This 1999 Pontiac Grand Prix came in with the Service Engine Soon Light on (SES) and a code P0125. The code refers to an engine coolant temperature excessive time to enter closed loop fuel control fault. Simply put the engine did not come up to operating temperature as quick as the computer thought it should. Testing is pretty simple. Make sure the system is full of coolant. Start a cold engine and see if the upper radiator hose warms up quickly. If it does the thermostat is stuck open and not allowing the engine to build heat. Sometimes it is as easy to drive an already warm vehicle and watch the gauge if present or in my case observe coolant temperature in data stream. If it fails to reach and maintain between 185 and 195 again the thermostat is stuck open. The next step of course was to change the thermostat. It is located in the very convenient position of under the throttle body and behind the exhaust crossover pipe. This procedure applies to most of the General Motors Vehicles with the 3.1 liter and the 3.4 liter engines with the thermostat hidden in the same place. The first thing I had to do was to drain the coolant level. The radiator drain is at the bottom of the radiator, driver’s side, on the engine side of the radiator. I was able to loosen the drain by sliding my hand between the engine and the fan assembly to reach the drain and loosening it with a 1/4″ ratchet and short extension. I let the coolant drain into a pan. One could of course gain access from the bottom of the vehicle as well.
After the coolant had started draining, I next removed the air inlet hose. This required disconnecting the intake air temperature sensor and the mass air flow sensor electrical harnesses. The crankcase ventilation tube also had to be pulled from the rubber hose. I also remove the air filter and filter cover for more elbow room.
Next I removed the throttle cable bracket and unhooked the cables from the throttle body.
With that out of the way I closed the drain on the radiator and removed the upper radiator hose from the thermostat housing.
I then went to disconnect the coolant lines that go from the heater pipe to the throttle body. I found them to be very soft. I decided that I would be replacing them so I cut them rather than remove them. I remove the heater hose at the rear of the pipe and removed a hold down nut at the front valve cover. Now here is where it gets a little tricky. many manuals say to remove the exhaust crossover pipe and in some cases I have had to but most of the time I can remove the top bolt with ease.
And then using a wrench I can slide it under the crossover pipe, place it on the bolt head and break the bolt loose.
Sometimes I have to break it loose from the top. Which ever way works best. Once it has been loosened a couple of turns I can usually unscrew the bolt with one finger from the top and one finger from underneath. I try not to remove it completely. Instead I back it out most of the way and keep working with the thermostat housing until it will come out of the hole. The housing is slotted for this purpose. If I do take it all of the way out I simply reinstall it prior to replacing the thermostat. About two threads is all that is needed.
I try to pay attention to the position of the thermostat so that I can install the new one exactly as the old one. This makes it easier to install the housing.
The new thermostat is set in place ready for reassembly. When replacing the small coolant hoses at the throttle body be sure to cut the new ones to the exact length of the old ones or you will have to remove and reinstall again.
Your Welcome and 3.1L engine.
Hi Augie,
Thanks for the donation. Does your car have the 3.8L or the 3.1L engine? The noise in the exhaust was likely caused by shifting of spark timing due to the 18x signal dropping out.
Well Sparky you nailed it again, i put in the new temperature sensor and everything is working the way that it should. We spoke awhile back in your changing the ignition switch post about my P0336 code, i changed the crank sensor behind the balancer and the code keeps coming back, the wire on the old crank sensor wasn’t damaged, while i was testing the temperature sensor i think i saw the problem, i put in the new temperature sensor and i was giving the car gas trying to bring up the temperature to see when the fans would kick on, they kicked on at 215 degrees so i stopped giving it gas and when it went down to 185 degrees the fans turned off, i let it run some more and after a few minutes of idling i saw the pulleys and the belt slow down and i heard a popping sound coming from my exhaust, not like a backfiring sound but like a clogging sound, when i went to shut the car down the service engine light was back, i took it to get scanned and it was P0336 again, any idea of what i should change next? Thank you for all of your help keeping me and my family on the road, i just sent you another donation.
Ok Sparky i will order a AC Delco temperature sensor today, after i changed the thermostat (AC Delco), i just ran to Autozone and got a Duralast temperature sensor and i probably did over tighten it (it didn’t come with any torque specs). I bought the car 5 years ago and until a couple of months ago i never saw the temperature go above 185 and i always had heat, as soon as the temperature started hovering around 215 i had no heat so i decided to change the thermostat and antifreeze, the antifreeze had a good amount of sludge in it (i hope i don’t have a head gasket issue as well) but now i have heat. I’ll let you know what happens after i change the sensor and if i have to take it in to have it scanned then so be it.
Hi Augie,
The slight movement with the key off is normal. I went back through your comments. It is going to be very hard to figure out what is wrong without a scan tool. My best guess at this point is that the new sensor is either faulty or was over tightened when it was installed.