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.