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Erratic temperature + can't get new thermostat to open. 1988 5.0L

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Erratic temperature + can't get new thermostat to open. 1988 5.0L

Old 10-27-2015
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Question Erratic temperature + can't get new thermostat to open. 1988 5.0L

I'm stumped. Tried to pass smog. Was told there was a vacuum problem ("likely a leak") for the EGR valve.


Temp gauge went up only half way to normal during idle (but went to bottom of normal range while driving). Haynes manual says engine must be warm to test for vacuum to EGR valve. And although the manifold-spider vacuum was nice and normal, there was no vacuum to EGR with temp gauge at "half-way to normal" while idling.

Well, the gauge used to regularly go all the way to the bottom of the normal range after I installed a 180 degree thermostat several years ago.

But now, with the gauge going only half-way to normal when the engine is idling, I figured I must have a stuck, partially open or otherwise defective/erratic thermostat. And I was hoping that replacing the thermostat and getting a warmer temperature would also get the EGR vacuum working correctly.

So I put in a new 195-degree thermostat.

I refilled the cooling system. But now I can't get the temp gauge to go above barely warm -- that is, to go off of squarely on the "C". And the upper radiator hose does not get very hot although it does get fairly warm. I can hold onto the hose. My impression is that it should get too hot to hold on to.

I've burped the cooling system every way I know how. And I've turned on my heater and moderately warm air comes out. I've been guessing that there is a bubble of air around the coolant temperature sensor (at the top of the manifold -- excluding the air-intake superstructure -- seemingly one of the last places that would get wet when refilling the system with coolant) and that the bubble is essentially insulating the sensor from the coolant. I've also been guessing that something similar is happening with the thermostat motor.

It also seems that I have a small leak dripping under the truck. After letting the engine cool overnight I felt under the thermostat housing and found a drop of wet coolant.

I hoped that maybe the leak was coming from the bypass nipple/hose connection (and not from the interface between the housing and the manifold). So I tightened the clamp a little more. And that seemed to reduce the dripping under the truck, at least for a while. But there is still a bit of extra dripping under the truck when I run the engine.

So that's where things stand at the moment.

==>Could it be that the thermostat is not seated well and that the thermostat flange is causing a gap between the thermostat housing and the manifold? It was extremely difficult to goop up the gasket with gasket sealant and assemble the thermostat and the gasket and the housing together -- AND -- yet try to be sure that the thermostat was well seated in the housing, where there is a recessed area where the thermostat flange is supposed to seat.

Besides, the Haynes manual says to insert the thermostat into the opening in the manifold (that is, not into the housing). According to Haynes, the housing goes onto the manifold after the thermostat is placed in the manifold. So that sounds like the thermostat gets automatically positioned correctly for correct seating. Anyway, that is what I assumed.

==>So could such a leak between the housing and the manifold also be the cause of the thermostat failing to open in order for hot coolant to get to the upper radiator hose?

==>Another possibility that comes to mind is that I forgot to clean the hardened stuff (possibly gasket sealant?) off of the bypass nipple on the housing before I reinstalled the housing. So after I applied a bit of (silicone! = nice to rubber) tune up grease to the nipple and the bypass hose, I just hoped that the stuff between the nipple and the hose would not be a problem and that the rubber hose would simply conform nicely. If this hardened stuff is indeed the source of the leak, I suppose I might try to use a battery-terminal rotational wire brush to try to clean off the nipple without removing the housing -- provided the battery-terminal brush will fit into that area.

I also wonder if maybe I should just apply fresh gasket sealant to the nipple, over the old, hardened stuff, after wiping off the tune-up grease. If that would work well enough, I suppose I could avoid having to remove the housing again.

It would be a substantial rigmarole to remove the thermostat housing again. So I want to be sure that there is no other cause of the symptoms before I go that route; and I would want to be sure that removing the housing again is necessary.

And even if I do remove and somehow check the thermostat for proper seating and clean the bypass nipple, how can I do that efficiently -- especially now that the gasket and the mating surfaces are already gooped up with gasket sealant?

I'd appreciate some pointers here on technique.

Many thanks for any help!
rfhaney is offline  
Old 03-16-2017
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Just replace it with a quality thermostat...
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