[JinKC]

Hey all, I could use some more brainpower as I'm officially out of ideas.

My early 64 mustang with the 260 is acting like its overheating but I'm not convinced that is what's actually going on.

Details:
1.The car will blow coolant out of the radiator cap right when the thermostat opens (and only at the moment the thermostat opens). Occurs roughly 10 minutes into the drive depending on the weather. Also will occur if I just let it idle in the driveway until it warms up.
2. Once the thermostat opens (and blows coolant everywhere), I can refill and continue driving without issue for at least an hour (haven't tried longer).
3. The car will not blow its coolant if I remove the thermostat (ran for ~month that way).
4. This behavior exists regardless of the outside temperature (winter/summer)
5. Temp gauge in the car never shows it getting hot (replaced temp sensor for good measure)
6. IR thermometer on the block doesn't show any obvious signs that its actually overheating

This issues started shortly after having some engine work done (cylinder bored, heads cleaned up, new valve seats; nothing crazy just a rebuild of the original engine as it was getting tired). Its acting like the block is getting slightly pressurized which then blows past the radiator cap once the thermostat opens. Head gasket maybe? Why does its seem to run fine with the thermostat removed? I've noticed my PCV valve fits a little loosely in the valve cover. Could a leak there be the culprit?

Anybody have any ideas?

Thanks,
JinKC

Simple things I've tried:
1 Thermostat has been replaced twice
2. I've personally verified that the thermostats opens at the right temp.
3. I've tried adding bleed holes in the thermostat to make sure I wasn't getting a vapor lock
4. I'm confident I have gotten all of the radiator/block after the coolant fill
5. I've pressure tested the cap (and replaced for good measure)
6. I've pressure tested each cylinder (no issues)
6. I've pressure tested the radiator (no issues with the radiator but found a small heater core leak (new one in route)

 

[6banger]

couple of questions. How much coolant does it blow out before it stops coming out? When it stops coming out, will it overheat then if no more added? How large of bleed hole did you put in the thermostat? I once had a chevy 6 that I had to put a 1/4 inch hole in to make it work right.

 

[CJM68GT390]

Are you overfilling the radiator? The coolant should be just above the radiator's tubes when cold so that there is room for the coolant to expand when it gets hot.

 

[Klutch]

4. I'm confident I have gotten all of the radiator/block after the coolant fill

In statement 4 above, are you saying you got all the "air" out of the radiator/block? If so, I would suggest burping the system just to be sure. I thought I had removed all the air from my Mustang cooling system. But I was having issues with it spitting coolant. I got the front of the car as high as I could, removed the radiator cap and ran it until the thermostat opened. When I looked into the radiator, I could see the coolant was filled with bubbles. It took a while, but eventually, those bubbles were gone. Now it doesn't spew coolant like it used to. And yeah, my coolant level is just barely above the cooling fins and that's where it's happy.

 

[JinKC]

Thanks for all the replies!
1. I've tried one-to three 1/8" diameter holes in thermostat. I haven't tried a larger diameter yet.
2. Once the coolant comes out I can drive without any issues - it does not seem to overheat at that point.
3. I fill it to just above the tubes as you've suggested CJM
4. I've had issues in the past with the system burping coolant after a coolant change. In the past, idling with the cap off fixed my issue as you suggested and it would be good until the next coolant change. This time that hasn't worked. With the cap off and idling, you will start to see some bubble as the engine works up. Then a huge rush of coolant (1/2gal+) will shoot out the open cap as soon as the thermostat opens. I've tried burping it several times with the front of the car slightly high (sloped driveway) but I haven't tried jacking up the front yet. I've even tried using a large spill-free funnel during the burp but the flood still shoots out.

I've had the car for nearly 29 years and haven't seen this issue before. To be sure, it did like to be burped after a coolant change but I've never had this much trouble. I've tried to burp it well over 10 times now and have gone through over 5 gals of coolant.

I'll try jacking the front up today and see if that helps. After that, I'll try bigger holes in the thermostat.