If the Stat has never been changed, you're lucky to get that amount of service from the original.
Consider the unit has to move almost continuously while the engine is up to heat. The movement is likely stressed off center and is binding holding it open.
The reason the engine will be hard to hot re-start is the cooling signal the computer gets indicates the same as a cold start.
This sets the fuel rich and causes a near flooding condition a hot motor does not need in addition to a greatly different ignition timing.
Driving with a cold cooling system also causes the fuel consumption to rise because the sensor still detects the temperature as too cold and keeps the fuel too rich.
All this can be seen looking at live data with a Scanner.
The code 125 just sums it all up for you.
Change the Stat with a 195 unit.
When refilling the system, take time to be sure the air gets out by leaving the cap off, topping off as needed or you might have overheating or other issues causing confusion and air locking the hearer core.
Good luck.

 

Great advise and analysis from you all. Bluegrass, why would the engine get a cold start signal when the coolant is hot, regardless of the thermostat's condition. According to my gauge, the temp seems to be the same as it always was. It's raining like hell, have to do the thermostat replacement and some of the testing tomorrow. Keep up the great advise y'all.

 

Every time the engine is shut off, the next Key on before cranking, the PCM looks at the values of the Intake Temperature Sensor and the Coolant Sensor to determine the Fuel and ignition timing requirements.
It is the only way the system has to know what these should be and difference between cold and hot restarts and all in between the two extremes.
Added into cold start is the extreme cold end where the idle is raised to as much as 1200 to 1500 RPM, then slowly comes down as the Coolant sensor detects the rise and the Ox Sensors heat enough to take over fuel control in closed loop...
If any area has a fault, the results can vary from very lean to very rich fuel and starting difficulties.
Add to this any issue with fuel pressure.
Note that if the Coolant sensor signals a cold value under hot restarts and temps, the fuel will way too rich and may flood the engine during cranking.
At the same time the Alternator has to output [as a load on the engine], to re-charge the battery.
Everything has to be in order for smooth starts and cold drive-aways in cold or hot temps without staving or flooding or hesitation.
If the PCM is powered down, the system has to rebuild the tables for the existing conditions and slowly varies as those conditions change.
A/C and Lighting loads, power steering loads also affect the start setting and high idles etc.
A lot goes on few know about.
If there are any other issues, they have to be recognized and delt with.
Fuel regulator failure, ignition etc.
The dash gauge is nearly useless doe to how it was employed.
Both the coolant and oil pressure is on the same circuit, as either failure is serious to the engine.
The movement is does not reflect actual coolant temp. Don't put you faith in it's reading.
OIL pressure the same thing. The oil pressure is just a switch closed or open to a very low pressure and not the real value.
Good luck.

 

I replaced the thermostat because it was fairly easy to do, even though I had serious doubts to begin with that it was the problem and it wasn't. There was nothing wrong with the thermostat. Given the fact that my coolant was always at the proper level and always at the proper temperature according to my gauge which is as accurate as a Rolex, the status of the thermostat seems to be irrelevant. Stuck open or not, who cares. If the coolant is around 195 F, the ECM or PCM should be happy with that unless it gets the wrong information and that it did. After changing the thermostat the same identical problem was still there, so I went ahead and changed the coolant temperature sensor, which I should have done in the first place, because it makes more sense, is easier to do and without the loss of coolant. The moment I started the car (at normal operating temp) it started after 3 or 4 turns and 30 seconds later the excessive RPM came down to almost normal, I knew then that this was the problem. I drove it around the block, came home and turned it off, waited a few minutes and started it again and..........bang it was right there, problem solved. It starts and runs flawlessly. There is one little problem though, the Check Engine light came back on, even though I disconnected the battery for about 1/2 hour. I thought by disconnecting the battery it would clear the code. Am I wrong? Will the check engine light clear itself or do I have to go and have it cleared. Just to make sure that I don't have a new different code I will go to O'Reilly tomorrow and have them take another reading. Btw. for anybody that doesn't know where the sensors are and what they look like, the 1997 4.6 ltr engine has 2 sensors. They are both on the manifold to the left (standing in front of the car) of the thermostat housing. The one most left is black and is the sender for the temp gauge. About 4" to the right from that is the temperature coolant sensor, which is grey.

 

ueww40, great to hear that you solved your issue. I can't say whether the code will clear itself or not. Have your ever thought of buying yourself a code reader? The basic units are only 35 bucks at Harbor Freight (maybe there are cheaper ones out there, I haven't looked) and you can cancel existing codes and clear the check engine light. If your problem isn't really fixed, the code will come back. I find the check engine light to be a royal PITA. Before I got my code reader I was always tempted to put masking tape over the damn thing. It's like an itch that you can't scratch. Hope your truck continues to run well.

 

No Don I haven't, but now that you mentioned it, I am thinking about it. I love H.F. and I just got a flyer from them giving 20% off for Father's Day. I think I go over there and see what they got.

 

I did get a nice code reader (Zurich Z8s) at H.F. I got a great deal on it over Father's Day weekend. Now I have to learn all its bells and whistles. I mentioned in post #14 that I still had my check engine light on and I wasn't sure if it still was the same P0125 or something new. Well, it was something new. I now had a code P0118 indicating a problem with the coolant temp sensor again. The sensor I just replaced. How could that be? Well, it was my own clumsiness that destroyed that nice new sensor. While installing it I was not careful enough and sort of broke the plastic connector part from the bottom metal sensor part. Apparently when installing it I must have pushed on it with the 19 mm deep socket and broke it loose. I didn't notice that when I connected the cable and it kept throwing a P0118 code. I replaced the sensor again, this time very carefully, making sure that the socket didn't touch the grey plastic part at all and that fixed it right away. I cleared the code with my new scanner and finally all is well. Problem solved, no check engine light. Car runs great. Thank you all for all your help.

 

Your code118 description tells you the circuit was >open.
Make sense?
If the Scanner did not tell you this, you have to be sure by looking up codes that have the full descriptionso.
To many ignore the real description and blame the sensor that reported the issue.
In your case, you created open ckt. right at the sensor.
Good luck.

 

Yeah, the scanner did tell me that. it said: Engine coolant Temperature Sensor Circuit High Input. When I looked into it it also did mention all other possible problems and Open Circuit was one of them (created by my sloppiness, which is a great teacher. It won't happen again)