Troubleshooting overheating

[Hack]

My '70 is overheating this spring.

It hasn't done it in the past. It happens mostly idling in park, not when driving.

The only thing I've changed recently is some adjustment of the carb. I have to admit I hate carbs and I stink at getting them right. I got a book on how to do it and the car ran like crap when adjusted per the book (idled way too fast and the electric choke made the idle way too high).

Last year I improved it by looking at a friend's carb and matching the adjustments. The carb is a Holley 600. It had been very rich and I think I got it to run quite a bit leaner. Also, the electric choke was still set too high. Towards the fall I turned the choke down too low and had problems with cold driveability.

This spring I adjusted the choke back up a little.

I have no idea if it's the carb or just the radiator cap happened to go bad this spring. The cap starts leaking gradually and it makes a humming or squeaking noise as it lets fluid past. The cap is not very old, maybe two years.

I've got idiot gages only so I'm not sure how hot the car is actually getting. I don't think it's getting very hot as it just pushes some liquid out, not huge clouds of steam or anything.

Any thoughts?

[waterbug1999]

Doee it have electrical fan or the clutch fan?

[Hack]

It's a 302 from a '91 Mustang and I left the plastic clutch fan on it. I haven't made any changes to the fan.

[waterbug1999]

Well, the clutch could be going out on it... Might be why it overheats standing still. I had one do that on my old cheby back in the day. Just a thought.

[xjfish]

Replace the radiator cap. Should be cheap.

[Hack]

Well, the clutch could be going out on it... Might be why it overheats standing still. I had one do that on my old cheby back in the day. Just a thought.

Makes sense. I should at least check the clutch.

[dupcla]

Can you hear/see the fan running at idle?

[xjfish]

I'm sure the fan is spinning.. Check that there is resistance when turning the fan by hand, it shouldn't freewheel. If the rad cap isn't holding pressure, you're going to have cooling issues.

[Fernanernie]

I'm sure the fan is spinning.. Check that there is resistance when turning the fan by hand, it shouldn't freewheel. If the rad cap isn't holding pressure, you're going to have cooling issues.

X2

[Hack]

Can you hear/see the fan running at idle?

Yes, it spins at idle.

[Max Power]

It spins at idle, but with the clutch engaged, it will spin at engine speed instead of freewheeling.

I guess the thing you are worried about is what is happening now that wasn't happening last year when it didn't overheat with the same set up.

Carb adjustments shouldn't effect temp at idle, so you are down to the cooling system itself and the mechanical components. The fan seems like a likely culprit for failure. You could test it with a non-clutch style of fan to verify, or just replace it. Your car shouldn't overheat even with the cap off at idle, but you should get a good cap anyway.

Often the shroud is the problem in a set up like yours where a swap occurred, but since it didn't happen last year, that's probably not it. Likewise you probably have enough radiator as it didn't overheat last year either.

I would suggest a new fan, a new stat, and a new cap. If it still overheats, I would guess maybe head gasket or something internal.

Did you use the original 6 radiator or upgrade at the time of the swap?

[dupcla]

If the engine is real lean it will overheat, but that doesn't sound likely from what you've said.

I'd start with a cap and slowly spend more money. If you want the fan clutch get a new one but I'd say remove it and deal with the lost mileage/noise. I doubt the fan needs to be replaced but if it is the original fan it may not be sized correctly. Get a good mech fan from a v-8 car/truck.

[Hack]

Thanks for the help!

I kind of cheated, since I had some parts laying around. I checked and found that the clutch fan from my old '83 fit inside a shroud I also had previously bought. I changed the fan and put the shroud in the car.

The cap was the problem, though. After swapping the fan and putting the shroud in, the cap was still weeping just slightly after shutting down or sitting idling in gear for too long. The cap that was on the car was a 7 pound unit. I went to the parts store and their stock replacement was a 13 pound cap. I replaced the cap and put quite a few miles on the car tonight with no problems.

I still have no idea why it ran fine last year, but oh well.

[dupcla]

The seals on the caps go bad when they sit. Coolant is corrosive.

[Pauly]

Springs get weak you may have been pushing fluid before and didn't realize it.