Head gasket sealer recommendation needed

[Rootbeer Roadster]

I can drive my car for an hour without any issues, but if I go on a longer drive of 11/2 to 2 hours the overflow tank will fill up to the top and sometimes spay all over the engine. The tank is the plastic 2 quart type and is plenty big enough. I've messed around with the pressure testers, dye and even got the head gasket test kit. Originally I found a leak at the T filler with the pressure tester, but since then the dye and test kit had negative results. At this point I don't know what is wrong except for symptoms that won't go away. My lazy side says to skip the longer drives and keep on driving, but I've already done that for a year and driving season is here again. I'm considering buying some head gasket sealer and giving it a try, but I don't know what to buy. Do you have a recommendation?

[ryanmac]

Test or replace radiator cap. Could be leaking by into reservoir once up to full operating temp and pressure.

[Rootbeer Roadster]

I should mention that when it overflows there is air in the system.

[CraigS]

If you use a Ford OE or similar replacement radiator be sure to check the OE rad cap that is on the driver side top. I use a 20# cap there and 16# on the T filler. I tried a rubber plug but couldn't get it to stay in place. Also cap off the over flow nipple just below the pressure cap. I used a 2inch piece of hose clamped to the nipple w/ a bolt clamped in the end. If this is left uncapped and if the pressure cap there is the style that let's coolant suck back into the engine, it will suck in air w/o the nipple being capped off. If you decide to try a sealer remember that the car needs to be driven for one of your longer trips so the stuff dissolves and flows to where it needs to go. It has been too long to give you a specific recommendation but I have used sealers to good effect back in the day.

[Rootbeer Roadster]

I've replaced the cap a couple of times and will change the T filler and cap again. My confidence level is low which is why I want to try some sealer. Has anyone tried this stuff and can recommend a brand?

[NAZ]

You may very well have a combustion leak as some of the symptoms you describe are consistent with a leaking head gasket. But I would have expected it to show up on the combustion gas test (assuming that's what you did with that dye you mentioned). You can pour some chicken giggle concoction in the coolant system but that's not the proper way to repair a leaky head gasket. That said, I always carry some of that aluminum powder stop leak in my off-road vehicles and it has gotten me home a few times over the years. But a "mechanic in a can" is a desperate way to handle a repair. However, before taking any drastic action, is your system pressure (cap rating) adequate for the temperature you're running? You didn't mention the temperature the system was running at when the pressure relief valve (radiator cap) starts bleeding off pressure. You've done a good job of systematically troubleshooting the problem so far but coolant system temperature is also a good thing to check. You can find temp / pressure charts online that will tell you the boiling point at a particular pressure. If your temps are below the boiling point for the cap rating then the extra pressure in the system is likely from combustion gas leak.

[Rootbeer Roadster]

NAZ the thermostat is rated at 82C and the car runs between 81 to 84 all the time. During steady cruise it is steady at 82. It never gets any hotter even when it is overflowing the tank. Like I mentioned earlier I can drive for a little over an hour without any issue. The weird thing is the two times it has done it on long cruises I was able to drive another hour to get home after refilling and burping with my Lisle funnel. I believe I should test for exhaust gases again and probably will, but I wonder if I will need to do it after a very long cruise. The last time the results were negative, but I started the test on a cold engine and just warmed it up as the directions stated. I know that the sealers aren't the "right" way to go, but I'd like to give it a shot if someone can recommend a brand. The idea of taking the engine apart isn't that interesting to me. I did it the first time while the engine was on a stand. While it's not difficult the parts are heavy and leaning over the car to remove and replace them sounds like a good way to get a sore back for a week or so. If I go this way does anyone know of a good brand?

[NAZ]

At 84C (183F) you’re running ~46F below boiling assuming a 50% mixture of antifreeze and a 16 PSI cap. Even more if you have a 20 PSI cap or more than 50% mix. It’s not likely you are simply boiling coolant causing a pressure rise. A combustion leak has the potential to add a great deal more pressure than the cooling system can handle. That helps me confirm your fears about a head gasket leak. If you’re set on adding chicken giggle to the system I’ve heard good things about Blue Devil but haven’t used it myself. Good luck on solving your cooling issue. I'm sure you'll be back on the road soon.