This may be common knowledge, but I worked out a way to test the headgasket failure on a recently aquired '95 4Runner with the 3.slow engine (3.0 V6) which ought to work on any vehicle. I have been trying to find a cylinder leakdown tester in town but half the parts stores (Napa, Checker) have no idea what a leakdown test is (much less the tool), or they don't have it in stock (Advanced Auto). The leakdown test appears to be the preferred test method for diagnosing a blown head gasket. I ran a compression test, but it isn't necessarily going to show what's wrong other than compression isn't right. Soooooo, I stared at my compression tester (from sears, about $20), stared at my 2 gallon mini air compressor from Menard's, and realized I could put the two together! The best part is, I didn't even need to buy anything more. I figured I'd share this since most people probably don't have a leakdown tester, but many people probably do have an air compressor, and a compression gauge is much easier to find in stock and cheaper ($20 vs. $75+) than a leakdown gauge. I wasn't feeling too good about paying $75+ for a tool I'd only be able to use for specific tests. A person can just about buy a 1-2 gallon air compressor and a compression gauge for $75, and there will be many other uses for them (at least for the air compressor).

My main goal was to confirm if I could pressurize the cylinder with air and push air back into the cooling system via the suspected cylinder. I figured I had a blown headgasket and didn't really care what leakdown rate was, I just wanted to know that it leaked. Any air escaping the cylinder into the cooling system = very bad, right?

The gauge on my compression tester unhooks via standard air tool quick disconnect fittings, so I took the gauge off and just used the hose to hook up the air compressor. I made sure the cylinder was at top dead center (to close the valves so air can't escape back through the intake or exhaust system). The tank on the compressor was already filled up to about 10PSI, so I didn't turn the compressor on. I hooked it up as-is, and immediately I heard glug glug glug glug gluuuuuuuuuuuug through the radiator (cap removed).

I then turned the compressor on and let it run until the tank was up to 25PSI, then I shut the power off and just let it drain. The line pressure never got above 15, and the tank drained within about a minute in a half. I could hear water rushing through the intake, coolant tubes, and glug glug glugging out the radiator. I think I can call that a blown headgasket. Because of how little air volume exists inside the cylinder, especially at TDC, I can't see any reason to need to buy a huge 60 gallon $$$$ compressor for this type of thing.

Please note that an actual leakdown gauge would certainly be better, especially if your headgasket is only barely leaking. I figured mine leaked, I just wanted to make absolutely sure. The 4Runner blew about 1 gallon of coolant out the overflow reservoir and actually blew the cap right off. There's a terrible miss, theoil cap has milkshake residue on it (oil still looks good though), and there is copious white clouds of smoke coming out the exhaust within about 3 seconds of starting up a cold engine (no, I wasn't stupid and ran it very much after the gasket blew--and it went fast). The suspect cylinder would collect water droplets on the end of the compression tester and give compression readings anywhere from 160 to 230PSI, while all other cylinders were around 165 and just got oil on the end of the compression tester hose/fitting. Oh, and I used a mini wand LED light and was able to stick it into the spark plug holes to see the tops of each piston. The suspect cylinder had a cleaner piston than the others. Lastly, the spark plug from this cylinder was cleaner than the others, and turned a but rust colored (likely from water sitting on it and causing rust stains on the metal).

I'm no expert, but I figure ANY air getting pushed into the cooling system from the cylinder is VERY BAD. If you try this yourself, I wouldn't recommend going over 80-100PSI. That's the range I've read on most websites that give basic instructions on doing leakdown tests. I didn't have to go anywhere near this high. My pseudo-leakdown test also won't be super helpful for small leaks or diagnosing between intake or exhaust valve leaks or ring leaks. In those cases, a real leakdown tool with an incoming line pressure gauge and a cylinder pressure gauge would be best, to tell you how much pressure is being lost. Again, I just wanted to confirm either air was getting into the cooling system or it wasn't. Also, make sure the cylinder is at top dead center, or you'll be losing air out the intake or exhaust valves accidentally, and the cylinder won't pressurize enough to test the cooling system. If you do try this, make sure all spark plugs are removed. Also, I've read that higher PSI such as 80-100 will rotate the engine (even quite fast), so you may want to put a breaker bar on the crank bolt and fasten it to the frame or something. At 25 PSI, the engine never even thought about rotating. Lastly, make sure the battery is disconnected before doing any of this (standard precaution for most any kind of work).

I know the 80's have their issues with headgaskets, but man, I really HATE the 3.0L V6's. I've owned 6 4Runners/pickups with this engine, and 4 of them blew headgaskets. Two of them had already been on their second set. And despite a special service campaign (aka recall) on most 1990-1995 V6's, Toyota says my '95 isn't covered (bastards). The 3.0L is notorious for blowing the gaskets at the rear cylinders and the theory is because there is a cross-over exhaust pipe pumping a lot of heat into those cylinders. Combine that with the fact toyota was forced to stop using asbestos in their gaskets in 1988 (first year of the 3.0L), they had some poor gaskets.

If my procedure is a little flawed, please feel free to chime in. It was so easy though, I feel dumb for not figuring this out years ago.

Oh, one last thought. If you use a compression gauge and it's like mine, it will have a schrader valve at the very end where the spark plug hole fitting is, and another schrader release valve up by the gauge itself. My air compressor hooked up to the end of the guage's rubber hose where the gauge itself went, and the air must flow backwards through this connection to the spark plug fitting connection and into the cylinder. The schrader valve that is found at the spark plug hole fitting should be removed since it's not meant to leat air flow backwards through the hose stolen from the compression gauge. Leave the other schrader valve in (the one by the gauge itself), because you don't want air escaping out before it even gets to the cylinder. You can also skip the compression gauge assembly and make your own hose assembly assuming you have hose, a fitting to go into the spark plug hole (napa), and a quick release air line fitting to connect to your air compressor hose. I basically just stole the rubber hose assembly from my compression gauge assembly, but you can make one too. If you thought you could thread it in without this short length of hose, you could get a spark plug hole fitting and a screw in schrader valve and hook your air compressor line right up to that. My air compressor hose is that curly yellow stuff, so I'd never be able to get it that close to the spark plug hole.