The '95 Z28 has been having this problem, and as my sister is raving about at the moment... she had to have it towed to the nearest GM dealership.

The issue...

The check gauges light comes on and the engine temperature gauge shoots up to the red zone. The car is NOT running hot, the coolant level is perfect, and the water pump was recently replaced.

According to my sister, this time the coolant level engine light came on the dash as well, which prompted her to have it flat-bed towed to the dealership.

Background...

The check gauges light came on about 3 weeks ago and I had to go rescue my sister from her friend's parking garage. I checked the car out and as I said above, the car showed no signs of being hot (visual or smell), the overflow tank was at the level it's supposed to be, and there were absolutely NO signs of coolant leaking out underneath the car. No coolant on the ground, and no coolant on the underside of the car. No smell of coolant.

I traded her cars and drove the Z28 home 10 miles. I drove it with the heater on the hottest setting and the fan speed on the 2nd fastest. The car drove absolutely perfect, like a brand new car the entire way home.

The only thing that struck me as odd is that the heater was blowing just as hot as my GXP's. The last times I drove the car was several times in the winter and I'd be home before the heater would even get warm. She remarked that she thought it was weird the car was actually blowing hot air too.

Background on parts...

The radiator was replaced in May, 2010 due to the original one being rusty.

The water pump was replaced January 2011 due to the old one having a very slow leak from the area on the bottom of it. The radiator drain plug (petcock) was replaced at the same time because the other one wasn't sealing right. When the water pump was installed, the dealership broke the original coolant temperature sensor and had to install a new one. The coolant was flushed and refilled. Up until the last month, there hasn't been an issue with it.

My speculation...

The car still has its original 1995 heater core, and as such, the heater has not been very hot or useful for the last few years (which isn't a major deal here in Dallas). Replacing the radiator did nothing, and several non-chemical flushes of the cooling system has done nothing. Now all of a sudden, the heater works like a brand new car. I speculate that a big dried up booger/turd of gunk has come loose and is floating around the cooling system now causing the car to think it's getting hot or low on fluid every time the booger/turd gets caught somewhere else.

What do you think? Broken level sensor? Broken temperature sensor? Some other glitch?

 

The cooling systems on our cars are tricky.

Here's some notes I've learned over the past 6 years.

• LT1's do NOT like air in the system, bleed the air out using the bleeder screws while the engine is running
• Check Engine light will come on when the temp is in the red zone, this is just to alert the driver that something is wrong
• Low Coolant Sensors short out very easily on our cars, I wouldn't trust one if it said you have low coolant, but if the sensor is broke you should obviously keep a close watch on your temp / radiator levels

Besides that... heater core could be clogged, thermostat could be stuck, radiator cap be bad and no longer holding pressure. I'm sure there's other things to check.

 

I agree to check to see if there is an air pocket in the system open bleeder screws, one is located just before thermostat and other is on hose going to waterpump. I would also do a check to make sure your head gasket isnt gone.

 

How would I check to see if there is a head gasket issue? Any method without taking the engine apart and thus having to replace the head gasket...?

The car isn't having oil mixing with coolant and mechanically the car seriously runs and drives like a new car (or it would if it'd quit having these glitches and didn't have a gasoline smell that the dealership and I both can't trace and explain), so I hope it isn't the head gaskets. The car has never technically been over heated and we've owned it since '95, so I hope it isn't head gaskets.

The radiator cap and thermostat were replaced the same time as the radiator, so should be okay I hope. There could be air... I'd check if it wasn't sitting at the dealership right now.

I'm seriously hoping there's just a 10 year old turd floating around in there that can hopefully be easily flushed out. The replacement of sensors is getting expensive (particularly the labor).

If anybody has any other ideas (or has had this problem and solved it), please post up!

 

Definitely make sure the primates that serviced the cooling system bled it properly.
The temp guage gets its info from a sending unit in the D/S cylinder head. The PCM gets its temp info from the sensor in the water pump. Use a scan tool to monitor engine temp via the sensor to test the temp guage sending unit. I believe the check engine light may illuminate if a IP warning light is on. If they disagree I'd suggest the sendng unit since much else in the cooling system seems to be serviced or replaced recently.
Something else is easy to check is the thermostat. put it in a pan of water on the stove w/ a thermometer. set on high. Make sure it opens and it opens at the correct temp.
There are two types of GM coolant level sensors--Those that have failed and those that will. You can pull it and clean the electrode with a Scotchbrite pad or sand paper if you want to try to save the 50 clams a replacement will cost. Shoebox's site has the repair info and pix. Your theory about the heater not working and then working may explain the coolant level sensor failure since crap from the heater core may have coated the electrode.
HTH
Tim