Before I get into anything, let me list the things I've checked/done to the cooling system:
Upgraded rad to 2 core Champion unit
Replaced 195 thermo with 180 thermo
Replaced water pump with ACDelco unit

So, this all started a few months ago when the water pump blew out. I was driving in the neighborhoods when the pump started screaming and the temps shot up. I had to stop every 5 mins to keep from overheating and when I finally got home I replaced the pump. For about 1 week the car ran cool but then it started running hotter than I'd like it to (about 240 with ac on at idle) so I replaced the thermostat. Again, it ran at about 200 for a week and started running at 240 again. At this point, after replacing almost everything, checking for debris in front of the rad (there was none), and making sure there were no air pockets in the system, I assummed the radiator to be the cause and replaced it. Still, there was no improvement.

I am basically at wits end on how to get this thing under control. The fan works (I don't have an aux fan though), the pump moves water, there are 0 leaks (doesn't drip water, no traces of water in oil and no smoke from exhaust), there is no air in the system, and the rad is unobstructed. I read somewhere that the temp guage can malfunction and this seems to be the only thing I can think of now except that it doesn't act like anything is wrong with it.

 

running 240 is no big deal. your fans do not kick on till then. when you get to 240 are the fans coming on and the temps droping if so you are in fine shape

 

I have the fan wired to the ignition so it always runs. The temps never drop unless I'm driving.

 

MIght wanna presure check the system

Pullyour plugs namely 5and 7 see if they are "super clean"
Someof these had a habit of eating head gaskets

Fact you have a good cooling system and blewthe water pump out may be suspect of combustion pressure in your cooling system. Hope thats not the case happened to me though.

 

I'll check them. But the water pump was not in good shape at all. It had 105k miles on it and the impeller showed it. I'll pressure check it anyways though.

 

1. What is the coolant level in the overflow bottle when the engine is hot?

2. When the engine cools down, what is the coolant level in the radiator, and in the overflow bottle?

A blown head gasket can allow exhaust gases to enter the cooling system, and cause immediate overheating. If this has occurred, there will be no return siphon from the overflow bottle back to the radiator.
If the radiator is full after cooling down..... no head gasket problem.

 

Hi

I have come across this in the past, overheat the car once or twice is enough to screw the temperature sensor (between plugs 1 and 3)
The temperature would display a lot higher than actual, even in winter the temp would display 20 degrees higher than usual.
Put a new temp sensor in and that fixed it all back to normal.

Note check the oil temperature, it is usually 10 deg lower than the coolant temperature to confirm if the engine coolant temperature sender is faulty.

If i blow a hose or overheated it just once i replace the temperature sensor, they are cheap.

Another thing to check, if there is more than 50/50 coolant/water the engine temp will be affected.
Water wetter is something else to look into.

 

When the engine is warmed up and the cooling system is pressurized, the level goes up. When the car cools down the level returns to normal. When the car is cool I can squeeze the upper rad hose and hear the water moving so I know that it's taking water back in and the water level in the rad shows it.

The coolant is premixed 50/50 so it should still be 50/50/... right?

Since it seems cheap I'll try the temp sender first and see where that takes me. Also, I was thinking my trans may be the problem?

 

Yes, no problem there.

Remember, cooling is just air flow and coolant flow..... stop one, or both, and the vette overheats. Of course, a malfunctioning sensor can cause a faulty reading.

New radiator, tstat, pump, coolant, fans continually on (bad idea), should have solved the problem.

Do both digital and analog readings agree with the high temp?

 

I haven't taken an analog reading yet. The only temp reading I have is the one on the dash so I'm becoming skeptical of its accuracy.

 

The digital sensor is the more accurate of the two...... read the digital for comparison. A faulty digital sensor will set a code, a faulty analog sensor/sender will not set a code.

 

No analog display on an '86 - it's all Digital.

Compare the Display to the Coolant Temp Sensor reading with a Scanner. While observing the Scanner, make sure the fan comes on at 226 to 228 degrees. With an a/c request showing on the Scan and a Manifold Gage Set hooked up, Fan should come on with 220 to 230 psi on the High Side.

This Year only needs the 1 Fan and with a/c, it should be below the threshold for non a/c Fan Operation because the Fan is going to come on the minute the liquid it's making in the Condenser hits about 120 to 140 degrees (instantaneously) and keep running until it drops to just above 100. That's well below the thermostat which will maintain adequate coolant temp but around town, you'll see 205 maybe 212 with a clean system - and that means both the a/c and coolant. Highway should be at the thermostat.

Assuming your Coolant Temp Display is accurate (no more than 5 degrees from the Coolant Temp Sensor), I'd guess wrong Pump or damaged shroud - or (if it were me) the last thing I did is the reason the next thing is broken - or there's an internal problem so checking compression isn't a bad idea.

 

Ok, I replaced the sender and it's still hot. I noticed that I can get the temps under control when the AC is off. Should I get the AC checked?

 

I have also noticed that the trans cooler lines that go to the radiator are particularly hot. Would a trans be causing this issue?

 

It should be cooler, so assuming a new Radiator and the shroud is in good shape, I'd check it out. Condenser temp is well below the Thermostat - that's why the Condenser is in front - so if it's plugged up or overcharged or the Fan isn't controlling it's pressure/temperature, it will overheat.