I just had a motor rebuild done and also clutch, pressure plate, flywheel resurface, etc. Before I left the shop we noticed that the fans wouldn't come on and the engine was overheating. Apparently there are 2 temp/coolant sensors, one on the front of the water pump and the other on the right side of the engine block. When we unplugged the one on the front of the water pump, the fans came on. When we plugged it back in, the fans died. So we assumed it was the sensor. Now I have replaced both sensors and all the relays on the fuse panel and still no fans. The only thing I haven't replaced is the thermostat. Does anyone have any ideas before I give up and take it to the dealership? Thanks in advance for your help.

Jim in Tennessee

 

I have read numerous posts so here are a few more items that may shed some light on the problem. Headers were installed same time as the motor rebuild and rear o2 sensors aren't functional so I do have a ses light constantly right now, which could potentially make the fans run constantly, but not so. Also, I have not had the a/c gassed up since the rebuild so this might be a potential problem. This is a 96 z-28 with 6-speed manual, trans. with 150k miles. Any help is appreciated. Thanks.

Jim in Tennessee

 

Overheating to some is not overheating to others. What really happens?

Your test with the sensor was not valid. Any car with a good sensor will do the same thing.

 

Each time I have tested the fans(after replacing each sensor and after replacing the relays) I drove the car to warm it up, parked it and watched the temp get very close to redline. A couple of times it was hot enough to smell antifreeze and even belch some out underneath the passenger front fender. I have almost redlined sitting in the drivethrough of a bank and a fast food place.

I guess the test where we unplugged the sensor was just to prove that the fans were not bad. So the fans are functional, both sensors are new and the 3 relays are new. I even removed the sensor on the water pump and removed the teflon tape and reinstalled it just in case the tape had anything to do with it. I have not replaced the radiator cap nor the thermostat. The radiator cap did have some corrosion on the top from the battery, which I cleaned off. Could the cap make that much of a difference?

 

IMO, what you need to do is use a scanner to view engine temp and see if the fans are commanded on when they are supposed to be. The fan system is capable of working-proven by your previous tests. You can find cooling system info on my site.

 

Do you mean a scanner that hooks to the computer or one with the infrared dot that reads temp.? I only have access to one of the infrared types, unless the type of scanner that auto zone has is what you are talking about.

Can I rule out the a/c system or radiator cap as potential problems. Your tech manual seems to mention that either may affect the cooling system. In using the scanner to test the engine temp., are we making the assumption that the dash gauge is inaccurate? I have no reason to believe the gauge is inaccurate because the sensor is brand new and the wire between the sensor and dash is is great shape. When the gauge was showing 240-250 degrees I had a car that was hot enough to blow antifreeze out the overflow tank.

If the scanner proves that the fans are still not coming on at the proper temperature, what is my next move? Thanks for all the information and help so far. This may or may not mean anything, but I did not have this problem before the engine rebuild. The only problem I had then was a severe knock.

 

The dash gauge does not use the sensor that you replaced. So, you need a diagnostic scanner to read the PCM to see what it sees. We don't assume the gauge is wrong, we just want to verify what the PCM is doing, because it is what controls the fans.

 

Did you also check the fuse under the hood.

When I had a similar issue with a bad sensor, I also blew the underhood fuse, which I didn't realize was there at first.

 

Problem has been solved. Turns out all that was needed was to bleed the air out of the coolant system. I hadn't added any coolant or water and I had assumed that was already done by the gentleman that did my motor work. I should have known to check this because I also have a 96 impala SS with the LT-1.

Most likely, neither of the coolant temp. sensors were bad in the first place, yet I replaced them both. None of the relays were bad and I replaced them also. I do agree that different people may have a idea of the definition of "running hot" because I do see that these cars run hotter than what most people are used to. However, as it turns out, mine was in fact getting too hot as evidenced by gauge, smell and excretion from the overflow tank. Lesson learned: problems aren't always what they seem and never overlook the simple things.
Thanks for the informative responses.

Jim in Tennessee