Intermittent Overheat Problem

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Guilford, CT
2006 Ford Ranger 4.0 SOHC V6, 62k miles.

I have an intermittent problem where the coolant temp gauge gets into the hot zone for a few seconds and goes down. It'll go like this: Start truck up after it's been sitting overnight. Coolant gauge slowly rises as engine warms up like normal. Gauge gets to normal (middle of gauge) and keeps going, and going, till it maxes out. It stays there for a few seconds, and then instantly drops right back to the normal zone, right in the middle of the gauge. It stays normal the whole rest of the trip. It only happens as the engine is warming up. I'm a little doubtful that the engine is actually overheating, and I think there's probably just something wrong with the gauge or sender. It's very intermittent and has only happened about 6 times in the past 2 months or so. And only twice it maxed out the gauge; the other times, it started creeping past the halfway mark and went back down. Nothing seems to be wrong with the engine. No coolant loss, coolant is relatively fresh.

Could it be a thermostat intermittently sticking closed, and finally opening up after the engine gets too hot? Or do you think it's a bad sender? I really hate intermittent problems; they're such a pain to diagnose.
mad.gif
 
Sticky thermostat would be the first thing to consider. Cheap enough to just replace unconditionally.

Also if the radiator fan is electric, it may not always be starting when it needs to. This could cause overheating when sitting still or in slow traffic, but not at speed.
 
There's no electric fan. It just has a mechanical fan with fan clutch. The problem happened this morning while I was driving 65 mph down the highway, so it's not an airflow problem or fan problem.
 
Originally Posted By: mk378
Sticky thermostat would be the first thing to consider. Cheap enough to just replace unconditionally.

Also if the radiator fan is electric, it may not always be starting when it needs to. This could cause overheating when sitting still or in slow traffic, but not at speed.


I agree with the sticky thermostat as a very likely possibility. Also, if you have any air trapped in the cooling system, it can keep the water pump from being "Primed" initially. Burping air out of some modern cooling systems is more art than science.
 
If you find the heater blows cold air even though the engine is hot, that would suggest an air bubble like 4wheeldog said.
 
Its most likely the stat get an OEM one, or at least a plain one, the failsafe ones seem to fail way more often... at least they are mostly open

The mechanism here is that the seal is sticking or some of the wax ball has leaked and it pops open at a higher temp, then it stays open ...

If you want to confirm, get an ir thermometer, and watch the warm up and see if it exceeds 210° f or so
 
I haven't touched the cooling system in a few years. Can air work its way into a closed system on its own? 'Cause unless there have been air bubbles in the system for the past few years, and only just now somehow started causing problems, I don't think that's the problem.
 
Air can work in if there is a leak.

But low coolant level and a stuck thermostat are the first two things to check. If you've checked the coolant level and it's good, then check/change the thermostat.
 
If your engine has smaller hoses leading to the pressurized coolant reservoir, check them and make sure they are flowing free. If they are not, you will encounter all sort of cooling problems.
 
Originally Posted By: DoubleWasp
If your engine has smaller hoses leading to the pressurized coolant reservoir, check them and make sure they are flowing free. If they are not, you will encounter all sort of cooling problems.

Cooling reservoir is not pressurized.
 
Originally Posted By: exranger06
Originally Posted By: DoubleWasp
If your engine has smaller hoses leading to the pressurized coolant reservoir, check them and make sure they are flowing free. If they are not, you will encounter all sort of cooling problems.

Cooling reservoir is not pressurized.


That is odd; mine is.
 
Originally Posted By: simple_gifts
Originally Posted By: exranger06
Originally Posted By: DoubleWasp
If your engine has smaller hoses leading to the pressurized coolant reservoir, check them and make sure they are flowing free. If they are not, you will encounter all sort of cooling problems.

Cooling reservoir is not pressurized.


That is odd; mine is.

The Duratec 4 cylinder was the ONLY engine in a Ranger to ever have a pressurized cooling reservoir. All other engines have a non-pressurized reservoir.
 
exranger06: Test the new tstat before you install it.
Just boil some water await for it to cool to ~200F pour it in a clear class and toss in the tstat; it should open pretty quickly.
Do it to the old one too and see if its stuck open a "little" or closed.
 
Originally Posted By: exranger06
I haven't touched the cooling system in a few years. Can air work its way into a closed system on its own? 'Cause unless there have been air bubbles in the system for the past few years, and only just now somehow started causing problems, I don't think that's the problem.



Yes, through the water pump suction hose joints.

I would have it tighten to eliminate the possibility.
 
Originally Posted By: exranger06
Originally Posted By: simple_gifts
Originally Posted By: exranger06
Originally Posted By: DoubleWasp
If your engine has smaller hoses leading to the pressurized coolant reservoir, check them and make sure they are flowing free. If they are not, you will encounter all sort of cooling problems.

Cooling reservoir is not pressurized.


That is odd; mine is.

The Duratec 4 cylinder was the ONLY engine in a Ranger to ever have a pressurized cooling reservoir. All other engines have a non-pressurized reservoir.


Is there a way to convert using a radiator from another engine? Or is it as simple as it is in the Cherokees and just put on a normal radiator cap?

Duratec rangers are getting pretty cheap on CL. I wouldn't mind one to pull my popup. But ... pressureized reservoirs are a no-no for me.
 
I literally just replaced the thermostat in my 2002 3.0 Ranger last month for the exact same symptoms. For grins, I couldn't boil the old thermostat open on the stove, so it was definitely done after 170K miles. I also took the opportunity to flush the cooling system, and switch from the old school green formula coolant to Zerex G-05. Note that mine was delivered with the green (owned it since new). They switched to G-05 later in the 2002 production run as I recall.
 
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