My car keeps overheating. I've replaced both temperature sending units (gauge and ECM), checked/replaced the thermostat, and bled all air out of the system. The fans do not turn on at ~220*, but they DO turn on if I attach my code scanner (shorts out the ALDL connectors). Oddly enough, I don't get any heat, either -- that would likely mean the heater core is clogged, right? What could be the problem? I can't think of anything else. The car has started overheating ever since I replaced the Optispark, which involved removing the water pump.

 

I know its a PITA to remove the heater core but you might have to in order to clean it out. It is a pretty common problem on pre
96 LT1 cars.

 

You need to get it on a scanner so you can see what temp the pcm is seeing. You can always try resetting the pcm, too.

 

how can i get to it?

 

With no fan activiation or heat I would look carefully at the thermostat. The PCM temp sensor is in the waterpump (as opposed to the dash's sensor in the cylinder head). Sounds like some sort of circulation problem in that area. Just a WAG

 

im sure the themostat is working perfectly.

 

i'd hate to suggest you clogged your water pump. Maybe too much RTV when you put the water pump housing back on. see if you can snag a coolant pressure sensor. It might be worth pulling a coolant hose to see if the water is flowing. make sure the hose going to the thermostat end of the coolant line is lower than the outlet hose from the water pump. if the hose going to the thermostat is higher than the bleeder it won't really help much. Make sure the coolant boil-off lines coming from the back of both heads isn't clogged too, i.e. might be time for a throttle-body bypass.

just a few thoughts

 

first off, pop out your thermostat and run the car w/out it. I agree that you need to know what the computer is seeing. If you know anyone that has a scanner or a laptop and cable call'em up. make sure you let that thing bleed for a while. Also, to lessen the chance of overheating while you're "toying" around, unplug your MAF. This will put the computer in limp home mode and turn both fans on right away. Before you start the car take the cap off the radiator. Then start it and make sure you are seeing flow through the radiator. Start playing from here. Do this and let the car warm up, carefully watching both the computer readout of temp AND the guage. Once it gets past the 160 mark you should be able to detect heat. If not, problem numero uno. I also agree that the hard coolant line that goes from the back of the heads up to the t.b. is a problem area. Since this is a return line to the radiator I would recommend disconnecting it as it comes out of the bottom of the Throttle body and routing a long extension of hose to a 5 gallon bucket. You should get relativly good flow into the bucket, in other words it'll start emptying out the system fast so only do this on a cool motor for 45 seconds tops to be on the safe side.