Rebuilt engine overheating

The 289 V-8 of my 66 Mustang was rebuilt and now runs very hot. I have difficulty starting it whether cold or hot - any suggestions?

If you start it cold and drive away immediately how long before it begins to overheat? Maybe it only overheats idling. Maybe only when driving at high speed. Have you checked the coolant? Is the fan belt tight? GIVE ME A HINT.

Remove the spark plugs. Now try turning the engine over by hand at the crank bolt. If the engine is hard to turn over, something went wrong during the rebuild.


You’re kidding right? Any answer from put coolant in it to ask for your money back on the rebuild fits for the overheating.

Any answer from get a new battery to ask for your money back on the rebuild works for the “difficulty starting”

How much have you run the engine since it was rebuilt? It sounds as though the engine is tight. I don’t know if the engine needs more time to break in or if there is a problem with the rebuild. I assume that this Mustang is not a daily driver. I do remember this would happen with engines that were overhauled in the old days (meaning the 1940’s through the 1960’s) and the engines would need a little time to loosen up.

I know nothing about 1966 Mustangs. I had two GM car motors rebuilt over the years. Something like 1978 and 1986 Pontiac Bonneville wagon, big ones, with V-8 motors.

Both overheated when I got them back. The same thing fixed both of them.

Those old Pontiacs had silicon operated fan blade clutches in them. In both cases the clutch was reused, and in both cases replacing the clutch with a new one totally eliminated the heating problem

Here is my theory. I believe when they rebuilt the engine, they naturally took out the fan blade assembly and laid it down. And, just enough silicon oil leaked out to render it inoperable. Perhaps because it was worn, I don’t know.

I have no idea what a '66 Mustang has, but look at the fan blade. If it has a silicon operated clutch, and they re-used it, replace it, don’t try to guess as to whether it is bad or not, because you probably can’t tell.

If it is direct drive, that is not your problem, forget I said anything.

Rule out some bad stuff and get a kit that identifies the presence of hydracarbons in the coolant. Can’t think of to many things that will heat a car up quicker that exhaust gasses contacting the coolant.

How about the head gaskets being put on the wrong sides? Restrictors in the front…if wrong…no flow.

coolant and belt are good - it overheats while driving at any speed.

thanks, we’ll try this.

I’ve probably drive it about 1,000 miles - here in snow country, I’m a “fair-weather” driver of the car. Thanks for the encouragement that we may just need more break-in time.

we’ll look at this - thanks.

we’ll look at this, too - thanks

and this - thanks!

Flipped head gaskets was my first thought as well because I have seen that done before, thankfully not by me. The down side is that checking this involves a lot of work and new head gaskets.

How is it determined the engine is running very hot? By an original gauge or new aftermarket gauge, boiling over, running hot at idle or highway speeds, or both, etc, etc.?

There’s not enough info provided to make much of a guess at all but you state it’s hard to start hot or cold. This could even be interpreted to mean the starter motor or starter solenoid is failing.

If the engine is legitimately overheating then your next step should be removing the spark plugs and running a compression test. The results of that test should tell you whether it’s back to the drawing board or not; which means rip the engine back apart.

If the ignition timing was way off, it would be hard to start and overheat. It may also be that the thermostat was installed upside down or something else was overlooked in the final reassembly.