Leak Down #'s Opinions Wanted

[e30sh]

91 M42 302K miles

Compression test hot 1 to 4.

175 / 170 / 170 /175 psi

Leak Down Cold @ 100 psi

18% / 22% / 23% / 19%

2 and 3 have no bubbles in the coolant but the coolant level slowly rises and overflows out of the expansion tank when pressurized. 1 and 4 are normal.

While running with the valve cover vented to atmosphere and the throttle nipple capped, I'm getting some steam / blowby out of the valve cover.

Car runs well, doesn't overheat and the plugs look normal. It looses a few inches of coolant out of the expansion tank every 500 miles.

No oil in coolant or coolant in oil.

Crack / Profile gasket?? Car wasn't overheated. Do they normally crack in two cylinders? I assume the head gasket is OK based on above.

[e30s50dan]

I'm in the middle of a rebuilt,and had the same numbers.on the engine stand while doing the leak down test I had bubbles on both intake and exhaust valves ,while spraying carb cleaner.

Result...carbon build up!!

[e30sh]

Any theories on how the cooling system is getting pressurized only on #2 and 3? The profile gasket would show on all 4 I assume.

[jaredmac11]

It sounds like youre thinking it has a crack? Have you done the test to see if there's actually exhaust in the radiator?

https://www.amazon.com/Block-Tester-BT-500-Combustion-Leak/dp/B06VVBSFTF/ref=sr_1_4?ie=UTF8&qid=1520788172&sr=8-4&keywords=head+gasket+test

This is very comprehensive as well:

https://www.youtube.com/watch?v=uVn-IDi7da8

But are those numbers still within spec? 22-23% doesnt seem bad at all.



Have you ruled out coolant leaks due to the plastic pipe, bad hoses, etc? Have you tried the radiator leak tests?

[roguetoaster]

They normally crack between a valve seat and a coolant passage in cyls 2/3.

So, when the coolant overflows the level is way below cold when the car cools? Have you double checked that it's bled really well?

[e30sh]

Have not done a chemical test to the oil or coolant but they pass the visual test.

The coolant level will be at the line in the expansion tank. Pressurize the cylinder and in ~30 sec it will overflow with the cap off. Once pressure is removed from the cylinder, the coolant level does not go back down. Crank it up and drive it a bit and the level returns to normal.

It's been slowly loosing coolant for over a year. I just top it off and go. It doesn't get driven that much, ~4k miles / year.

Do they normally crack in two cylinders? I assume if #2 is cracked, # 3 will read normal or do the cooling passages tie together between 2 and 3 but not 2 and 1 etc.

The no bubbles thing is confusing. I had an M44 years ago with a warped head. It blew bubbles in the expansion tank with #2 pressurized.

[jaredmac11]

They normally crack between a valve seat and a coolant passage in cyls 2/3.

So, when the coolant overflows the level is way below cold when the car cools? Have you double checked that it's bled really well?

FWIW, I pulled a head with a crack at this exact location.

[e30sh]

Originally Posted by roguetoaster View Post
They normally crack between a valve seat and a coolant passage in cyls 2/3.


FWIW, I pulled a head with a crack at this exact location.

In both cylinders or just 1

[e30davie]

Unfortunately if you are getting any sort of issue between the cylinder and the cooling system, its time to pull the head off and investigate.

Cracks in heads is not the end of the world. But will require some rectification in the machine shop.

[jaredmac11]

In both cylinders or just 1

The crack was not between the two cylinders but was at the exhaust valve port at the surface of cylinder 3.

It was not between the 2 & 3. Just because cylinder 2 & 3 are low pressure, I wouldnt automatically state that the crack is between those two cylinders necessarily. Im not a mechanic but that test isn't end-all. Youd have to pull the head to verify.

The good news is, that would be the time to do a full refresh if you havent yet.

[TobyB]

the coolant level slowly rises and overflows out of the expansion tank when pressurized

Time to pull the head.

t

[roguetoaster]

In both cylinders or just 1

Usually only one cylinder area on the head exhibits a crack.

[TobyB]

...and it's not unusual for the head gasket to lift in the middle, so your bubbles make me
agree with everyone above, and guess that you'll find a failed head gasket
(and possibly warped, but not cracked, head)


t
prognosticating

[e30sh]

...and it's not unusual for the head gasket to lift in the middle, so your bubbles make me
agree with everyone above, and guess that you'll find a failed head gasket
(and possibly warped, but not cracked, head)


t
prognosticating

I had a M44 with a blown head gasket between 2 and 3....it blew bubbles in the rad while doing a leakdown. This M42 does not...just raises the cooling system level.

Since it was on 2 cylinders, I was hoping it was the profile gasket somehow.
Air gets past the rings into the block back up to the profile gasket. Cylinders 1 and 4 don't leak as much past the rings to allow air back to the profile gasket. That's why I was curious if they normally crack in 1 or 2 cylinders.