OK guys, as I posted in an earlier post I have very low compression in 7 of 8 cylinders. I did a wet compression test this morning with no change in any of the affected cylinders. hoping that rules out rings. I was getting ready to do a leak down test and pulled the right side valve cover.


ALL of the rockers look level and I can twist 7 of the 8 push rods with my fingers. That doesn't seem right. I would think at any given time you shut an engine off you are going to have a few valves that are being pushed open, right? They all appear closed or very close to closed. Maybe my low compression is caused by not enough air being allowed into the cylinders?


It's a new cam. Wondering if maybe the PO didn't get the lifters primed right? could he have done something wrong during break in to flatten the cam lobes? What test(s) would you all recommend I try next?


Thanks!

 

"Prime lifters" is not the problem.

Yes it is possible that he, or you, have flattened a bunch of cam lobes. Let's not go there just yet.

Probably not a valve adjustment problem either, but, very eeeeeeezy to adjust the valves, and rule that out:

Use the "EOIC" method.

With any given cyl's exh valve just beginning to open, adjust that cyls intake (EO, Exh Opening); and with any cyl's int valve just finishing closing (IC, Int Closing) adjust that cyl's exh.

Great place to start is, put the engine at #1 TDC by turning by hand with a crank turning tool, NOT THE DAMPER BOLT, with all the plugs out, while watching the valves; with the timing mark approaching, watch for the #1 exh to close and the #1 int to begin opening. Continue to turn; observe the #1 int to open, reach full opening, and begin to close. Keep watching until it just closes. (Int Closing) At that point, adjust the #1 exh valve by jiggling the push rod up and down and just barely taking out all the play, NOT BY "TWISTING"; once all the play is removed, tighten it ½ turn more. Rotate the engine EXACTLY 90°: this will put it at EXACTLY the same sequence of events for cyl #8. Adjust it using the same method. Continue through the entire firing order, 18436572. As you are turning the engine toward #2, watch the exh valves; look for one to begin opening. It should be #6, might be either #3 or #5 depending on the cam. After you do the #2 exh, watch the exh you just identified, and look at THE NEXT ONE in the firing order; and turn the engine until that one just begins to open. As you just begin to see the push rod come up, adjust that cyl's int. Rotate the engine in 90° increments adjusting int valves in the firing order.

http://www.summitracing.com/search/p...haft%20Sockets I suggest the Proform one there if the damper is installed (remove the pulley to use it), or the Sunex one if not.

Repeat the compression test, verifying with absolute certainty that all spark plugs are out except the one you are testing at the moment, the throttle is blocked wide open, and the battery is topped up for each cyl.

 

I already adjusted the valves with the engine running last week. Do you recommend I do it again using this method?

 

Not really; if you adjusted em running, and it all seemed to work right, then they're as good as they're gonna get. Especially if it didn't alter the issue at hand.

I STILL have a strong feeling that there's something wrong with your compression testing. Are you ABSOLUTELY POSITIVELY CERTAIN that the throttle is blocked WIDE OPEN during the test?

 

Positive.
I block it WOT with a block of wood. ALL plugs are out and it's a brand new battery. #1 jumps right up to 150PSI every time I test it. The others...Well not so much...

 

Try doing them in a different order.

 

Results of my latest test before calling it a night. My back is killing me!


Got the engine to TDC. Put 100+ lbs of air into the #1 cylinder (which is the only one with good compression) It actually turned the engine about 1/4 revolution. Good compression right?


Then I rotated the engine one full turn which should put #6 at TDC (Which is my lowest reading of 70psi) The air blew back into the plenum and I could feel it also coming out of the #4 spark plug hole. The intake valve is open at that time in #4.


I can feel the rockers on #6 are slightly loose and will jiggle and push rods can be twisted so that tells me the springs are closing the valves as much as they can. WTH?

 

When you're pushing compressed air into the cylinders you need to have the piston at TDC compression for that cylinder. If you don't, it'll push the piston down the bore until either the intake or exhaust valve on that cylinder opens (depending on which way it rotates), allowing air to escape past the valve.

Getting it dead-on TDC compression can be tricky, though. Alternatively, as a foolproof way of doing this, you can just fully loosen both rocker arms on that cylinder so it doesn't matter if it pushes the piston down the bore- the cylinder should still be sealed up no matter.

Now, I'm a little confused about how you describe pushing compressed air into #6. You say it's at TDC/compression of #6 when you applied air pressure. ASSUMING THE MOTOR DIDN'T ROTATE (timing pointer on the balancer should still be dead-on the TDC mark), getting air blowing back into the plenum would be a VERY BAD SIGN. That would mean the intake valve isn't sealing up on it's seat. Which would mean the head has to come off so you can figure out the cause.

I just want to make sure I have a clear picture of what you're describing before I jump to that conclusion, though.

 

that's exactly what I'm saying. when #1 was TDC it moved the engine. When #6 was at TDC it blew back into the plenum. yes, I rotated the engine one full turn from #1 TDC so the timing mark was back at TDC (180 off for #1) that put's #6 at TDC compression.


I have 7 of 8 cylinders with low compression. I don't mind pulling the heads if I can get this fixed. I need the diagnosis, then I can fix it. I plan on testing the others tomorrow.

 

I know you just adjusted your valves, but being you have this maybe major prob, I say loosen all of your valves up before doing the leakdown test/ blowing compressed air in.

With what you said about #6 cyl blowing into the intake sounds bad so at this point take the rockers off or atleast loosen then to give one less thing that could throw of your results.

 

I will loosen the hell out of each rocker before I do the test. Shouldn't that tell me if the valves aren't seating properly?

 

Yes sir it will. That was one of the reasons why I recommend doing it at this point.

The wet compression test proves the rings are not to blame and most likely cause for your low compression would be valve related.

By loosening your rockers and doing a leakdown test would take the camshaft, lifters, pushrods, springs out of the mix... At that point you will know if it's a valve seat sealing prob or a cam/valvetrain part prob

 

Latest update...I just got in touch with the PO who told me he did not do anything to the heads, he only had the lower half redone. Sounds like he just slapped the old heads back onto his rebuilt lower half. It's starting to make sense now..

 

Remember that TDC compression on #6 is 360* off from TDC compression on #1. NOT 180* off. It's one full engine rotation apart.

If you only did 180* from #1 TDC compression the #6 piston will be at BDC and one of the two valves will be almost fully open (intake or exhaust, depending on which way you rotated the engine).

Still, if you set these valves with the engine running using the "back off the rockers until they click and then tighten 1/2 a turn" you shouldn't have low compression unless there is a MECHANICAL problem with the motor.

 

Yeah, today I loosened every rocker till they almost fell off and tried it again. All 7 leaked back into the intake. Then I spent several hours getting ready to remove the heads. The intake base is all I have left to remove before the heads.


I will update when I get the heads back and installed. Thanks for the help everyone...