No compression in one cylinder.. anything I can do before pulling head?

kerb12

Well-known member
So I finally put the top end of my engine back together (a '66 200 with a later model head) after pulling the head to get a crack in the intake manifold fixed. While the machine shop was fixing that they also milled it a bit (it had previously been milled as well). I had asked the machinist to try to get to 48cc's.

When I finally went to start it, it actually cranked over but it idled really poorly, so I ran a compression test and cylinders 1-4 and 6 were all 140-150... BUT cylinder 5 has zero compression. It doesn't even bump the gauge.

From what I've read, this seems to point to a valve stuck open since if the head gasket didn't seal properly it would most likely cause low compression in an adjacent cylinder too. Does that sound right? Anyone have any advice on how I could troubleshoot this a bit more before pulling the head again?

I took the valve cover off and when I crank it, both springs on that cylinder compress and pop back, but I'm not sure exactly what I'm looking for. Nothing stands out as looking really wrong compared to the other cylinders though.

I know some people have shimmed their rocker arms after milling, which I didn't do, but if the problem was that it's too close and not letting the valve come back up all the way, it would affect all of the valves not just for one cylinder right?
 
Either way you're going to pull the head. Might be a valve problem, might be ring lands gone on the #5 piston.
 
Sometimes when installing the rocker shaft the pushrod can get out of the lifter cup and ride at a higher location and hold the valves open ...Check that this is not your problem.
Shimming the stands is not the right way to fix too long pushrods, it will change the as designed rocker to valve geometry.
Your push rods should be fine, you milled the head but used a thicker gasket netting no real change that the lifter cannot handle.
Edit...I see that you have changed the cam and lifters from stock , it is possible that with those changes that the pushrod is to long now.
Try temporary shimming the stands to see if that helps ..Still check the pushrod out of proper location first.
 
Put air into the cylinder via the spark plug threads & listen for where the air escapes. Out the carb, intake valve, exhaust pipe exhaust valve, crankcase breather, piston or ring problem.
 
Thanks guys, I think this is my plan - First loosen the rocker arm and wiggle the pushrods around and see if they seat better, retighten, and compression test again. If that doesn't make a difference I'll try the air in the cylinder test... maybe also loosen the rocker arm again and redo the air test and see if the valves close when there's no pressure on them from the rockers.

I just want to make sure I exhaust all the easy tests before tearing everything down again...
 
wsa111":3qhws6yq said:
Put air into the cylinder via the spark plug threads & listen for where the air escapes. Out the carb, intake valve, exhaust pipe exhaust valve, crankcase breather, piston or ring problem.

That's the Numba One answer right there.
 
kerb12":8d03vk9z said:
Thanks guys, I think this is my plan - First loosen the rocker arm and wiggle the pushrods around and see if they seat better, retighten, and compression test again. If that doesn't make a difference I'll try the air in the cylinder test... maybe also loosen the rocker arm again and redo the air test and see if the valves close when there's no pressure on them from the rockers.

I just want to make sure I exhaust all the easy tests before tearing everything down again...

I think that's a good plan. But outside of some problem you can see, find and fix within the valve train, you're pulling the head when you know the problem lies within. You'll see right away what the problem is when you pull the head. There's really no point in testing to figure it out beforehand imo with zero compression and no decipherable problem under the valve cover. If you have a good borescope you could stick it in the spark plug hole and it may tell you something. You can do a leak down and if it's a valve it may ease your mind, but you're still pulling the head.
 
It may take more than wiggling the pushrods, you have to knowingly feel that it is in the cup...The reason I know about this is It happen to me and it was also #5...Your problem may be something else but it does happen.
 
Take off the rocker assy & do a cylinder air check. If the problem is fixed then you have a pushrod & or a rocker arm problem.
Do you have adjustable rocker arms?? If not by milling the head you have a pushrod in that cylinder that is too long.
 
Ok so here's what I just did... I loosened the rockers enough that I could move the pushrods around and made sure they were all sitting in the cups - no difference. Then I took the rockers off completely and put a straightedge across the tops of the valves and to my eyes they looked good, but I guess it could be off just slightly enough that I can't see it. Then I swapped pushrods from #5 (the bad cylinder) to #2 (which measured good) to try to rule out a bad pushrod. Torqued everything back down and #5 was still bad.

I tried the air test, but when I'm shooting air in it's pretty loud and I can't for the life of me hear if there is hissing from anywhere... but I got bad ears.

I guess my last try is to temporarily shim the rockers and see if it changes anything. What's a reasonable size? I have some hardened 1/8" washers leftover from my head studs. Is that too tall?
 
wsa111, I do not have adjustables, I have the stock unadjustable and stock pushrods.

By the way, the reason I'm avoiding just pulling the head is because I only have about 5k miles on the rebuilt engine, so unless something went really bad I can't imagine it's the rings/piston. And I just had a really reputable machinist rebuild the head before I put it on. I'm sure he could have made a mistake and not noticed a bad valve or seat, but I'm still pretty new to working on engines so I think it's far more likely that I just screwed something up on my end.
 
Did the engine run ok for the 5K??
Sorry about your hearing, but the proper way to put air into a cylinder is remove the check valve from your compression tester & put 100# plus into the cylinder.
Open the throttle to full & if its an intake you will hear it easily. Same with the exhaust & since you have the valve cover off you will easily hear a leak.
As a professional i could diagnosis your problem within 30 minutes.
You might need a second opinion.
Good luck.
 
Pull the valve cover and turn the engine over with a wrench, cw, until the you are sure both valves on #5 are closed. Like right after the exhaust valve on 5 closes. Then you can pressurize the cylinder for a leak down test like Bill is describing. If the test proves a major valve leak, then pull the rocker arm assembly and retest. If the problem goes away, you know there is something wrong with the valve train on #5. There are only two things I can think of(outside of a long pushrod) that would cause a valve to hang open on your engine, one would be if the pushrod is not in it's cup in the lifter, the other is a problem with that lifter causing it to somehow act like a solid lifter and have it's plunger stuck fully extended. You should definitely check out the valve train. Don't get me wrong, if you can avoid pulling the head you should. Nothing like taking a major step that was totally unneeded and find it was just the seating of the pushrod. You may be able to see with a flashlight if the pushrod is seated into the lifter properly though the little hole. Easy fix. Unfortunately, if it's a lifter problem you'll have to pull the head, but at least you will be pointed in the right direction in the unlikely event that that is the case. Do the leak down as described above, get some help if you need to and it will help guide you into the inevitable solution. Hopefully it's the pushrod. It's all fixable no matter what the problem turns out to be unless there's a hole in the cylinder wall ;) I'm joking of course.
 
Seth, it may well be a hardened exhaust valve seat coming loose in the head????
 
Yeah there's another one. If it's had seats installed. That's always kind of worried me about exhaust valve seats. Maybe it's irrational but it kept me from doing it. Well that and the added expense. Especially after my machinist said not to worry about it anyway b/c I didn't need hardened seats in his opinion. Heads gotta come off for that one to. I suppose he could have a bent valve as well. Maybe the pushrod wasn't in it's cup and it bent the valve stem?
 
Thanks for the more tips. I'm going to see if I can borrow an actual leak down tester from someone and try it with the rockers on and off. Will let you know what that shows.
 
"Can't think of(outside of a long pushrod)..."
bent PR?
 
If you bent a valve, pull the spark plug & shine a led light onto the piston top. You will see a mark or dent in the crown if so. I wish you the best.
 
Alright I have some updates. I was not able to get a leak down tester but I was able to get a hold of a mechanic smoke tester... I already had the rockers off, so I pumped some smoke in and did not see any smoke come out of the carb or exhaust. Good sign since they should be as closed as they can get. Just as a sanity check, I rotated the engine until one of the valves was open and tried again, and saw smoke come out of the carb. Then rotated until the other valve was open, and saw smoke come out of the exhaust... so far so good.

Put the rockers back on and rotated until both valves appeared to be as closed as I could get them, and... still no smoke out of either carb or exhaust? I pulled the oil dipstick and no smoke out of that either.

One thing I did notice was an odd gurgling sound but I couldn't tell where it was coming from.

Could it be that my head gasket didn't seal properly and that the leak is towards the cooling channels and my air is pushing into the coolant?
 
You can get a tester on loan from your local parts house for a test of a leak from the head to the cooling system. I think they call it a coolant system pressure tester or head gasket coolant pressure tester... something like that. Basically you screw it on in place of the radiator cap and follow the instructions. If the head gasket is leaking into the coolant it raises the pressure and I think there's a chemical test to that can be used while performing the test that's even more conclusive. There's no way that would cause a dead cylinder in a compression test. Or you'd already know the head gasket is a huge problem.

It sounds like maybe it was the pushrod. You tested it w/ the rockers off and all was good. Put the rockers back on and got nothing out of the crankcase. I've got my fingers cross for you.
 
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