Overheating problem

My son has a Mustang GT 5.0 that is overheating. I replaced the radiator with a heavy duty 3 core radiator. It had underdrive pulleys so I replaced them with stock pulleys. I put dual electric fans on it. I flushed the block and put a new thermostat and replaced all the hoses (made sure the bottom radiator hose had wire in it so it wouldn’t collapse). I put a new water pump and drive belt but it still overheats when sitting in traffic and overheats when driving with the A/C on. Anyone have any suggestions

When it overheats do the fans come on? If they do, have your radiator fluid tested for the presence of exhaust gasses. If yes, blown head gasket or worse.

What year is the Mustang?
How many miles?
Has it been modified?
When you put the new fans on, did you also update the shroud?
Have you verified that the fans are operating, that the temp sensors aren’t bad?

Post the answers. If the engine has been modified, be as specific as you can.

Oh yeah, and by all means pick up a lab test kit and check for presence of hydrocarbons in the coolant. It’s easy, affordable, and if there are nothing you’ve done will help. Post the results.

Water pump? And the only other thing that hasn’t been mentioned in a long time is if the water pump is going in the right direction.

Excellent thought, bing. It’s also possible that the impellars are eroded.

OP put in new water pump

Might still have rust in the block and heads, even though he flushed it

Bad cap?

Might even have an air pocket. The other day at work, after replacing a leaky water pump, the truck ran hot in my stall. After getting rid of the air pocket, everything was good again

I’ve encountered a few aftermarket thermostats which were the sole cause of overheating

Really Stupid Question: What’s in the radiator? At least once a year, I run across somebody with similar problems who recently flushed and refilled their cooling system with just antifreeze instead of a 50/50 mix of antifreeze and water. Antifreeze is good for three things; keeping the cooling system from corroding, keeping the coolant from freezing in cold weather, and slightly elevating the boiling temp of the water to reduce boil-over in hot weather. But antifreeze is TERRIBLE at carrying heat away from the motor; run straight antifreeze, and even with the heaviest-duty radiator out there, it’s going to overheat in any conditions warmer than Prudhoe Bay.

Are you certain that both fans are moving air the correct direction?

Good point by Evil Wrenchman.

Rod Knox might be on to something here. My son installed a heavy duty cooling fan in his diesel GM truck a couple of years ago and it continued to overheat. While troubleshooting the problem…I noticed a curious wind blowing on my trouser leg. A quick reversal of the electrical leads to the new fan got it going in the right direction.

Thanks for all the responses. The car is a 1987, it has 89000 miles (found it on a farm in a barn and checked vin # and miles are right), it is stock not modified, factory exhaust manifolds, fans came with new shroud and both are working and blowing right direction. The water pump is new (not eroded) and turning right direction. Put new cap on when put radiator in and bled air from system, running 50/50 mix, and the thermostat is a stant (checked and is opening and closing). When I flushed I pulled everything off I could to make sure there was no rust. I got a test kit for exhaust gas and will test tomorrow. The one thing I didn’t mention the heater core leaks and the hoses are blocked off (didn’t think this would matter).

OK did the test for exhaust gas in radiator and it was negative. I’m replacing the temp sending unit this weekend. Does anyone have any other suggestions.

you did connect the inlet and outlet from the pump and motor with a short hose when you disconnected heater right?

"The one thing I didn’t mention the heater core leaks and the hoses are blocked off (didn’t think this would matter). "

This could very easily be your problem. BYPASS the heater, don’t block the hoses. Connect the heater hoses to each other to allow the coolant to flow.

It might be worthwhile to get an infra red temperature sensor and check the heat flow.