Where is my compression leak is coming from?

[Scooter311]

Dooods Happy Friday, woooo
Okay, so my 2056 runs, and I've been driving it a little, but it's still not 100 percent. I got around to doing my compression test, and after the headwork, overall compression was bumped from 124psi up to 145! However, that pesky #3 cyl is still low, at 80. When I had the heads off, I didn't see any cracks or telltale signs to explain it, so I buttoned it back up after getting them cut, thinking it was the little warpage that caused the initial problem.
Now that it's together, is there a trick to determining what the cause could be? I tried squirting a few drops of motor oil into the cylinder and testing again, however it stayed right at 80. Is there anything I can do that will tell me what the cause is, before I pull the motor again? (getting really good at it now )

Tell me, why am I still having fun with this thing?


Cheers
Jeff

[Jason Porter]

Are your valves adjusted properly? After about a week (at the most) of driving, you should retorque your heads, and adjust your valves again. Just don't remove the bolts for torquing...

[SGB]

Yes, retorque those heads. I suggest you reset valve lash to really big openings to be certain there is complete seating pressure before redoing c-test on that cylinder. My "burned valve" turned out to be a too tight intake valve.

[SteveStromberg]

You do need to adjust the valves and retorque the heads as Jason said.

You lapped the heads to the barrels or used the sealing rings?
Did you stagger the rings?
Did you check the ring clearance in the lands?
How was the cross hatching on the cylinder walls? did you hone the barrels while you had it apart?
What cylinders are you using with the 96mm pistons?
What was the compression like before? were they all fairly close but then #3 was lower?

[norustscott]

"re-torque the heads"...thats funny...it also would mean droping the engine and stripping off all of the injection and cooling tin... not practical...I would also not suggest only re-torqing the "bottom" studs...

[SteveStromberg]

Thats why you need to run the motor on your test stand before you install it.
I use a center mount weber on FI engine to test run out of the car, seeing as it a little impractical to have all the FI out to do your test runs.

[Jason Porter]

Originally posted by norustscott
"re-torque the heads"...thats funny...it also would mean droping the engine and stripping off all of the injection and cooling tin... not practical...I would also not suggest only re-torqing the "bottom" studs...

basic assembly follow-up is not practical...OK...

And Steve is correct, this should be done prior to installation...

[norustscott]

Basic assembly follow-up? do you mean checking your work? Sure...once done correctly though, what changes, 1 hour... 12 hours...3 years after you originally torqued the heads... and have not run the engine? Test stand...would be nice and if you have one then by all means re-torque the heads...


80 psi is bad...My thought on causes:

valve adjustment/ clearence (go ahead and open it up to .008 - .010" for the test)
broken ring...from installation
bad lifter...search past posts on my famous growing lifter...
Ring end gap stager/alignment...hmmm. may loose compression but 60spi is unlikely any body havea data point on this?
poor valve to seat seal...did you lap the valves?
Poor cylinder to head seal...did you lap the pistons to the cylinders?
Some kind of wierd bent valve hang in the guide
Rocker arm binding on the shaft
Pushrod stepping out of the rocker cup



If it is not a valve clearence/adjust problem then you are unfortunatly going to have to drop and inspect...I have been there...I feel you pain...

[schosh]

Scott,
What exactly is "pushrod stepping out of rocker cup"
Is it common? What is the cause?
Can it be resolved without yanking the engine?

Bill

[norustscott]

I have had a pushrod walk out of the cup on the end of the rock. However, this was caused by excessive valve clearence and or a rocker arm that was too tight on teh shaft( binding) or a valve to valve guide clearence problem... The push rod catches the edge of the rocker arm causing the vlave to never close...If this happens you will know it. As long as it is not a valve guide problem then Yes, you can fix it with the engine in the car. I don't think this is the problem...but without being there I thought I would List all possiblities...

[SteveStromberg]

If you dont have a test stand you can still run the engine. I have set the engine and gearbox ready to install on a piano dolly and fired it up.

If you are fixing a oil leak always run the engine before reinstalling to make sure the leak is fixed.

[Scooter311]

YOU GUYS ARE THE BEST
I've gotten the flu bug and am home sick, so no messin' with the old girl 'till Monday, crapola......
A little quick background info - I'm running KB P/Cs, and stainless valves, and everything was running smooth-as-a-pooch about 2 months ago, until a clogged fuel filter and low fuel pressure combined to overheat the motor - at about 3 grand I think it was, on the x-way. So I pulled the head, (#3 was showing 60psi at the time, when the others were at 124/125), there was a slight warped spot, and some blowby stains on the head, so I figured that was the problem. Valves looked fine, nothing 'looked' bent or broken, and I pulled the cylinder too and the rings looked great, as was the cross-hatching still on the cyl wall. (motor only has 1700 miles since rebuild, and I'm not running head gaskets, oh, and I lapped the cyls into the chambers as suggested as well.) Everything is new, and still clean and straight.
I reassembled, with a fresh flycut on both heads, and have the same problem (but it doesnt' seem to run all that bad, believe it or not. ) I left my valves a little on the loose side, but I'm a hack and could've easily messed it up as well, hmph.... I'll loosen those puppies up and do another comp test before pulling the motor again. Maybe I'll get lucky, and just have to retorque the heads, Can THAT be done while the motor is still in the car?
Thanks for the info so far,
Cheers,
Jeff

PS - dont' get sick

[John Rogers]

Compression leaks can come from several places:
- Past the valves, squirting oil in will not help increase it.
- Past the rings, squirting oil in usually raises it.
- At the cylinder/head joint, if you squirt oil in it should leak out when warm and show up.
- Out a hole (casting defect) in the piston or cylinder casting and possibly into the crankcase. If you remove the oil filler cap you can hear the chug-chug-chug of blow-by.
- Past the injector seal-ring, if hard or cracked or the plastic tip has disappeared and the small seal-ring is blowing by and it will also suck air inside, possibly causing backfiring on deacceleration.

If it were me, I'd check the last one before you tear into things.... Good luck

[Dave at Pelican Parts]

Uhh, John--that last won't cause a compression leak. The injectors are in the manifold, not in the combustion chamber. You'd get a vacuum leak/air leak/manifold leak (whatever ya wanna call it) but not a compression leak.

The only leak I can think of to add to John's list is having a hole in the piston. It'd have to be a mighty small one to still leave you with 80 PSI compression. Rings or valves are much more likely. Were the valves inspected while the heads were off the engine?

--DD

[Jason Porter]

Scooter's not using injection you 2, moot point...

Jeff, any updates???

[John Rogers]

Now here is something to ponder. As Dave noted the injectors are in fact on the manifold but when I worked on our stock 74 the compression was 30# lower on the cylinder with the leaking injector and after replacing it the compression came back up?! I also noted a lot of blow by around the injector which has also gone away.

[Scooter311]

Well folks I'm back, still not feeling too good though - cough cough ........
HERE'S THE SCOOP
I messed around with it last night, and I found the source of the leak! I bought a compression tester that had an air nipple on one end, and then removed the schrader valve so I could pump air into the cylinder - I have an air valve on my compressor, so I could put air in slowly. Well, that didn't matter at all because the leak was SO BAD, it was leaking out as fast as it was going in! I couldn't believe how fast it was coming out - FROM THE CYL/HEAD JOINT! The valves are fine, the rings are fine, no cracks, just this frickin' tornado from right on top, where the head meets the cylinder. I'm actually a little relieved - I was worried about having a bent valve, a big ol' crack in the seat, or some other horrific problem. This probably isn't the worst that could happen.
I took apart everything topside to yank the motor again, so tonite all I need to do is take off the exhaust and stuff, and do the dance again. Before that though, I'm gonna heed one last piece of advice - I'm gonna check the torque on the heads, just in case - it would be nice if that's all it is!
No biggie either way, I actually, in some bizarre, sick way, ENJOY doing this.......
Oh yeah, I'm taking pictures too, so I'll post 'em after I get it apart completely.

Cheerio

Hah, just reread my post and I said I was worried about a big ol' crack in my seat.