Blown Head Gasket - DIY? what else to inspect/replace?

[David T]

We assume you checked all of the cylinders the same way. If you didn't do it "correctly" at least they should all be done the same so you can compare the results even if the absolute values are not accurate. By that logic cylinder # 6 has a problem. BTW, to do a "proper" compression test you start with a fully charged battery. Remove all of the spark plugs, turn off the fuel pump, and block open the throttle. Count the number of "bumps" and do it the same for each cylinder. Repeat the test after squirting in some oil into each cylinder and compare the results. A significant increase with oil is indicative of leaky rings. Further testing can involve a leak-down test to figure out exactly what is leaking excessively. A leak-down test is where you pressurize each cylinder at TDC and see where the air is escaping, through the rings and into the crankcase, through the intake valve into the intake manifold, or the exhaust valve into the exhaust. Besides being able to measure the size of the leak, you can actually hear the air leaking. Is the motor using oil? That is a clue to the rings leaking. Does it backfire? That would be a valve. Check the valve adjustments on that cylinder. All we can see from the plugs is that the motor was running very rich.

[JCochrane]

Sorry it's been a while since I posted back.

Low compression does NOT confirm a low cylinder, it only indicates further testing. Low compression could be a bad valve or a cracked ring. A coolant combustion gas tester is a definitive test for a blown head gasket. Either way it looks like that head is coming off!

So I finally put the plugs back in to run the coolant combustion gas tester. First of all, no white smoke when starting it up, warming it for test, or running it - curious. Test came back negative. Let it sit for a few hours, then warmed it back up for a while and ran the test again - still negative, and still no white smoke - or at least not a lot like before.

Oh yeah, and the test showed no apparent leaks either - bulb would stay compressed and not be able to draw more air through if I held tester tight to the container. I'd then loosen up a bit to release pressure and try again, but it never lost pressure if I kept it tight to the reservoir and pumped the bulb a few times.

For what it's worth, I'm using this kit: https://www.amazon.com/gp/product/B06VVBSFTF/

This isn't clear to me; are you saying you tested cylinder 6 with no o-ring on the compression tester?

Sorry, there was still an o-ring on the tester itself, but I had been using the deep-socket adapter, which I could screw in first (by hand at first, almost entirely) and then screw the compression tester into that. Once the o-ring was ruined on the adapter/extender, I just twisted the compression tester (rubber hose with metal plug end and o-ring, so still enough clearance, just not wrenchable at all) directly into the plug socket. I also repeated measurements on all other cylinders to make sure the new configuration was acceptable - that I was getting it tight enough and whatnot. Same readings, so I assumed it was sufficient.

We assume you checked all of the cylinders the same way. If you didn't do it "correctly" at least they should all be done the same so you can compare the results even if the absolute values are not accurate. By that logic cylinder # 6 has a problem.

Yep, rechecked the same way when I took the adapter out of the picture.

BTW, to do a "proper" compression test you start with a fully charged battery. Remove all of the spark plugs, turn off the fuel pump, and block open the throttle. Count the number of "bumps" and do it the same for each cylinder. Repeat the test after squirting in some oil into each cylinder and compare the results. A significant increase with oil is indicative of leaky rings.

The battery probably needs to be replaced, but had a charger on it prior to running the test, and after running the test, and I came back to some of them a while later (after recharging) just to ensure that the battery state also wasn't causing differences in measurements.

Is the motor using oil? That is a clue to the rings leaking.

Nope. No appreciable oil usage, at least in the little it's run recently.

Does it backfire? That would be a valve. Check the valve adjustments on that cylinder. All we can see from the plugs is that the motor was running very rich.

Nope. Doesn't run the smoothest, but purrs when idling and don't recall any backfires at all.


Should I drive it around town a bit and see how it does? Maybe the oil change fixed it, at least enough for now - obviously a cylinder measures low so there will be work at some point, but it may have been like that for years I think, based on my experience with the car and my dad's recollection and suppositions... I should also drain the super-old fuel fully and replace...

[David T]

If it is not using oil and there are no combustion gases in the coolant I suggest doing a valve adjustment. That just might get the compression back on that one cylinder. If it doesn't help, then try some STP or Marvel Mystery Oil, maybe a stuck ring or a leaky valve. A leak-down test will tell you what the problem is with that one cylinder. You might just get by without having to remove the head. It will never run as smooth as it should with that one cylinder so low.

[JCochrane]

If it is not using oil and there are no combustion gases in the coolant I suggest doing a valve adjustment. That just might get the compression back on that one cylinder.

Looks like that's not too hard, but still have to take off the head, so new valve cover gaskets & such, right?

If it doesn't help, then try some STP or Marvel Mystery Oil, maybe a stuck ring or a leaky valve.

STP/MMO is just pouring into spark plug hole and letting it sit 24 hr and checking that the level drops, right?
That seems like an easier/quicker check. Probably not the 'right' solution, or you'd have mentioned that first, but would it be unwise to start there and proceed to more-intrusive valve adjustment if it doesn't help?

A leak-down test will tell you what the problem is with that one cylinder. You might just get by without having to remove the head. It will never run as smooth as it should with that one cylinder so low.

Would I do the leak-down on just the one cylinder? Since the others seem to have good compression? If that may indicate the problem without having to remove the head, would that be a better place to start than just attempting the STP/MMO route?

[David T]

To check and, if necessary, adjust the valves, you just have to get the valve covers off. You slowly pour the STP or MMO in the intake while the motor is running. You can do a leak-down check on only one cylinder. I would check ALL of the valves, do that first. If you need a lot of adjustment on #6 then that may fix the low compression.