Different Overheating Issue than Most

[90gmcmoneypit]

I have a 1986 Ciera with GM's 4tech engine. Recently, I noticed that the temp gauge (which is part of the digital instrument cluster) will sparratically, and under no particular driving conditions, jump way up to hot, then after a moment, go back down to normal. This happens VERY rarely, and I know that either the fan or fan relay is bad, and have yet to replace it. The radiator fan does not kick on at all, but normally, the car runs so cool that it probably wouldn't kick on, even if it could. I've checked the coolant temp manually immediately after this happened the last time, and it was at 180 degrees, so it wasn't running hot.

I'm wondering if there is a sending unit in the fan relay that may be telling the computer to kick up the temp gauge in response to the fan not kicking on, regardless of the coolant temp. I'm planning on replacing the fan relay and if that doesn't work, obviously the fan, itself, but in the meantime, I'm curious as to if this issue can be attributed to the fan problem, or if there is another problem in the works. The head gaskets seem fine, no oil in the coolant or vice versa. Water pump is new, thermo is new, rad cap is new.

Anyone?

[Ian Szgatti]

Hey there, check your coolant temp sensor, and the wiring. There is a reference voltage going to the sensor that should read five volts. To find it with a multimeter, disconnect the wiring to the sensor and connect positive lead to a terminal, the negative to ground.. probe both connectors until you find the reference voltage.
If that isn't in spec, trace the wire back the the PCM.. For the sensor itself, check the resistance across the sensor terminals.. heat the sensor to a known tempreture and compare to resistance at that temp.
If you want a real easy way, buy a new sensor since they are so damn cheap anyway and swap out the old one. Cant hurt anyway. This sensor can also prevent your fan from comng on... although you seem to know your relay is bad. Anyway, you should replace the relay and the sensor anyway, so that you have a nice new cooling configuration. Post your findings please and thanks!

Oh by the way, test the wiring going to the fan, and then obviously test the fan before replacing it. Check the fan by measuring resistance across the terminals and comparing to specs. An even easier way is to connect a jumper wire to the fan from the battery to see if it will spin.. just dont chop yourself, cuz i expect the fan will work.

[90gmcmoneypit]

Well, I've replaced the fan relay, still overheats. Went ahead and played it safe, replaced the thermo again, still overheats.

I ran it without a thermo in it at all, and it didn't overheat once. As soon as I put the thermo back in, it's not overheating, but the temp still jumps to hot, then back down to cold again.

I know that it's a good idea to bleed the cooling system, and I'm thinking that maybe that's the problem, however, there's no freakin bleeder valve (at least if there is, I sure as fire can't find it) either on the thermo housing or otherwise.

If anyone has any insight, PLEASE share. I'm dying over here trying to figure this out. At best, just to know where the bleeder valve is so I can give that a whirl would make me happier than a pig in slop.

[Ian Szgatti]

Originally Posted by 90gmcmoneypit

Well, I've replaced the fan relay, still overheats. Went ahead and played it safe, replaced the thermo again, still overheats.

I ran it without a thermo in it at all, and it didn't overheat once. As soon as I put the thermo back in, it's not overheating, but the temp still jumps to hot, then back down to cold again.

I know that it's a good idea to bleed the cooling system, and I'm thinking that maybe that's the problem, however, there's no freakin bleeder valve (at least if there is, I sure as fire can't find it) either on the thermo housing or otherwise.

If anyone has any insight, PLEASE share. I'm dying over here trying to figure this out. At best, just to know where the bleeder valve is so I can give that a whirl would make me happier than a pig in slop.

Top up the resiviour, and take the rad cap off. do this with a cold engine. Make sure its looking full in there. Run the motor hot until you see coolant come out, you want to allow the air to escape in this way if possible. Using the throttle plate, rev the engine.... let the throttle off a bit and you'll see you'll be able to get the cap back on

[90gmcmoneypit]

Bah, head gasket.

I noticed the coolant beginning to burn rapidly. Nothing in the oil, no oil in the coolant. However, I'm attributing the fluxuating temps to steam pockets in the cooling system, caused by emissions leaking through the head gasket. The old gasket looked pretty bad.

That, and when I gassed the engine, the overflow would bubble with emissions. So, the head is off, the headaches have begun, but hopefully this will remedy the issue.