Cooling system woes.

[Skeith]

I am having some trouble with the cooling system on my 93 lumina. I recently swapped a seized engine out from an 93 cavalier and ever since I have overheating issues.

The cooling system had a leak, so I pulled the old steel pipe that goes from the water pump to the heater core and replaced it with a rubber heater hose. But my rad fans wont come on.

I thought it was the coolant temp sensor so I replaced it. Still no rad fans. The temp sending unit seems to be working I did a resistance check on it.

The fans work, relays and fuses are good. I have voltage and ground at both sensors, and if I pull the plug to the coolant temp sensor the fans will run constantly (fail safe).

I figured okay maybe my engine hasnt been getting hot enough or whenever I checked they were just off. But today I drove down to my parents for dinner (20 min or so drive) was there for 5 or so hours, and when I left at a red light the engine temp light came on.

Im assuming the engine was running cool from the air from driving being pulled through the rad and when I was at the red light the temp rose, plus heat retained from the drive earlier. It was a hotter day today, about 31*C with a humidity index of 41*C. So I pulled over got some gas and popped the hood, sure enough no fan activity, I killed the engine pulled off the coolant temp sensor to get the fans running. Started it and the fans came on perminantly. (check engine light on because of sensor disconnection). drove it home and the temp light slowly faded out.

The the H*** is wrong? I cant figure it out.

On top of that there is a plugged vac line somewhere in the EECS causing the fuel tank to draw a vacuum. (havent had time to check that one, just been cracking my gas cap a smidge to release pressure while driving for the past week) This weekend Im going to repair the EECS but the cooling sys has me more worried.

Where do I go next? What else should I check, the ECM can obviously see the sensor there since when I unplug it the check engine light comes on and the fans run. Relays and fuses are good. The temp sensor is brand new and the temp sending unit is good as far as I can tell, resistances are in tolerance and the temp light comes on so the temp sending unit must be good.

any Idead? this is driving me mad!!

[bszopi]

Have you done a really good purge on the system to make sure there are no air bubbles?

[idontknow]

The fans, fuses, relays, and wiring all work as evidenced by them coming on once you unplug the sensor. The only thing left it could be is the ECM not firing the relays when it should. Or another bad sensor, or the sensor itself not getting enough water contact.

[Skeith]

Im pretty sure the system is free of air, I bled it pretty good at the T-stat bell cap untill a clean steady stream of coolant came out. The T-stat works as the hose stays cool for a couple min when started cold untill the engine reaches operating temp and the stat opens, the hose then warms up. T-stat is also brand new.

Im going to check the resistances on the ground and sensor lines to make sure they arent too high in case one of the wires is faulty.

Jumping pin A+B on the OBD interface to get the idiot light going, it flashes code 15 (engine coolant low temp) this is with the sensor unpluged after driving it. So the fans are probably keeping it from reaching operating temp.