What happened?

[Wedgetail]

I replaced the ignition coil on my 40 Fordor Deluxe and took it out for a test drive. About 10 miles in, the engine started missing and running a little rough, and then it was leaking coolant and the temperature gage went up. When I got it home, there was coolant all over the engine compartment and the right wheelwell. After it cooled a while, I added a full gallon of water and started it again. It still sounds rough, but no coolant leaks out from anywhere. I can't tell where the coolant came from, and I don't know if the running rough is related. Any thoughts?

[CA Victoria]

Check the cap to see if the disc under the gasket is in good shape.

[Ggmac]

Did you confirm its run level or just fill it to the top ?

[Charlie ny]

Sounds like the coolant overflowed onto the coil and shorted things. A good wipe down and overnight sit and she'll be raring to go. Fingers crossed.
Charlie ny

[Wedgetail]

With the engine running and the radiator cap off, I can see that the radiator fins are covered by about a half inch

[rotorwrench]

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On an engine with no cooling system issues, it takes certain conditions to get them to overheat to the point the cap releases the excess pressure. Running at idle too long on a hot day can do it or running up a long steep hill. If it overheated under regular driving conditions then I would suspect a cooling system problem.

[Wedgetail]

I took the car for another short drive today. The radiator cap is fine, I think the coolant is coming out of the overflow tube, which at speed on the road gets spread around by the fan, making a mysterious mess. The engine got hot after a mile of driving, and I can see steam or smoke wafting out of the oil fill cap, which can't be good... I bought the car in the fall, so this is my first experience with warm weather driving, it is in the 70s.

[wga]

Driving it in the 70s is perfect weather.
Check the oil, if the color is milky brown, that
would indicate water in the oil.
Ignition timing set correctly?
Do you have a vacuum gauge to use to check engine operating conditions?
Is the cooling system clean?
Thermostats opening correctly?
Double check the gasket contact on the radiator cap to be sure that it is making
full contact in the radiator. Should be a 4 lb rating.

[flatjack9]

40 cooling system has no pressure cap. Dump the anti freeze and try running just water. Same thing used to happen to my 39.

[Wedgetail]

Great list, thanks WGA

The oil looks ok.
I just changed the ignition coil, so I will figure out how to check the timing, it was starting to miss, maybe something is off.
The vacuum gage is on the list, just rose to the top.
I'm going to replace the lower radiator hoses because they are old, mushy and cracked, so that will prompt some radiator flushing too. The coolant looks clean-ish with a bunch of rust that settles out in the bucket.
I have thermostats? Good to know.
The radiator cap looks in good shape, not sure how to check it, there is coolant blowing out the overflow tube at the top of the radiator. Does that have a pressure valve too?

Thanks again for the list, lots to look into!

[TJ]

Pushing coolant or water out the overflow tube tells me the radiator is partially plugged. have you ever taken the radiator out and had it tested or rodded out. If the hoses are old, cracked, and mushy I suspect it's been a long time since anything has been done with the cooling system. Just simple flushing may not solve your problem. If you just bought the car in the fall who knows how long it sat with junk accumulating in the cooling system.

[rotorwrench]

There could be a radiator tube crud build up or it could be a leaky head gasket (internal). A good look at the spark plugs may give an indication. A steam washed plug will be cleaner than it should be. The plugs should have signs of normal combustion. White colored water vapor from the exhaust pipe is a very good indication of coolant getting into the combustion chamber. Any sign of coolant leakage is a sign of a problem. Even sticky distributor spark advance can cause overheating. Water pump seals can leak but should be noticeable under close inspection.

Worst case would be a cracked head or block from a freeze crack or from an overheat crack in an exhaust valve pocket. Flatheads are vulnerable to cracking if extreme temperatures have an effect.

The pressure cap came along around 1941/42. If it just has a gasket and no thermal spring in the cap then it is still a non pressure system. Some replacement radiators were set up for a pressure cap back in the day. Skip has a pressure valve that can be installed on the overflow tube but the radiator may need some attention before going that route. It's hard to tell if the tubes are clogged on some radiators. Having them rodded out it the only way to know for sure.