Pressure in radiator after engine is totally cool

[GRTfast]

Hello all,

I've noticed that when I take my radiator cap off a day or two after driving, there is still some fairly significant pressure in the system, enough to make a popping noise when the cap releases. I haven't noticed this before. Aside from this issue, everything seems to be normal.

How I discovered it:

The other day my carburetor guy was over at the house tuning my freshly modded 850 DP (annular boosters, screw in IAB's, etc..). I noticed a little bit of water dribbling out of the overflow tank (that has never happened before) when the water temp got to about 185 as the car was sitting and idling. The water temp has never been over 200. For the rest of the day when we were working on it, it worked fine, cycling between 180 and 195 degrees as the electric fan kicked on and off. I took it on a long test drive after we were done setting the idle circuit and everything was fine (no overheating, no funny smells, no overflow out of the reservoir). The next morning (engine completely cooled) I went out and opened the radiator cap and there was a good amount of pressure. coolant level was normal (about an inch down under the neck) where it always is. The next day I went for a 30-40 mile drive and came home. Coolant level in the overflow tank was normal, temps were normal (180-195) and it ran great. The next morning, I opened the cap and had the same pressure. Motor has about 10k miles on it, and has fresh Felpro 1027 head gaskets as of 1k miles ago (had to have the valve guides replaced). Heads were skim cut before I put it together.

I suspect a minor head gasket leak, allowing combustion gases into the cooling system, but I cant understand how it is holding pressure overnight if there is a gasket leak. I ordered a coolant testing kit from amazon that will indicate if there are exhaust gases in the coolant. I will perform that test when the kit arrives tomorrow.

My build is as follows:

Mark IV 454 0.030" over
Brodix BB2 rectangular port heads (312cc intake runners)
Forged Pistons 9.5:1
Comp cams Thumpr hydraulic roller (0.547" lift, 227 @0.050" intake, 241@0.050" exhaust duration (advertised 283, 303), 107 LSA)
RPM air gap intake
850 double pumper holley with 1" phenolic spacer
1 7/8 header primaries 4-1 headers with slip on side pipe mufflers.
Eagle forged crank and H beam rods
Meziere 55 gpm electric water pump
AFCO racing downflow aluminum radiator

Thoughts?

[mag2555]

No 2 ways around it!
If your coolant is down to room temp then there should be no pressure in the system!
Have you confirmed that the hoses are still pumped up, as a popping sound could be Vacuum also?
My money's on a head gasket issue and hopefully not a crack somewhere or leaky valve seat .

[GRTfast]

yeah, the upper hose is pumped up. It's just odd to me that it seems to be a one way leak, because it's holding pressure for hours on end. I guess if the leak is only happening under combustion chamber pressure, then the head gasket could be acting as a one way valve, allowing pressure into the cooling system, but not back into the cylinder.

[GRTfast]

Assuming it is a head gasket leak, what gaskets would you guys recommend I use when I fix it? I'm not having a warm and fuzzy with the 1027's if they only lasted 1000 miles on a fresh install with recently decked block and freshly cut sealing surfaces on the heads.

Also, assuming there is a leak from the combustion chamber into the cooling system, how can I diagnose if it's the head gaskets or a crack of some sort?

[gramps]

Had similar pressure problem, it ended up being the water additive we were using in race motor. Tested for compression gases in water, even pulled the top end to check gaskets and cracks, found nothing, put everything back together filled coolant system with plain water no more pressure after cool down. We think we added distilled water to system which was a no no with this additive.

[GRTfast]

Had similar pressure problem, it ended up being the water additive we were using in race motor. Tested for compression gases in water, even pulled the top end to check gaskets and cracks, found nothing, put everything back together filled coolant system with plain water no more pressure after cool down. We think we added distilled water to system which was a no no with this additive.

hmm interesting.. I used a different additive (Hyp-er lube supercoolant http://www.hyperlube.com/c3/super-coolant-c8.html) this time because they didn't have any redline water wetter at my local store. I didn't use distilled water though, just regular bottled drinking water. I wonder...

If my combustion gas test doesn't reveal anything, maybe that is the issue.

[mag2555]

I can't see how anything you add to any type of coolant that does not make it hot or make it expand can produce and pressure rise in a system once the motors at room temp, or hold pressure up once at room temp!

Freshly cut block decks and heads decks are meaningless if they are not mirror Baby's Ass smooth, the ruffer they ( many say they grip the Gasket better) are the faster heat up and cool down session with Alimunum heads will tare / saw a gasket up and make for leaks when cool , if not while running!

[GRTfast]

I can't see how anything you add to any type of coolant that does not make it hot or make it expand can produce and pressure rise in a system once the motors at room temp, or hold pressure up once at room temp!

Freshly cut block decks and heads decks are meaningless if they are not mirror Baby's Ass smooth, the ruffer they ( many say they grip the Gasket better) are the faster heat up and cool down session with Alimunum heads will tare / saw a gasket up and make for leaks when cool , if not while running!

I tend to agree, unless the additive is reacting with something in the water or the cooling passage metal and creating a gas as a product of the reaction. I know it's a long shot, but it is theoretically possible. If you've ever put drano and aluminum foil in a plastic water bottle and closed the lid, you will know what I'm talking about. (Don't try this by the way, it makes a small bomb!!)

[mag2555]

Your issue sounds like the pressure when cool is due to a cylinder on the compression stroke while both valves are closed being able to pumping into the system only when cold!
If the issue was when hot and running you would be blasting the Rad cap open !

[Keith Morganstein]

I can't say you don't have something going on, but I've seen this many times on perfectly healthy engines/cooling systems. This happens a lot on larger diesel systems. The engine may not have run for weeks, take of the cap and you may get an unfortunate coolant bath. Cars too, esp if they have a coolant recovery tank.

[GRTfast]

I can't say you don't have something going on, but I've seen this many times on perfectly healthy engines/cooling systems. This happens a lot on larger diesel systems. The engine may not have run for weeks, take of the cap and you may get an unfortunate coolant bath. Cars too, esp if they have a coolant recovery tank.

Yeah it's weird. If the coolant test shows no combustion gasses, I plan to drive it and keep an eye on everything. If it's a leaky gasket or a crack somewhere, I'm sure it will eventually reveal itself in a more spectacular fashion.

[mag2555]

Agreed!

[GRTfast]

If I need to replace the head gaskets, what would you guys recommend (Mark IV block, Brodix aluminum heads)? I'm using Felpro 1027's now.

[mark_follweiler]

I can't say you don't have something going on, but I've seen this many times on perfectly healthy engines/cooling systems. This happens a lot on larger diesel systems. The engine may not have run for weeks, take of the cap and you may get an unfortunate coolant bath. Cars too, esp if they have a coolant recovery tank.

come on guys....Keith is correct. this is VERY common! there is nothing wrong with pressure, even after several days.

[Schurkey]

coolant level was normal (about an inch down under the neck) where it always is.

Why is the coolant an inch down instead of full?

All the air should be blown out of the radiator into the overflow bottle when the system heats up and expands. When the system cools, only coolant should be "vacuumed" into the radiator. The radiator should be "full" with no air.