Bad Thermostat?

nghtshd88

nghtshd88

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Replaced my entire coolant system about 3 1/2 years ago all OEM parts. Today at highway speed I noticed the guage dropping a little. Slowing down going downhill it sank even more. Finally upon parking facing uphill it shot back up to normal. Nothing above that luckily. Zero leaks and no coolant loss it looks. Dont think its the CHT sensor? Id hope not to have to remove the upper manifold again...
 
Is this a Gen 1 or Gen 2?

V6 or V8?

Did replacing the entire cooling system include the DCCV and aux coolant flow pump?

What about the heater core?

Typically with a thermostat that sticks shut... you'll see a temp spike as the engine warms up... and then a sudden drop in temp... with it gradually coming back up to full normal temp.

With a thermostat that sticks open... the engine will not get up to full temp.

By your description... it sounds like there's air in the system, and it is low on coolant.

By "all OEM" ... did that include the $400+ degas bottle?
 
Gen 2 3.9. Yes air is what I thought but again no loss of coolant and air hasnt been introduced in a very long time. Yes an OEM degas also. There are no visible leaks. Didnt do the pump dccv or heater core.
 
Well the fact that when you parked it on a hill, (nose up I assume), suggests there is air in the system. When the temp came up... it was due to the angle of the car... and the fact that the coolant contacted the CHT.

I don't think the CHT is the issue... I think it's air.

Find a level spot and top off the degas when cold... and mark it with a marker. Drive a couple days and let it cool off completely... then check the level again. You will probably see it has dropped again. Checking while not completely cooled off can be misleading.

Might want to pull the shield under the engine and take a look at the DCCV and Aux pump. Keep in mind they are mostly plastic to.

You could also get some UV dye for cooling systems... and a black light.

If it ends up being the DCCV or Aux pump... best to replace both at once... but you know that.

There are 4 hoses down in that area too. 3 "quick connects" that go to the aluminum lines that run along the passenger side shock tower... and one coming off the wye on the upper hose.
 
When I say temp up I dont mean overheating just the guage back to the normal position.

Checked twice cold and zero loss of coolant.

Hooked up my SCT for a 45min drive and avg temp driving is 182-184 degrees I hit a max of 188-190 idling. Lowest was 178-180 coasting down hill off the throttle.

I think the guage itself just became hyper sensitive. Going up hill on the gas and parking nose up I assume is just the temp reaching 182-184 again which I saw. Coasting down off the throttle it dips 2-4 degrees. Still have never seen the guage act so erratic before.
 
Those temps sound low for a 3.9 ... so the stat could be failing by opening early... or be weak.

Do you have the proper mix of water/coolant in the system?

Do you have a way to check engine oil temp... compared to coolant temp... on your scanner?
 
Yes proper mix of motorcraft fluid. Not sure about the oil temp didnt see that.

I agree it is def low vs the temps i saw post coolant system replacement. I do have a torrie tune but the fan was not running. Probably a faulty thermostat. Better stuck open then closed...
 
Old thermostat was stuck open. Heats up faster now ofc and am running between 194-199 driving around. Not a hot day.

It took about 20mins to remove but ofc with a mess of coolant. Compressing the new thermostat into the housing is a major pain. It got away only once luckily.

Still with the original radiator. Hope that isnt next.

20200602_140551.jpg
 
Yeah... that can be a concern. I think it's an issue with the wax in the thermostat... or whatever they use to actuate the plunger. I could see that breaking down over time.

I think most of the rest should still be good... and the T-stat is fairly easy to replace.

Hopefully you got new O-rings... and didn't reuse the old ones. Tried that once and it didn't work out.
 
04_Sport_LS said:
Yeah... that can be a concern. I think it's an issue with the wax in the thermostat... or whatever they use to actuate the plunger. I could see that breaking down over time.

I think most of the rest should still be good... and the T-stat is fairly easy to replace.

Hopefully you got new O-rings... and didn't reuse the old ones. Tried that once and it didn't work out.
Click to expand...

I reused the O-ring lol. There is only 1 if not counting the cap.
 
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