4.3LX -Alpha 1 overheats at idle only

[Chris1956]

My 1988 4.3LX Mercruiser overheats only at idle. This occurs while in neutral or while in gear. As soon as RPM exceeds 1000 in gear, motor cools off. I recently needed to change the circulating pump pully from the old rotted out (discontinued) 5 1/2" model to the new 7" model. I realize this slows the speed of the pump somewhat, but never expected this result. It was also recommended by Mercruiser.<br /><br />Has this happened to anyone else? Is it possible my water temp gauge is inaccurate at idle speed. I replaced the thermostat and checked the risers and manifolds for leaks. Both are good.

 

[tommays]

Re: 4.3LX -Alpha 1 overheats at idle only

how old is the pump in your drive it sounds weak<br /><br />tommays

 

[Chris1956]

Re: 4.3LX -Alpha 1 overheats at idle only

Tommays, The outdrive pump is new this season.

 

[Wellcraft 89]

Re: 4.3LX -Alpha 1 overheats at idle only

Chris have you ever had the water pump(on front of engine) off itself?<br />maybe the steel vanes on your pump have rusted away<br />Also ever poped a freeze out plug out to see if the water jackets are full of sand or rust.<br />One other thing on the 4.3 in the thermostat housing there is a device that has 2 plastic balls and spring and rod between them,maybe it has rotted away and is causing a flow problem.<br /><br />Rich

 

[Wellcraft 89]

Re: 4.3LX -Alpha 1 overheats at idle only

Chris on edit you may have the older style thermastat housing with no ball and spring.<br /><br />Rich

 

[Chris1956]

Re: 4.3LX -Alpha 1 overheats at idle only

Welcraft, I have the two balls and springs. The housing is original (1988) and I run it salt water, but it seems servicable. AS for the engine circulating pump, it was bought from a marine place no more than three years ago, so the vanes should be both bronze and OK. As for the water jacket being full of rust/sand. It is certainly possible, but why would it overheat at idle only?

 

[elpaso]

Re: 4.3LX -Alpha 1 overheats at idle only

Pull the hose that runs from the transom to the oil cooler -<br /> to the thermostat housing.<br />Start engine and watch water flow - at idle and at 2000 rpm.