View Full Version : Cylinder pressure test unsuccessful-missing something?

06-26-05, 11:46 PM
This evening I tried air pressure testing my cylinders to verify a suspected head gasket failure. Bbob says this was supporsed to be simple, easy and cheap. I built a air fitting with pressure gauge as per his instructions and went in search of TDC. We tried everything, but never got a single cylinder to anymore than 10 lbs. despite having the compressor set at 120 psi. We would find TDC using a marked dowel, but not knowing if it was exhaust or intake, we'd try to pressurize it and if nothing, we'd undo the air fitting and turn the same cylinder to the top again. Still nothing. We did this on several cylinders over the course of three hours and finally gave up. What are we doing wrong?!?!? I sure would like to confirm a headgasket problem before committing to this expensive repair. Thanks in advance for all your good advice. Larry

06-27-05, 12:15 PM
Something is wrong here. If you are at TDC, you are either on the compression or the exhaust stroke. If you are on the exhaust stroke, you should be able to hear air hissing at the exhaust pipe as the exhaust valves will be open. Could you have been a few degrees off TDC and had a valve cracked open just a bit? Try getting it to where it is holding 10 psi again, then rotate the engine just a degree or two one way or the other. You should hear it pressurize pretty quickly when the valves are closed. That is the only explanation I can come up with.

06-27-05, 01:34 PM
If you are sure you are at TDC on the compression stroke and that you only get 10 psi in the cylinder, then something else is wrong.

Air is going somewhere. Is your test fitting leaking? Can you plug the fitting and get 120psi on it?

06-27-05, 02:21 PM
If you did what you said with the engine then something must be wrong with your "test equipment"..... If you turn on the air with the fitting lying on the floor (not in the engine) I assume air will escape from the end of the adapter you made in copious amounts...??? If not, then there is a restriction/blockage in the fittings somewhere.....

If you put the fitting in and turn on the air you should easily hear air escaping into either the intake or exhaust if the valves are still open....i.e..not at TDC firing. If you are at TDC then it is either TDC exhaust (in which case both the exhaust valves and intake valves are likely cracked open) or TDC compression.

You are putting the piston at TDC, right, with the dowel..?? The dowel should move upwards (BTDC) further and futher out of the hole until it stops (TDC) and then starts to move downward (ATDC). When it stops going down and starts to move upward you have found BDC. Are you finding BDC by mistake...??

There is always going to be a little air pressure loss in the cylinder due to ring leakage and valve leakage. If the air supply to the adapter is restricted then the leakage could be greater than the supply volume and cause the severe pressure drop. Ditch the regulator and any associated plumbing. Pipe/plumb/hose directly from the air supply to the adapter to make sure that there is no restriction. You just need "about" 120 PSI to the cylinder. No need for a regulator or anything on most shop air systems....unless your "air supply" is above 500 PSI I wouldn't think you would need a regulator. Possibly the regulator is a very low capacity regulator and causes a sufficient restriction to cause the pressure drop.

06-27-05, 04:00 PM
What size air compressor are you using? A very small air compressor will supply 120 psi but win not supply the volume of air needed to over come normal static cylinder compression leakage. In other words your air compressor may be too small to complete the test. Like bbob said it sounds like the problem may be with your testing equipment.

06-27-05, 07:00 PM
Yeah, at first read it does seem to be the equipment. Not knowing just how you made it is a problem for everyone trying to help. If you have a fair amount of volume in the air supply tank, 12 gallon tank would be as small as I'd go, plumb in a straight shot at 120 psig, have a good approximate TDC, both valves closed, and the drive belt off it should actually blow the piston down turning the crank. Any time I've ever done it one of the first things I learned was to put a breaker bar on the crank snout bolt and lock it up somehow to avoid the piston from moving downward.
Have you removed a valve cover to see that the valves have seated? If not, then do so. I'd estimate that the exhaust valve will start to open at about 15 deg. ATDC in crank degress, which isn't much.
Bypass loss could range quite high in comparison to overall sealing percentages or calculated cylinder efficency but shouldn't be anything near what you are experiencing. You should be hearing a lot of loss if any of the valves are open, even just a bit, like .020" will lose alot of volume.
If after you have checked/replaced your fixturing/tooling for the test and you still experience loss, it could be leaking from cylinder to cylinder which would definetly be a headgasket burn or pushout between cylinders. That could also explain the volume loss.
Other that what me and other's have suggested I'd say I'm at a loss as to what is the problem. Like Sponge Bob Squarepants say's, "Well,Good luck with that".

06-27-05, 07:56 PM
I'm most inclined to go with you guys that it is in fact my air supply. We did exactly as described to find TDC, put in a dowel and turned the serpentine belt pulley until it reached maximum distance out of the cylinder (ie dowel at its highest point). I do not have a regular compressor w/tank but one that generates high enough pressure but at low volume. What boggles my mind is that at TDC there is such a small void to fill that I figured most any compressor could do that. My air delivery tool is working, and it doesn't have regulator on it, just a pressure gauge to measure if any pressure is being generated. Without that pressure gauge I would have been staring into my coolant reservoir for three hours to no avail and mistakenly thinking I had pressurized the cylinder. I've decided this is beyond me and am having my mechanic do the cylinder pressure. This suspected head gasket thing is turning into a bit of nightmare. For my own piece of mind, I need to definitively diagnose it before diving in for $3,000 of work. Appreciate all the help and advice.

06-29-05, 11:48 AM
The chamber volume is small, true, but that is not the governing factor. The inherent leakage around the valves/valve seats and around the piston rings is always going to be there and you need a steady flow of air from the supply source to maintain pressure. If the "leak" past the valves and rings (normal) is greater than the compressor capacity then it will not hold pressure. If there were no inherent leakage then even a tiny compressor would eventually be able to pressurize the chamber no matter how large it is...the volume is not the issue, the flow rate of the compressor is very important. It does need to be huge but more than a mini-compressor will supply.