Running hot, lots of work done, no joy

Mostly stock.

Car had been running sort of hot but not completely overheating, for a while.

The timing belt and water pump are about a-year-old and were changed proactively.

Since the engine had been running hot, I went ahead and pulled the head and had it checked and re-surfaced. It was about 0.007 out of spec, but one of my manuals states the head can be out 0.008. The old gasket did not have an obvious sign of being blown through. While the head was pulled, I polished all the valves and valve seats. Also changed the valve seals. It’s only a four cylinder but it does feel like the acceleration is back to what it was when it was new.

I have made sure that all of the air is purged so that only coolant is flowing.

I should have performed a leak down test before taking the head off, but I did not. I did perform a couple of leak downs after replacing the head gasket. I used about 90 psi pressure and the rings are leaking a little bit which is fine. All four cylinders are losing about 12%. I did not hear any noise coming from the intake or the exhaust manifolds. No bubbles appeared in the coolant and I left the leak down running for a while on each cylinder, heck I even did that a couple times. No bubbles.

I’ve already sent the PCM off to the folks at NEAT Computers have it checked and they said that it was in perfect condition so I reinstalled the PCM. Of course, they recommended that I check the engine coolant sensor and the fan control module.

I tested the coolant sensor using a pot of hot water on the stove and the results seemed okay. The coolant sensor is aftermarket, so this week I’m going to go to the dealership and pick up one that is factory. In the third and fourth points below, I note the temperatures that the PCM is providing based on the engine coolant sensor resistance so it seems like the engine coolant sensor is working properly. Regardless I will change it with one from the dealer.

I think the fan controller is okay based on the results below, but I’m curious to see what you think.

- The voltage at the fan control module from the PCM is about 0.04 V when the fans are not running.
- The fans will both run and receive 5.1V from the PCM when the air-conditioner is on. Both fans run out of very strong pace with this voltage.
- If I just let the engine run with no heat being pulled on the inside of the car, then the engine will gradually warm up. Eventually it will reach about 208° according to my Bluetooth OBD reader app. At that point, the fans will kick on and will receive about 2.3V from the PCM, both fans run at a LOW speed until the engine temperature drops to approx 203° and the fans turn off. It would do this over and over again if I let it.
- When I unplug the coolant temp sensor, then both fans will start and run at full speed. The voltage from the PCM at the fan controller is 5.1 volts. My OBD Bluetooth reader will go from showing the engine temperature to showing -40°F. I can plug the coolant sensor connector back into the sensor, but the fans will continue to run at full speed until I clear the P0171 code or restart the car. When I plug the connector back in, the engine temperature will return to normal in my app.
- If I let the car climb to 208° and then unplug the coolant temperature sensor the fans will kick on at a high speed and within a minute the engine temperature will drop to about 195° and will stay there as long as the fans are running.


I’m thrown off here because with my Bluetooth OBD reader I’m able to see the temperature of the engine based on the engine coolant sensor. So if I am able to see the engine temperature that means I’m getting the temperature from the PCM. The obd reader app shows the engine temperature drop to the fake -40° when I plug the engine coolant sensor. It just seems like if I can see the engine climb to 208° then the PCM should see that as well and turn the fans on long before that high temp is reached.