Compression Problem Revisited - Update - See p2

[Pilotdave]

I have a couple of questions about a compression issue that I hope you guys can help me with.

1. Our '30 Tudor has low compression in one cylinder, okay in the others: 50-55 in #1, 3, 4 and 15 in #2. I had previously found a valve problem and attempted to correct it. After replacing both valves and the head gasket, compression was no better.

Yesterday I performed a leak down test. With 50 psi on #2, I could hear air rushing out #3 spark plug hole. With the plug in place, I could hear air at the tail pipe. My diagnosis is that neither valve in #2 is seating properly.... does this make sense?

2. I did encounter one challenge performing the test. The engine tried to rotate every time I pressurized the cylinder even though we had the car in gear, e-brake on, and wheels chocked. Finally got it to hold still so we could run the test on #2. However, when I tried to run the test on #3 I couldn't prevent the engine from rotating - enough to move the distributor rotor about half way back to #2 terminal. I could hear air slowly leaking from #3 when this happened - is that because a valve had started to open? And is there a way to prevent the engine from rotating when doing this test?

Thanks for your help!

[James Rogers]

If you have leaking from one cylinder to another, you have a gasket problem or a warped head. The gap between 2 and 3 is such that only a warped head or very bad gasket would cause that kind of leakage. The reason you had the motor trying to turn is the seal on #3 is better than #2.
Another way to check this is to hold your thumb over the #3 spark plug hole and turn the engine with the starter. You should hear air escaping from the #2 spark plug hole if this is the problem.
The problem is not in the valves.

[glenn in camino]

Maybe a broken compression ring but I don't know how to test for that.

[Bob Johnson]

The method I use to keep the engine from rotating during a leak down test is to use the hand crank. I put in the hand crank and the use a 4X4 wedged between the end of the hand crank and the ground. This prevents the engine from turning. Make sure that the piston for the cylinder you are testing is just BEFORE top dead center. The hand crank should be set in the 9 o'clock position (passengers side).

Bob

[columbiA]

Get#2 cyl a couple degrees before TDC on comp stroke & stick a bar in crank pulley spoke & rest other end of bar on wood block at the right height on rh side.Having piston in that position should stop bar from flying around the other way.When you put the air to the cyl,have a valve that you can open slowly until cyl is up to pressure.If a valve is leaking,you will hear air coming out tail pipe or carb.If a bad ring/s or scored cyl,you should hear air blowing out filler pipe.If leaking into cooling system, you should see bubbles in rad.

[Pilotdave]

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Thanks for the ideas for preventing engine rotation. I can make one those work!

I was not getting air bubbles in the radiator. The air coming out of #3 spark plug hole was loud enough that I couldn't hear any air leaking from the carb. I did have the throttle wide open when doing the test.

I'm a bit perplexed by a warped head/leaking gasket being the cause. Wouldn't the compression be low in BOTH #2 and #3 if air is leaking from one cylinder to the other? I have 50+ psi in #3 and 15 in #2. I don't know very much about this and am looking to get educated.

My theory about valve issues in #2 was based on the assumption that the intake valve for #3 could be open while air is leaking out of #2 through an improperly seated intake valve. This was based on review of the valve clearance setting instructions in Les Andrews' book which shows both valves for #2 closed while intake for #3 is open. Am I misreading this information?

Thanks again!

[RichPA]

Leak down tests should be done with the piston at top dead center at the top of the compression stroke for that cylinder.The rotor should be pointing at that plug wire terminal, if not turn it one revolution. The valves will be nowhere near opening and exact TDC will prevent the air from pushing the piston down.Since cylinder bores wear at the top this is the best position to see if the rings leak.

[Tom Wesenberg]

"My theory about valve issues in #2 was based on the assumption that the intake valve for #3 could be open while air is leaking out of #2 through an improperly seated intake valve. This was based on review of the valve clearance setting instructions in Les Andrews' book which shows both valves for #2 closed while intake for #3 is open. Am I misreading this information?"

I think you could be onto something here!

[VWJoe]

Originally Posted by Pilotdave

Thanks for the ideas for preventing engine rotation. I can make one those work!

I was not getting air bubbles in the radiator. The air coming out of #3 spark plug hole was loud enough that I couldn't hear any air leaking from the carb. I did have the throttle wide open when doing the test.

I'm a bit perplexed by a warped head/leaking gasket being the cause. Wouldn't the compression be low in BOTH #2 and #3 if air is leaking from one cylinder to the other? I have 50+ psi in #3 and 15 in #2. I don't know very much about this and am looking to get educated.

My theory about valve issues in #2 was based on the assumption that the intake valve for #3 could be open while air is leaking out of #2 through an improperly seated intake valve. This was based on review of the valve clearance setting instructions in Les Andrews' book which shows both valves for #2 closed while intake for #3 is open. Am I misreading this information?

Thanks again!

50+psi on a cyl leakdown test is way to excessive. I assume you are not using a true cyl. leakdown tester with a built-in regulator. 10 to 15 psi is more than enough to show a potential problem and less likley to move the piston or blow the radiator apart.

Joe,,,

[RichPA]

Originally Posted by RichPA

Leak down tests should be done with the piston at top dead center at the top of the compression stroke for that cylinder.The rotor should be pointing at that plug wire terminal, if not turn it one revolution. The valves will be nowhere near opening and exact TDC will prevent the air from pushing the piston down.Since cylinder bores wear at the top this is the best position to see if the rings leak.

Doing this will eliminate any guesswork. If air escapes from the carb,muffler,oil fill cap,radiator cap or an adjacent cylinder thats where the leak is.It is possible for a head gasket to leak in one direction, acting like a check valve.

[Pilotdave]

Thanks, guys.

I will retry the test with lower pressure - had assumed (we know where this gets one!) that I needed to have test pressure greater than compression. I am planning to remove the intake/exhaust manifold and retest. If I get air out #2 intake port and not out of #3 spark hole, this would tell me that the problem is #2 intake valve. On the other hand, if it's still leaking from #3 spark hole and not from #2 intake port, it has to be head or head gasket..... yes?

[James Rogers]

I could be wrong before and probably am now. It is hard to diagnose problems from 1000 miles away with a sometimes vague description.

[James Rogers]

Originally Posted by Pilotdave

Thanks, guys.

I will retry the test with lower pressure - had assumed (we know where this gets one!) that I needed to have test pressure greater than compression. I am planning to remove the intake/exhaust manifold and retest. If I get air out #2 intake port and not out of #3 spark hole, this would tell me that the problem is #2 intake valve. On the other hand, if it's still leaking from #3 spark hole and not from #2 intake port, it has to be head or head gasket..... yes?

That's probably the best way to pinpoint the offending cylinder. Eliminate the conduit between the two so the transfer of air can only be by the gasket or out only one port. Either way you probably will need a new gasket, tho you may get lucky.

[Kurt in NJ]

If the crankshaft moves when you put air in the cyl you didn't have the piston at top dead center ---as it moves valves open making the test faulty, use a bent piece of stiff wire to feel the piston position through the spark plug hole.

With 15 lbs in #2 if it is a ring problem you will have a big puff out the breather every 2 crankshaft revolutions.

With #2 on compression both valves on #3 will be open a little, if you open the throttle and the "wind" coming out of #3 changes you probably have a intake valve leak on #2. ---there is alot of distance between #2,and#3, and given that you have good comp on #3& #1 I doubt that you have a headgasket problem.

[1931 flamingo]

When you initially repaired the valves and repl'd head gasket, what exactly did you do to the valves and what head gasket did you use?? Did you re-torque head gasket a few times?? Have you tried re-torqueing the head now??
Paul in CT

[RonC]

Dave what are the new compression readings after you replaced the valves?

[RonC]

Originally Posted by Pilotdave

I'm a bit perplexed by a warped head/leaking gasket being the cause. Wouldn't the compression be low in BOTH #2 and #3 if air is leaking from one cylinder to the other? I have 50+ psi in #3 and 15 in #2. I don't know very much about this and am looking to get educated.

Yes, the compression numbers you get are affected by the compressed cylinder volume too. If you had 100% leak proof cylinders but the compressed cylinder volumes varied from one cylinder to the next, as IS ALWAYS the case, even a little, your compression numbers will vary.

So IF you have leakage between cylinders your compressed volume would increase (in additon to leakage out the adjacent cylinder valves that are open) and consequently lower your reading for that cylinder. This is why a real leak down test is more accurate at determining true cylinder leakage.

Since you removed and replaced your head you need to rerun the compression test.

[Pilotdave]

Wow - you all have been busy while I've been off-line! Let me try to respond to the questions and comments:

1. I replaced the head gasket when I installed the two new valves. Used a copper-clad gasket for standard head. Also used a gasket sealer designed for this type of gasket.

2. When I replaced the valves, I lapped them in lightly using a hand-held suction cup. Saw a bright silver line around full circumference of each valve and called it done. I did not replace the valve guides. Set valve clearances at ~ 0.013-0.015.

3. Compression readings after installing new valves: #1, #3, #4 all at 50-55 psi. #2 at 15 psi. Same as before replacing the valves.

4. I re-torqued the head at least once, likely twice. I honestly did not keep track. I can re-torque it now and see if that makes a difference.

5. On #2 I did use a piece of stiff wire to try to get to TDC. Brought engine around until wire stopped coming up - the piston was essentially not moving up further even though I was still rotating the crank a bit.

I hope no one feels that I am rejecting their advice - it's way too easy to leave out the one bit of information that makes a difference in describing a situation! And sometimes the gaps in my knowledge may make it sound as though I'm not taking advice. It'd be wonderful for me - and a lot simpler for you - if a couple of you fellows could be right with me in the shop and not many miles away!! Anyway, thanks much for the help thus far.

[V4F]

have you tried the "wet" test to see if the rings are truly the problem ? wet test is with a little oil in the cylinder to seal the rings if possible ............ ive never had a crank move . i have 100# / cyl ..........

[Pilotdave]

Steve, at one point I tried putting a teaspoon or two of oil into #2 through the spark hole. That didn't make any difference in the compression result.