when a/c is on car overheats

[Xenostalgia]

battle battle

[instantkevin]

ok people.. this is what it sounds like to me.

the thermostat is not the problem if the car does not overheat without the A/C. if it was stuck or faulty then it would overheat regardless of the A/C.

I doubt the bad radiator cap. the caps do not hold pressure indefintely. the caps are meant to hold pressure to typically 12-16 lbs. when the coolant gets too hot, it exceeds this pressure which causes the valve to open in the cap, allowing the resovoir to be filled with hot/boiling antifreeze. the caps are designed to release the pressure this way to prevent a sudden blow-out. this is not necessarly the cause of the problem, just an effect of the problem.

you still didn't say what temp exactly the engine reaches, nor have you said whether the fan turns on with the A/C running.

[CamaroRS92]

The radiator should be actually at a cooler temp. with the A/C "on" since the fan is designed to run continuously in that mode to keep your condensor coil cool. othewise the refrigerant would be excessively hot and will reduce the cooling capicity of your A/C.
By this i am thinking that maybe your engine coolant system is already marginal and just getting by, and the small amount of extra heat generated from the condensor is enough to shove it over the edge.

Things to check:
1) be sure the condensor coil is clean. inside and out side surfaces. to do this you will have to remove the top support from the radiator and gently pull it back . Spray aluminim fin cleaner thru all surfaces of the radiator & condensor. (available at a refrigeration supply house) Then flush with clean water Spraying from the inside surface out (opposite of normal air flow) this will remove any film on the fins and increase the cooling capacity of both.
2) vacuum any debris out from between radiator and condensor. Leaves bugs dirt and a few things that will surprize you get sucked in there. (do this at the same time you are doing step 1
2-b) straighten any bent fins in the condensor and radiator
3) check your coolant should be 50/50 mix of water / antifreeze too much water will lower your boiling point. too much antifreeze and your heater wont get very hot in winter.
4) check relay on fan and check ground wire. be sure the fan operates with A/C "off" at the proper temp. first, then turn A/C "on" and check to be sure the fan runs continuously
5) check the fan motor itself. - Some of these eventhough they are running, do not spin at the correct RPM's. This is due to age and wear, and worn out brushes in the motor.
6) check inside the radiator for calcium deposits and corrosion. if you have been adding hard water these deposits will clog up the radiator . you may need to run some super flush/cleaner in the system to hopefully get rid of this buildup. If it is severe you may need to replace the radiator
Hope some of this helps!

[Viper_Storm]

Originally Posted by CamaroRS92

The radiator should be actually at a cooler temp. with the A/C "on" since the fan is designed to run continuously in that mode to keep your condensor coil cool. othewise the refrigerant would be excessively hot and will reduce the cooling capicity of your A/C.
By this i am thinking that maybe your engine coolant system is already marginal and just getting by, and the small amount of extra heat generated from the condensor is enough to shove it over the edge.

Things to check:
1) be sure the condensor coil is clean. inside and out side surfaces. to do this you will have to remove the top support from the radiator and gently pull it back . Spray aluminim fin cleaner thru all surfaces of the radiator & condensor. (available at a refrigeration supply house) Then flush with clean water Spraying from the inside surface out (opposite of normal air flow) this will remove any film on the fins and increase the cooling capacity of both.
2) vacuum any debris out from between radiator and condensor. Leaves bugs dirt and a few things that will surprize you get sucked in there. (do this at the same time you are doing step 1
2-b) straighten any bent fins in the condensor and radiator
3) check your coolant should be 50/50 mix of water / antifreeze too much water will lower your boiling point. too much antifreeze and your heater wont get very hot in winter.
4) check relay on fan and check ground wire. be sure the fan operates with A/C "off" at the proper temp. first, then turn A/C "on" and check to be sure the fan runs continuously
5) check the fan motor itself. - Some of these eventhough they are running, do not spin at the correct RPM's. This is due to age and wear, and worn out brushes in the motor.
6) check inside the radiator for calcium deposits and corrosion. if you have been adding hard water these deposits will clog up the radiator . you may need to run some super flush/cleaner in the system to hopefully get rid of this buildup. If it is severe you may need to replace the radiator
Hope some of this helps!

lol, thats a big list there, i hope after that dude does all that his car stops over heating.

btw how do you striaghten bent fins?

[CamaroRS92]

Actually there is a special fin comb made to straighten core fins (also available at a refrigeration supply house). bent fins = less air flow and less cooling capicity.
I have witnessed several who would have bet the farm on saying their radiator was unobstructed , but when the top cover was removed and the radiator pulled back there was a good 6" of debris in there. 6" of radiator that doesnt get air flow on a hot day is a lot.

[Topliss92RS]

I have a 92 RS 305 TBI with the same problem - replaced the radiator, radiator cap, and thermostat. The problem has gotten consistantly worse. I had the cooling system flushed, as well. I can see water moving in the radiator, so I know that the water pump is fine. The temp will peg my gauge, so we are talking well above 240 here.

As a workaround, I have wired my fan into the fusebox so it runs all the time with the ignition on as my car is my daily driver. I really don't like doing something like that, but it works for now, if that helps.

As for fixing it, any of you cooling experts ever run into a blocked ware passage in the block or some crazy thing like that? really the only mod I've done to the car (so far) is to change the stock 2.73 gears to 3.73. Can this be an ECU issue after that change? Now I cannot even run the air on the highway or I overheat within a few minutes. I have been battling this problem for almost a year now and would really appreciate any input.

[89IROC&RS]

did you ever change the speedo gears to account for the difference in rear gears? other than that it could be air in the block, or if the freeze plugs have ever been worked on someone may have just hammered the old ones into the block which clogs the passages and can cause over heating. only way to check is pull the freeze plugs.

[Topliss92RS]

Originally Posted by 89IROC&RS

did you ever change the speedo gears to account for the difference in rear gears? other than that it could be air in the block, or if the freeze plugs have ever been worked on someone may have just hammered the old ones into the block which clogs the passages and can cause over heating. only way to check is pull the freeze plugs.

Thanks for your reply - sorry was offline for a bit. Speedo gears not changed yet, on my to do soon list. Do you think that could be causing a problem? Now that you mention it, the problem really got bad after the gear swap. The car idles cool, but really overheats at highway speed so that seems logical. Had the system flushed at a radiator shop just in case i was doing something wrong, so air should not be a problem. The block is sadly still stock, so I'm sure that the freeze plugs should not be an issue.