My car gets hot on the highway with the ac on, it will go up almost to 235, but I manually switch the fans on when I notice it getting hot and it cools down to 180 in about 5 minutes. The air dam is still there, the cooling system has all been replaced in the last few years and the coolant is nice and clean and about a 50/50 mix. I have the dual fan setup from an lt1 car in there which is why it cools down so quickly when they are switched on, this happens when the cruise is set at 70 mph. The only thing I don't have on the car is the air diffuser that goes behind the chin spoiler. My air dam does have a little bend to it, would that cause this?

 

Air dam has to be sloped forward (like a cow catcher on a train) in order to scoop the airflow into the rad. If it's bent the wrong way it will sigificantly reduce cooling capability.

The other issue that might cause highway overheating is a clogging of the fins in either the condensor or the rad. Careful on this one, they can seems clean by external inspection, but they can still be dirty once you get a look at them when they are separated.

 

Just put the motor in a couple months ago and the condenser was new then and I had the radiator out and washed it all off. I bet its like you were saying and the air dam is bent too much.

 

the air dam doesnt actually "scoop" air into the rad. it creates a low pressure area that literally sucks air up into the rad. being slightly bent wont make much of a difference. how do you have the fans wired in? are they run off the computer or a fan switch

 

The fans both turn on at the same time, one plugged right into the single fan connector, the other one I used a fan relay from a parts car and got power from battery with an auto reset circuit breaker and that is switched on from ground like the stock fan and is spliced into the stock fan control, my override switch just turns the fans on anytime. So the fans still come on whenever the computer wants, I just can turn them on sooner.

I'm learning how to tune the car with help of someone in my local car club, and were gonna set the fans to come on sooner so I will be removing the override switch for a nice clean looking interior.

 

was your car originally a 2 fan setup? or 1 fan?

 

if it was originally a 1 fan set up, the ecu doesnt control it at all. is controlled strickly by the fan switch in the head.

 

single fan. It is controlled by the ecu. Nothing in the head but the gauge sender on the drivers side. Unless someone before me converted it over, which I highly doubt since it looked all stock.

 

ok, i pulled the wiring diagram. 86 did have a single fan controlled by the ecu

 

it only over heats on the the highway with the fans off? fans on the highway shouldnt make a difference to the cooling since the air comming in is moving faster than the fans can blow it.

 

That is correct, its only when the ac is on and the condenser is throwing hot air on the rad. maybe I need a bigger radiator? It cools fine when the ac is off.

 

stock 2 fan setups kick the secondary fan on with the ac. single fan setups should also kick the fan on with the ac. do your fans turn on when you turn on the ac? if not you might wanna wire one of them in to come on with the ac.

 

After '87.

Ecm fan is turned off after 35mph from anything I've read. Not personally confirmed though.

 

On my car without the override switch on at night time, I can see the voltmeter drop when slowing down below 35mph. This is the same with the ac on or off.

 

Someone who knows the ins and outs of the PROM code would be more qualified to answer.
As as guess though, I would imagine the ECM would still shut the fan off regardless of A/C ('87-older). '88+, the same pressure switch that sends the A/C signal ground, also grounds the secondary fan relay. So, even if the primary turned off at 35, the secondary would continue to run (with A/C on). Again, this is just a guess, not fact and I could be quite wrong.

 

That would make sense why it gets hot if the secondary fan stays running no matter what speed the vehicle is traveling.