Thread: My 97 GTP's Overheating Problem (3800 V6) - Solved !

My '97 GTP's 3800 S/C V6 engine started to overheat recently, but it took me a while to get to the root of the problem. Perhaps my experience will be helpful to others.

I have the set of 1997 GM manuals for the Grand Prix, and I have an Autotap AT-1 scan tool. Unfortunately, my scan tool is not communicating with my laptop, so I'm diagnosing the problem without the assistance of a scan tool, or knowing what DTCs are set. The MIL is illuminated, but this car has a history of the transmission setting codes, so there may be DTCs set that are related to the overheating problem, but I'm not sure.

Here's a synopsis of the problem and what I did to try and solve it before I found the cause...

- Engine was overheating in stop-and-go traffic, or when stopped for more than a minute or two, even in freezing temperatures. As soon as I started moving, coolant temperature returns to normal.

- Radiator coolant level OK, coolant overflow reservoir OK

- Coolant and engine oil look/smell OK with no contamination.

- No obstructions found in front of or behind radiator & A/C condenser.

- Replaced thermostat, to no avail. Still overheated unless I'm moving.

- REMOVED thermostat, to no avail. Still overheated unless I'm moving.

- Noticed that cooling fans were not running, even when instrument panel temperature gauge was indicating overheating (ie: needle at three-quarters scale), unless air conditioner or defroster was on. And when A/C was on, engine would not overheat, even when standing still.

- Water pump appears to be OK, since engine never overheats when car is moving, even in hot weather.

- Pulled and tested all three cooling fan relays, which were functioning properly.

- With fan relays removed from the underhood fuse box, I jumpered the appropriate circuits and proved that the fans WERE operating in both low and high speed modes.

- Significantly, I found that when I disconnected the wiring harness connector from the ECT sensor (something you only want to do with a cold engine, unless you like the smell of burning flesh, since it is right next to / behind the exhaust manifold cross-over pipe), the PCM commanded high speed fan operation !!! Apparently, the PCM is programmed to run the fans at high speed to protect the engine from overheating, if it loses communication with the engine coolant temperature sensor. The fact that the PCM reacted to my disconnecting the ECT sensor told me that I was probably dealing with a sensor reading that was outside of the normal range, as opposed to an open or short in the circuit between the PCM and the sensor.


I really didn't suspect the engine coolant temperature (ECT) sensor, since my instrument panel temperature gauge appears to be functioning correctly. I suspected a PCM or wiring harness problem, but the fact that the PCM could command the fans to operate when the air conditioner was running made that unlikely.

I had an epiphany when I studied the shop manual, and found that there are apparently TWO separate thermistors inside the three-wire ECT sensor which is located in the engine block, just below the thermostat housing. One sensor circuit (dark green wire plus chassis ground) is used to drive the instrument panel temperature gauge, and a separate sensor circuit (yellow and black wires, PCM temperature sensing and PCM sensor ground, respectively) is used to communicate engine coolant temperature to the PCM.

At this point I suspected that only the circuit that talks to the PCM was bad. But since GM placed the ECT sensor so that it would be as inaccessible as possible, it's virtually impossible to get test leads onto the pins, which you can't even see due to the position of the sensor. It's even challenging to get your fingers in there behind the crossover pipe to get the connector onto or off of the sensor. So, I purchased a wiring harness pigtail assembly, which contains the ECT sensor connector with 7 inch leads (AC Delco part # PT1825, or in my case "Standard" [aftermarket] part # S619), so that I could test the PCM ECT sensor (yellow & black harness leads) with an ohmmeter while it was still installed in the engine block. I could have accomplished the same thing by piercing the insulation on the yellow and black harness wires, but that would probably just cause problems down the road, when the copper conductors fail because they have been exposed to moisture or have oxidized due to the insulation being breached. GM designed their waterproof harness connectors very well, and for good reason. Gee... I think I just paid a compliment to GM ...or maybe to Delphi. That's not something I do too often.

With the pigtail connected to the ECT sensor, I found the resistance to be within the published temperature vs resistance values, with a cold engine resistance of about 6,000 ohms (with around 40 degrees F ambient), dropping to a few hundred ohms when the thermostat housing got a little too warm to touch comfortably, which I estimated to be a coolant temperature of 130-150 degrees F. This proved to my satisfaction that the ECT sensor (or sensors, since the sensor really contains two thermistors) did not need to be replaced. I also put a voltmeter between the yellow and black leads at the now-disconnected ECT sensor connector. I should have seen 5 volts, and the actual reading was 4.97 volts, if memory serves me correctly. I thought that was close enough to 5 volts to not be considered suspicious.

I was sort of hoping that disconnecting and reconnecting the harness connector from the ECT sensor might solve the problem (ie: a bad contact), but no such luck.

I now suspected that my problem might be in the harness, somewhere between the ECT sensor and the PCM. Oh joy. An open circuit (or even worse, a high resistance or intermittent connection) inside a wiring harness is the type of problem you don't wish on your worst enemy.

Another thing I found in the shop manual was that there was a 3-circuit in-line connector on the wiring harness, somewhere between the ECT sensor and the PCM. But where is it ??? I removed the plastic engine shroud, and followed the harness up from the temperature sensor. I found the in-line connector next to one of the fuel injectors on the side of the engine that faces towards the front of the car.

I pulled the "keeper" or whatever GM calls the gray plastic safety pin on the connector off, and separated the two halves of the in-line connector. I was going to try and clip my test leads to the yellow and black wires, and measure resistance at that connector, but I decided to just re-connect the two halves, and see if my problem still existed.

I looked down at the cooling fans, and they were running at low speed ! I've never been so happy to see a fan blade spinning.

I think I have solved my overheating problem, although I'll have to make a few more trips before I'm totally convinced. At least I now know where the problem is. I suspect that a high resistance in the ECT sensor circuit made the PCM think that the coolant temperature was lower than it actually was (hence no fans), but the resistance was not high enough for the PCM to think that it had lost the sensor signal, and turn the fans on at high speed.

I also suspect that a high resistance in the ECT sensor circuit did not show up as a lower voltage when I checked for 5 volts between the yellow and black leads due to the fact that resistance would produce a voltage drop only when current is flowing. Since an ohmmeter is designed to be high-impedence, virtually no current flows through it when making a measurement. Hence, no significant voltage drop measured.

And I convinced myself that I really do need to get a new scan tool. If I had seen PCM ECT sensor readings that were low, while the instrument panel temperature gauge told me the engine is overheating, it would have eliminated a lot of the usual suspects (water pump, thermostat, etc).

The only lingering question I have is this... I thought that in the past, my temperature guage was rising to exactly one-quarter scale and staying there once the engine reached operating temperature. Since the engine has a 195 degree F thermostat, I assumed that one quarter scale equated to about 195 degrees (it isn't marked with a number). Now, with the engine running and parked in one spot, I notice that the temperature gauge will rise to almost half scale (marked as 210 degrees), at which time the cooling fans start running, dropping the gauge back down to one-quarter scale in about a minute, when the fans turn off. Then the cycle repeats (gauge moving between one-quarter and one-half scale) as long as the car is not moving. I don't have the shop manual in front of me at the moment, but I think it said that the PCM does not command low-speed fan operation until it sees a coolant temperature of 207 or 209 degrees.

Could 1997 (or other model year) GTP owners please comment (by e-mail or by voting in the poll) on the behavior of their temperature gauge when they are parked with the engine running ? Does your temperature gauge move between one-quarter and one-half scale as the fans cycle on and off (perhaps that is how my gauge always behaved, and I never noticed it), or does it stay put at one spot all the time once the engine reaches normal operating temperature, and if so, where on the scale does it sit ?

Good job keeping after it until you found the cause of the problem.