Engine Overheating Badly Afther Thermostat Housing Swap...


New Member
Apr 18, 2016
My 1986 LX 5.0 was running perfectly...

But it had interior mounted mechanical gauges I wanted to replace.

I had electric gauges in my 86 GT... The TEMP gauge got it's signal from a sender that went into a threaded thermostat housing.

I have a high flow 165 thermostat that operated perfectly before the housing change.

I swapped the threaded housing, and applied a very light coat of red PERMATEX on both sides of the FEL-PRO gasket & let it set for two days.

When I removed the original housing, I naturally lost some coolant. I ALSO vacuumed some coolant out of the intake to ensure the "new" housing would have a dry surface to mate to.

Got my gauges hooked up, pulled the car out of the garage and parked it on a hill.

Then used the LISLE Spill-Free funnel to re-fill the coolant.

THAT'S when I had an accident. I dropped a quarter-sized rubber washer into the radiator. It vanished. I didn't panic because I assumed something would catch it inside the radiator without causing a problem with the flow.

VERY IMPORTANT TO NOTE that this car came with a MEZIERE electric water pump & electronic fan (BOTH WORKED PERFECTLY BEFORE HOUSING SWAP)

The car starts, and overheats... I know the thermostat is opening because I'm getting plenty of heat.

I know the electronic water pump is flowing because I can see the coolant spilling back into the top of the radiator.

The car continues to over heat and fill the puke tank.

As soon as I kill the engine, the electronic water-pump sucks in the coolant that is in the NO-SPILL funnel...

But when I refill the funnel and run the car, it spills the coolant back out and overheats again.

I'm left with three questions...

1) Could the dropped washer possibly get lodged somewhere and restrict flow (even though I know the electronic water pump is pushing coolant back into the top of the radiator)?

2) If I can see coolant being pushed back into the top of the radiator the pump is operating correctly, right? In the proper flow direction.

3) Could air be stuck in this system that I can't burp... Even with the Lisle No-Spill funnel?

I am beginning to feel quite discouraged about this. I like to fix things myself. I have rebuilt cooling systems in two Grand Cherokees (4.0 & 5.9) and a Delorean. I like to think I know what I'm doing, but I feel totally stumped and hopeless on this one.

Can anybody please help me?


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SN Certified Technician
Aug 27, 2012
In the garage
so the rubber washer didn't magically vanish, It is somewhere whith your radiator, or water pump, orthermostat.or engine. You do not want to continue to overheat your engine That is when bad things happen. YOu should have never run the engine wiht that piece of rubber still in there. It either most likely clogged the flow tubes in the radiator or flowed through the upper radiator hose to the thermostat and hopefully didnt make it past the thermostat.

If it were me, I would pull the radiator out and hook up a garden hose to it and see if it is clogged., then remove the thermostat housing and thermostat to see if it is there and pray I found the washer in either of those places.


New Member
Apr 18, 2016
I understand it's in there somewhere... It just seems strange that a hollow rubber o-ring could cause that issue. I'll have to take the housing off and snoop around tomorrow I guess. Given the flow path, it would be on the intake side of the housing, correct?
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