My 1997 Lariat, 4.6 ltr engine with 238000 miles on it runs like a dream until a few days ago when I noticed that after the engine was warm I had a hard time starting it. Now yesterday it threw a P0125 code (Insufficient Coolant Temp for Closed Loop Control). The car starts fine when cold. Now it is almost impossible to start it when the engine is warm. The coolant level is correct in the reservoir, the engine goes from cold to normal operating temp in the same time span it always did, the temp gauge shows normal as it always did, no changes there. There are no visible coolant leaks and the coolant level in the reservoir stays the same. Now, even though it is very difficult to start the engine when it's warm, but if and when I get it running it idles very high, like around 1000-1200 RPM. Once the engine runs again it acts normal from then on, drives beautiful without missing a beat. Any suggestions what it could be? All help is appreciated

 

For the P0125, replace the thermostat with a new Motorcraft part. Yours is failing, probably sticking open a bit.

Have no idea whether or not it will influence the warm starting issue or not. That could be a number of things including a vacuum leak or a sticky IAC valve.

 

Aside from the thermostat being an issue a coolant temp sensor will cause a no start condition as well. I had a very similar issue as you with my old explorer a few years ago and I believe the high idle is because your ECU treats it as a cold start (think winter time). I would be able to start it after a few tries. The gauge on your cluster is run off the cylinder head temp sensor so it's a different sensor.

 

I believe that neither the thermostat nor the coolant temp sensor appear to be major operations to replace (correct me if I am wrong). Do you think by replacing both most likely will fix this problem or could there be something else causing that? And what is the proper OEM thermostat for that engine, a 190, 192 or 195.

Thanks y'all

 

There's no indication at this time that there's anything wrong with the coolant temperature (ECT) sensor so leave it alone for now. Replace the thermostat with a Motorcraft RT1254 or Ford 7L3Z-8575-E.

Do only one thing at a time, that helps prevent confusion from getting introduced.

 

My automotive knowledge is very basic. That said, I agree with ProjectSHO89 that the P0125 code likely indicates that your thermostat needs replacing. I will be interested to know if a new thermostat will have any effect at all on the 'warm no-start condition'. Given that the thermostat regulates the coolant temperature, and if it is sticking open a bit as ProjectSHO89 has suggested, I believe that would mean the engine is running cooler that it ought to. I don't see how that would affect starting, because an engine should start when the coolant is cold, hot, or somewhere in the middle. I expect the no-start problem is unrelated to the thermostat but if someone knows better I'd be happy to hear about it.

 

Will follow your advise and change thermostat this weekend. I checked RT-1254 and it looks like a 190 thermostat. Strangely enough O'Reilly and Rockauto are the only stores that have that thermostat. All the others, like AutoZone and Advance have thermostats that are rated 195 and they claim that the 195 is the right thermostat according to their computers. Well, I am gonna take the old thermostat out before I get a new one and see what it says on it. It is the original thermostat the way it came from the factory. I am still surprised that nobody suspects the sensor. Why would the car barely start when warm and if I get it finally to start then idles around 1100 rpm. Shouldn't the car at least always start regardless of the condition of the thermostat? And P0125 was the only code the scanner read. So it looks like there are no other issues.

 

Just get the Motorcraft.

 

What the auto places call a 190 is actually a 192. The specs for the thermostat are not that precise, they actually cover a fairly wide range. 2003 is "starts to open 188-195, fully open 208 - 215". Read the threads about overheating. Non-Motorcraft thermostats are often involved.

You can test the sensor with a meter. Check the resistance cold and hot. The proper values are probably out there somewhere. If you think the sensor is open circuit then unplugging it should have no effect. A simple test.

 

Do one thing at a time starting with what is most obvious. Then, re-evaluate to see if anything else is needed and move in whichever way the data indicates. Don't guess, don't wander off into the weeds by yourself.

Any Ford dealer's parts department will have the thermostat, I supplied the Ford part number already.