with a 470, i was told to put the correct mercruiser thermostat in it and woola it worked.
I had a auto mobile thermostat and it was letting the water run throw the heat exchanger to quickly to cool.
Maybe they that and also a new lower unit water pump impeller, even tho your getting good raw water, it might be low preasure.

 

[achris]

.... the issue still happens with the thermostat removed....

The water circ systems on these engines rely on a working thermostat to be in place for the water to move in the way designed.... Part of the thermostats' job is to divert water along certain passages when closed. Without a thermostat in place, that can't happen, hence you see an overheat....

Chris....

 

[salty3rd]

Thanks for you ideas, When I replaced the head I put a brand new OEM 160 degree thermostat in. I only removed it to see if that was causing the overheating.I think Im gonna try a different t stat, double check the internal water pump and all the hoses. Just to be clear I ran a garden hose to the heat exchanger with the same results as running on the muffs. And i had plenty of flow out of the transom. Any other ideas while Im at it is appreciated.. Thanks Bill

 

[wrestling coach]

You might try confirming that your sending unit/temp gauge readings are accurate by using an IR temp gun on your thermostat housing as you see temps rising. Also have you checked the heat exchanger for broken pieces off your impellar? Usually found by removing the front cap..

 

[philbullet]

Do you have a 3" or 4" heat exchanger?
I had one with a 3" and continued to have heating isues when running on step. We have one with a 4" heat exchanger and it runs alot cooler.
Just a thought.

 

[stonyloam]

Well you THINK the raw water pump is pumping plenty, I'll bet it is not. The engine water pump is pretty foolproof, and thermostats in a antifreeze rarely fail, so that leaves the impeller in the outdrive. I would change it ASAP. One quick check is to see if the exchanger is getting excessively hot when the engine is at idle. Good luck.

 

[salty3rd]

Well you THINK the raw water pump is pumping plenty, I'll bet it is not. The engine water pump is pretty foolproof, and thermostats in a antifreeze rarely fail, so that leaves the impeller in the outdrive. I would change it ASAP. One quick check is to see if the exchanger is getting excessively hot when the engine is at idle. Good luck.

Thanks Terry I hooked the garden hose directly to the heat exchanger and got the exact same results therefore ruling out the raw water pump. I pulled the end caps off the heat exchanger and everything is clear. I have been running two of these engines for over 12 years now and seen a lot; but this has me stumped.

 

[achris]

Thanks Terry I hooked the garden hose directly to the heat exchanger and got the exact same results therefore ruling out the raw water pump. I pulled the end caps off the heat exchanger and everything is clear. I have been running two of these engines for over 12 years now and seen a lot; but this has me stumped.

ooooow. engine running, drive on, hose connected to the inlet side of the heat exchanger. All good..... BUT, did you have water running to the drive for the pump in the drive to pump? If not, goodnight impeller and housing, probably base too.

Chris.....

 

[salty3rd]

ooooow. engine running, drive on, hose connected to the inlet side of the heat exchanger. All good..... BUT, did you have water running to the drive for the pump in the drive to pump? If not, goodnight impeller and housing, probably base too.

Chris.....

Yes I hooked up seperate hoses to the heat exchanger and the drive, and let the heat exchanger inlet hose flow into the bilge so I would not burn out the impeller.

 

[JerryIrons]

Kinda sounds like something is restricting the flow of coolant, just enough that it gets overcome as soon as any pressure gets put on. Did something cause that hose to break, or when it broke something got stuck somewhere? I also use one of those radiator caps with a thermometer in it as a backup reading in case the gauge might be wrong.

 

[achris]

Yes I hooked up seperate hoses to the heat exchanger and the drive, and let the heat exchanger inlet hose flow into the bilge so I would not burn out the impeller.

:thumb:

 

[philbullet]

Could this be a possibility of a busted head gasket or cracked head?
Just something to think about.
I had it happen also.lol

 

[salty3rd]

I just finished for the night. I flushed the entire freshwater side of the cooling system, checked all hoses, the impeller on the cam shaft; all looked good. I put an old thermostat in; as opposed to the new one i originally had in there; and it ran for 20 min like it should, temp was normal. No smoking gun, but all is good, wont know for sure till a sea trial.. I was very close to pulling the head off again but I think I dodged a bullet. Thanks to all who responded..