Over flowing radiator

[Shug4]

When I first start up my m38a1 antifreeze pours out the overflow tube it also flows out the top of my radiator when I stop engine and undo the cap. This happens when antifreeze is cold. What causes this?

[Ben Dover]

Is it a 4# CAP Does it have a rubber seal on the valve or is it the early type that is used with the fiber seal in the filler neck?

[artificer]

First the coolant should not be filled right to the top but about 2 knuckles under the bottom of the filler neck.
Leave the cap off & startup if there are bubbles it is indicative of a blown head gasket.
If so, check by doing a compression test 'properly'.

[Ben Dover]

M-38A1 is usually not that touchy with correct cap.
Does your engine still have the bypass elbow between the head and the waterpump?
Again which cap do you have?

[Shug4]

Will check on the bypass elbow.

[Shug4]

It looks like I might have a head gasket blown because when I start it up it blows grey smoke out the exhaust. Also I have the cap with the rubber seat in it. I think it is smaller than the neck.

[artificer]

To work as intended radiator caps must be the correct length for the rubber or composite material to seat in the radiator neck.
Blown head gasket will show on a properly conducted compression test or bubbles will show up in the radiator on a leak down test isolating 1 or more cylinders.
Smoke can be indicative of many things & grey is not usually associated with head gasket, white is.

[Ben Dover]

Can you post a pic of the type radiator cap you are using? If the valve on the cap has the rubber seal, there is no ned for the fibre seal in the neck. Steam upon start up is not a good sign. Does it stop after a few seconds?
Evidence of coolant in a cylinder is indicated by a spark plug with little or no deposits when compared to the others, also, rust may be on the tip of the spark plug in the affected cylinder.
Removing the spark plugs will give you a quick answer.

[Bosage]

My 52 over heated today while heading to an event. Below is a picture of the radiator cap (all three parts) and I’m wondering if this is correct. When we replaced the temperature sender I watched the radiator while checking to see if it was working. No bubbles and it I could tell when the thermostat opened as it started circulating. The bypass hose from the pump to the block is in place and I observe no leaking anywhere. When over heating steam and fluid as escaping around the cap. I’m thinking the cap is my problem but wanted some expert advice too. Thanks!

[artificer]

The cap is definitely kaput & if you get a new one it may all be in one piece with a neoprene seal for the radiator neck so the fibre gasket which also looks kaput needs to be removed.
That doesn't mean that there is nothing else wrong but you won't know until the cap is replaced.

[Ben Dover]

Get rid of that old cap and remove the gasket from the filler neck gasket and go to your local Parts Store and buy one of these brands, AC-RC-5, GATES 31307, NAPA 4-1144, STANT R7, . These are all modern 4Lb. caps (Made for the M-38A1 Jeep that have the rubber seal made to the valve on the cap Proper fill level for the radiator is 1" below the radiator neck.(This is slightly different than the level than on MB/M-38.

[Bosage]

Just a quick update and thank you for the responses. I put a new cap on and appear to have the problem corrected. We've had temps in the low 90's with very high humidity. Drove it at highway speed (55) for several miles and back. Temp gauge went to 220 but when I got back to town, lower speeds and rpms, it dropped to the 190 range. No leaks or overflowing. Thanks again!

[Ben Dover]

Your M-38A1 left the fadtory for world wide service with a +160F Thermostat.

[Bosage]

Thanks - I’ve either got the wrong one or it’s bad.

[artificer]

I’ve either got the wrong one or it’s bad.

The thermostat in this case has nothing to do with this Jeep's higher than optimal operating temperature. So don't waste your money & time. Concentrate on what the real issue/s are, not red herrings!

The bad & replaced radiator cap solved only one of the 'cooling' problems.
Remember if the engine is not boiling & loosing coolant, the engine is not overheating, BUT....

Here is how, why & what is next:
220°F is above boiling if the cap is not holding pressure. That is why coolant was spewed out, lost then boiling continued & the cycle was ongoing/exacerbated.
220°F is below boiling when a 4# cap is fitted & working properly.
So no additional coolant is lost once the correct radiator coolant level is established, after expansion.

Even in 100°F ambient temperatures, 220°F is just a bit more than we need, really close to the limit [224°F] & is caused by less than optimal coolant flow [restricted] through the radiator core.
Absolutely nothing to do with running the vehicle under load with either a 160° or 180°F thermostat

Suggest the OP needs to check the radiator flow & most likely needs it cleaned of scale &/or accumulated debris. Rats nest etc. Also he needs to ensure the radiator shroud is in place, as this can contribute to 220°F temperatures, if not.