Leave spark plugs out, pressure overnight, then crank engine in the morning. You have a steam cleaned spark plug, excessive cooling system pressure, bubbles in the coolant etc. So pretty sure head needs to come off and you already have a good idea which cylinder to look at. Double check the instructions for the block tester and try again, maybe test it at the tail pipe to see if it works. You will feel better when you see the fluid change color and/or coolant sprays out of the sparkplug hole.

To me, all this does sound like a head problem, and I'll guess that somehow the coolant gases test is being done wrong. Are you pulling a vacuum to suck the gases through the test fluid?

Applying cylinder pressure and having coolant come out of the tank is pretty definitive.

You probably can't assume it's a head gasket only and not a cracked or warped head. Even the youtube 50sKid guy almost got burned by that - his machine shop found cracks in his head that he had missed, even after his careful video production.

A good machine shop should be able to clean, pressure test, and surface a head for about $150. That's what it cost me out here in CA, which is not known for being cheap.

Thanks. I’ve repeated the block test multiple times (squeeze the little bulb and pull fumes through the blue fluid) and yes if I suck exhaust from the tailpipe it does turn yellow. I had a hard time believing it myself since when I open the tank it seems to smell like exhaust. I know, it’s odd.

Maybe the test measures a gas that doesn't happen if it's running super rich or lean or such. Like mentioned above. It could be a broken head so take a step at a time.

Maybe but it does react at the tail pipe. And, the car runs fine except for the pressure in the cooling system.

Some engines have a path from intake manifold to the coolant. Not sure how that could add pressure to coolant since the intake pressure is less than coolant pressure. That would explain why no combustion gas in the coolant though.

If somehow was the intake gasket that is a lot less heavy duty than the head gasket

I make the Block Tester combustion leak test tool mentioned higher up in the thread and have a few additional thoughts/ideas to share.

First, our tool is very accurate provided that you get NO coolant up into the tester when performing the test. Coolant (even a tiny amount) mixed with the test fluid will tend to pull the color back towards blue (i.e. a false negative result if you do have a combustion leak). If you think the fluid might have been contaminated with coolant during the test, it's a simple check - after you do the test at the expansion tank, take the tester with the same fluid to the area around the tailpipe and 'sniff' the air surrounding the tailpipe through the tester while the engine is still idling. If it turns yellow, you know you didn't get a false negative while testing at the cooling system.

The other thing to note not mentioned in the instructions is that you'll want to drive the vehicle under load for 20 minutes or so at some point before doing the test if the coolant is new. The reason is that liquids (coolant/water included) can dissolve a certain amount of gases. If you have a combustion leak and run a test immediately after changing out your coolant, it's possible to get a false negative result since the fresh coolant could be absorbing (dissolving) all the combustion gases that are being introduced into the cooling system by way of the combustion leak. It's only when the coolant is saturated that the additional combustion gases bubble off to the high point of the cooling system allowing us to detect them during the test.

SO, with that in mind - the big question is, 'how can I be getting a negative Block Test result while still blowing out coolant in my expansion tank?'

If you ran the Block Test correctly with the two points above in mind and still got a negative result, I don't think the head gasket is the culprit. What we've seen in similar cases is that a cooling system that isn't bled properly can cause hot spots in the head. This produces localized boiling of the coolant and pressure build-up in the system without telltale combustion gases from a blown head gasket. The air pockets in the system cause further cooling woes as it interrupts coolant flow and causes cavitation at the water pump impeller.

What is a little puzzling about your description is that it sounds like the pressure build-up remains after the vehicle cools down. I may not have read that right, though.

Also, you didn't mention any current overheating - does the vehicle overheat when driving under load currently? If so, does it happen under all conditions (idle, stop & go, highway speeds)?

I'd really hate to see you tear into a full head gasket replacement if you don't need to!

Oh yeah, and the fuel mixture won't impact the Block Test - as long as there's combustion, the fluid will detect it.

A point above about blockage and localized boiling; it's why I asked about the coolant mix; if there was enough water displacing coolant, that would improve the chances, however if you put shop air into a spark plug hole and air comes out the coolant reservoir, clearly there is a pathway. So confusing is the test not showing combustion gas though!


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Thanks all.

Ok, about the block test.

1. Coolant is new as a result of changing the reservoir, Waterpump, thermostat etc.
2. Always bled the coolant system (remember on my fourth coolant tank very familiar bleeding)
3. I have done the block test enough to be on my second retail bottle. I have once or twice contaminated the fluid by drawing in coolant but cleaned it out and re-did. Never turns yellow although there was a time of two that the fluid was a duller blue it seems.
4. More than once after testing I put it up to the tailpipe and it did turn yellow then.
5. One time immediately after the coolant tank cracked I removed the cap and there was what looked like smoke and smelled like rotten eggs was coming out. Smelled like combustion. I sealed it, gotmy block test and drew some of it in. Nothing, no change.
6. The car does not generate excessive heat. Heater functions fine also. Does not get driven more than a few miles due to pressure but it will idle for good long time and no overheat.
7. The pressure hold over night if I drive it. Even two days later. Under load the upper and lower hoses get very tight, and next day hoses will still be 90% same tight.
8 One thing I experimented with was to remove a cup or two or three from the coolant tank and it would drive ok and the hoses would go soft within a few hrs as it should. But, my low coolant light was always on. I suspect the reason the hoses went soft was a hairline crack in the tank.
9. Lastly, I now suspect some sort of stop leak was used on this car. There is white residue around the transmission cooler, and in the skid plate.
10. My CCV at the valvecover is gummed up with “mayo.” But not seeing coolant in the oil.

Not sure what I can add but it’s raining all day, I’m all ears.