Mustang 5.0 overheats at 1,500 RPM+ but not at idle

PICKUPTRCK

MEMBER
1950 FORD MUSTANG

I just got this car and don't know the service history. The 1990 Mustang 5.0 V8, 76,000 miles can idle in my driveway and never overheat, but when driving 1/3 mile at 15-25MPH to the store, I'm at redline if I don't let it coast periodically. I'm too scared to see how it acts at higher speed. I've replaced the water pump, a 180 degree thermostat, 16lbs pressure-relief radiator cap and a new engine drive belt. The hoses seem fine, and what I can see of the head gaskets, they look near new. Every bolt-on performance part is already on it including pulleys and a lopey cam (holley statemmax II is printed on the intake & AFR is on the heads). It does leak about 2 cups of coolant every drive. The upper coolant hose is hot, the lower is barely warm to the touch. The radiator fan is an on-off switch wired model. It's always on just to help. Where do I begin? I've heard of blocked water jackets. How do I check for that?

1 Reply

MCKNIGHT77

MEMBER
13 POSTS

The differential in temp between the radiator hoses indicates that there's insufficient flow through the radiator. I recommend you take the radiator to a radiator shop and have it flow tested.

Or, since the engine had been modified, spring for a bigger aftermarket radiator to take care of the bigger cooling capacity requirements of the upgraded engine.

Another thing to check is if the engine has underdrive pulleys on it. They will slow down the rotation of the water pump. Great for power, poor for cooling.

Hope this helps.

Jim