Djhotenchilada

I have an 88 iroc z 5.0 MFI with the 5 speed manual transmission. I've been partially restoring the car so it's been in my garage. I've just replaced the entire cooling system and heater system. I did the water pump, radiator, all new hoses, thermostat, heater core, and heater control valve. Also I had help with the fan relays and they now kick on with the ignition. When I replaced the radiator I flushed the engine before installing all of the new hoses and the new radiator and heater core. For some reason though I've been getting very strange temperature fluctuations. I've been to the thread about bleeding the system and I've done all that and it's still acting funny. For about the first 25 minutes even in idle the temperature will go from normal operating temperature right to the bottom of the red on the gauge in about 30 seconds. In about the same amount of time it goes back down to where its supposed to be and it does this cycle every couple of minutes. The weird thing is that after the car has been running for about a half hour it goes to the normal operating temperature and then stays there until you turn the car off and it gets cold again. After the temperature calms down it doesnt matter how much I put my foot in it or what I do I cant make it get hot again. I've also replaced the temperature sensor in the last couple of days and it's still doing it. Any help with this would be appreciated. I'm not really sure where to go from here.

Aviator857

There are two temp probes, one grounds your fans when it gets to a certain temp to turn them on, the other feeds the gauge. Are you sure your replaced the sender to the gauge?

Could be the gauge itself. Buy a cheap laser thermometer off amazon or harbor freight and see if its actually getting that hot. Or buy a aftermarket gauge and hook it up and see.

Did you replace the thermostat?

If it is truly swinging that much until the car has ran some I would suspect blown head gasket if you are sure you have all the air out of the system. Bad EGR system or exhaust leak could also complicate it.

Djhotenchilada

the thermostat was replaced with the radiator and everything else. I've even try tapping on the housing every couple of minutes trying to make sure it's not sticking. Also I'm pretty sure I replaced the temp sender because the one I replaced was in the side of the engine right beneath the exhaust manifold with a single wire connector going to it. The old one that was in it was like one out of a second gen that looks like a nail head. I put in one of the new blade types, cut the old connector end off and put a female connector on it.

8Mike9

Passenger side is for fan control...it'll be a switch basically, drivers side is for gauge..it's basically a variable resistor.

Seems as though you put a fan switch on the drivers side maybe?

Djhotenchilada

No nothing has been moved. I did figure out what happened today though so my problem is fixed. There was a cut in the vacuum line that connects to the heater control valve. Not sure exactly why but I replaced it and then bled the cooling system again and it's fine now. I'm thinking that the last time I bled it and had the heat on it couldn't properly get all of the air out because of the leak from the vacuum hose. It seems weird but after I replaced the hose I did the same bleeding technique as before and it's running perfectly. I drove it all around town today and it never even got to 220.

TAbracadabra

Glad to hear, Great Job!
Also be sure you are running with the underneath air dam/deflector otherwise it will get hot at highway speeds.

That was a real head scratcher
Thanks for posting your resolution !