I have a 1997 Z34 (3.4 engine). Last week I heard a noise coming from the belt side of the engine and brought it in to my mechanic. He determined that it was the water pump and I then noticed a little coolant being sprayed by the belt. The pump was changed. The next day on the way to work, I got about 4 miles from home and saw the temp gauge was almost in the red zone (temp was fine before the pump change). Got it back to the mechanic, he changed the thermo...still got hot. He then changed the sending unit...still reading hot. He's taken the car out for a drive and taken the engine's temp when it reads hot. He says the engine is at a normal temp. He now thinks it might be the body control module sending false readings to the gauge. I've been taking my cars to this guy for years (he's a Chevy guy) and I do trust him. I just can't bring myself to drive it for more than 5 or 6 miles at a time right now. Any thoughts or similar experience? I'd really appriciate any help with this.

 

You have one of a couple of things. If this "running hot" began after changing the pump, bleed the air from the system (sounds like an air pocket). That check should have been done before any other money was spent. Also, if you have an air pocket, when the temp is running hot, if you turn on the heater full blast, it may not feel hot until the temp goes back down.

Before changing the temp sensor, if you or your mechanic has access to a scan tool, you should take the car for a ride with it. Most scan tools a shop uses can read the data stream from your sensors. Read the temp sensor. If your instrument cluster temp gauge is going beyond operating temp, immediately check what the scan tool is reading. The reading on the scan tool will be what temp the computer/PCM knows the car to be running based on the temp sensor. If the reading on the scan tool is normal temp and the gauge is HOT, then it's a problem with the gauge OR the communication from the BCM to the cluster. If the readings are the same, you have a problem with your cooling system.

Another thing to notate, are the fans kicking on?

 

He's used the scan tool and got normal temps when the gauge is reading too hot. He's also positive there is no air in the system and I also get continous heat, no fluxuation, so I don't think air is the problem. He say's it's OK to drivefor now but it still freaks me out when the HOT light goes on. Not sure if I should believe a human or electronics.

 

Based on what you just stated, the temp is running fine, normal. The temp sensor is feeding the car accurate data (otherwise, you would also have performance issues and probably a coolant system building way to much pressure if it was really overheating).

Sadly, a friend of mine has a '97 Grand Prix with this SAME problem. We even swapped the entire instrument cluster, no change. But, we know it's NOT over heating (heck, the cluster has an idiot light for when the temp is too hot, I can trigger it with the light test in the scan tool, it does not trigger when the cluster says it is too hot, it triggers when the computer says it's too hot).

We are not sure if his problem is BCM or communication from the BCM to the cluster.

At this point, you are at where we stopped. I've been temped to get the el cheapo temp gauge kit from Harbor Freight and stick in the Grand Prix for an independent reading.

Based on what you said, I feel your mechanic is correct the car is safe to drive, but it stinks to be at a loss for a critical read out (temp gauges have saved my butt more then once). For now, pay close attention to other signs of problems and see if you have a "too hot" idiot light on the dash.

And if you learn more about your issue, I'm very interested to know.

 

The HOT idiot light came on this morning when the gauge red-lined. I'll let you know if I find out anything. Any idea how to find the correct part number for a BCM for my car? The internet searches are way too vague.

 

Check your antifreeze level

Do a compression test

Check your tstat

 

Nitehawk, coolant level is good and consistant, no loss. Compression test - done and checked out as OK.

 

OK. How about the coolant mixture? If it's too weak, it could cause overheating.

It couldn't hurt to flush the system, definitely check your tstat though, if it's sticking, it could make it overheat (or underheat)

However it could very well be the BCM, or even the PCM failing

 

Coolant was flushed and changed last week when the water pump was installed. New t-stat went in 2 days later after getting the too hot reading on the gauge.

 

Have you checked the wiring behind the gauge cluster?