What could cause low compression after headgaskets

Mekanik · June 6, 2008

We have a f series with a 6.0L that had a routine headgasket replacement and is now missing on 3 cylinders, lots of white smoke. I'm not the tech working on this one, I'm just trying to help. It was missing on 1,5,6 (I think). A relative compression test showed cyl. 5 and 6 very low. The tech checked compression on cyl 6 and it was 300psi. A little low, but I think enough to fire the cylinder. I cranked the engine over by jumping the starter with the ignition off and I thought it sounded like it just had 1 cylinder without compression. He was going to check compression in cyl 5 right before I went home. If there was a valve stuck open I can see how that would affect other cylinders. I don't know how this could happen aside from carbon. The only thing I think could really go wrong is a pushrod not in the lifter properly, or a rocker that is not tightened down, but I can't see how that would affect other. cylinders. Any ideas?

mchan68 · June 6, 2008

ANYTIME there is a concern immediately after a repair, ALWAYS ALWAYS ALWAYS retrace your steps. It's something I've learned many times. I know it sucks royally, but it looks like he's going to have to start with pulling the valve covers and HP rails off. He may luck out and find one of the rocker arm bridges not seated properly on top of the valves or the plastic fulcrum(s) either broken, or not seating the rocker arm(s) properly. Failing that, I guess he's going to have to pull the heads off again. Good luck.

Jim Warman · June 6, 2008

Well, we've seen a few bent pushtubes... it can be hard for even some good guys get them in just right on these things (no not me - I'm saving myself for another "mother of all screw ups").

We had a new guy get a valve bridge on wrong... that cost a bent valve and a missing rocker tip... Any valve train woes *should* have an accompanying noise - even to some old fart with tinitis...

At least he knows which cylinders to concentrate on...

Brad Clayton · June 6, 2008

A very important step with doing head gaskets on a diesel engine is having the crank in the right position. With everyone doing motor work now some lack the experience and or advantage of having done apprentice work with a knowledgable motor guy or whatever. I mean there are a lot of techs being throw to the wolves nowadays. "Oh your diesel certified here ya go have at it."

Now I'm not saying this applies to your fellow working on this beast. But with this goofy rocker setup with the head bolts going thru the fulcrum plates, patience and having the pistons in the right spot are very important. That plus the fact the push rod ends disappear into the black hole of calcutta upon installation.

I bar the motor over many times with the valve covers off to see if my static settings will create any problems when dynamics are enabled. Most of the time it there is a problem with insatallation error it will show up right away before everything is buttoned up.

On another note these things are ticking time bombs and the truck could be put together perfectly and it just decided to give it up. /forums/images/%%GRAEMLIN_URL%%/shrug.gif

Mekanik · June 6, 2008

He has it apart right now. The #5 exaust pushrod is bent really bad. We can only assume that he missed the lifter. The crank was not in the proper position when the heads were installed. This is a step that I never really understood until now. I don't know if it has anything to do with the problem.

I have always put the crank right where the book says before installing the heads but thats just me.

BustedKnucklez · June 17, 2008

I once did a headgasket job and messed up putting in one of the pushrods. I missed the guide on the headgasket and bent the pushrod. Replaced it and all was well. I know look at every pushrod to verify the placement before proceeding.

Mekanik · October 2, 2008

So this thing ended up having a total of 3 bent pushrods. I never understood how this happened until a couple days ago when I was talking to this tech. As it turns out, he decided to leave the rocker arms on the cylinders when removing and installing the heads. I'm sure he probably left the oil resivoirs on the heads too. What does this have to do with the pushrods? All of the pushrods were standing up in the block when he put the cylinder head back on the block. Now I see how something disasterous like this can happen. /forums/images/%%GRAEMLIN_URL%%/surprise.gif

This tech also did not turn the crank so the dowel was facing down. I never totally understood this step, but always did it and never had a problem. I guess that because tightening the head bolts is putting pressure on the pushrods it would be a good idea to have the engine in a position where there aren't any valves all the way open. Can anyone else explain that better to me?

Tony302600 · October 2, 2008

So this thing ended up having a total of 3 bent pushrods. I never understood how this happened until a couple days ago when I was talking to this tech. As it turns out, he decided to leave the rocker arms on the cylinders when removing and installing the heads. I'm sure he probably left the oil resivoirs on the heads too. What does this have to do with the pushrods? All of the pushrods were standing up in the block when he put the cylinder head back on the block. Now I see how something disasterous like this can happen. /forums/images/%%GRAEMLIN_URL%%/surprise.gif

This tech also did not turn the crank so the dowel was facing down. I never totally understood this step, but always did it and never had a problem. I guess that because tightening the head bolts is putting pressure on the pushrods it would be a good idea to have the engine in a position where there aren't any valves all the way open. Can anyone else explain that better to me?

Can someone show me where the dowel is?

Can someone explain why after 10minutes of idling, the test drive results in a bent pushrod even after the truck was idling beautifully?