3.4 GM heads--stuck lifters.
I recently did a mill job on a set of 3.4 GM (cam in block) heads. They were just cleaned, and needed just .004" to straighten out the decks.
The shop installed the heads. No compression. They called me. I aske the usual "Did you get the pushrods mixed up?". Nope. They asked me how much I took off the heads and if that may be the problem. I told them less than .005" and that would not be a problem. After thinking about it for a while, I told them to squirt some carb cleaner down the pushrods and around the lifters. Voila! After doing that and cranking the engine over for a bit, they got compression.
My guess is that the lifters, with the spring tension removed, extended fully. As the heads were somewhat varnished, the lifters probably had a varnish or similar build up that caused the lifter plunger to stick all the way up, making for a solid lifter. The carb cleaner was enough to unstick the plunger.
I wonder if this is what happenend to the 6.0 Powerstroke diesel I was involved with........
No, or Lack of Compression?
It's most likely true your thoughts on the 6.0L.
AERA supplied the following information years ago in TB 1572. We get these types of calls all the time and our shops customers also assures them that they kept all of the push rods in the proper order.
Lack Of Compression After Engine Start Up
The AERA Technical Committee offers the following information concerning a lack of compression at engine start up. This condition is possible on engines that use a hydraulic lash compensator or lifter. No compression has been noticed on one or more cylinders after the cylinder head(s) have been installed and the engine has been started. This condition has been noticed mostly on engines that have not had the lifters replaced.
The cause of this lack of compression may be resulting from the lifter(s) preventing the valve(s) from closing. That condition may be caused by varnish buildup on the lifter bore or plunger. The oil clearance between these components is minimal (.0002") and very small amounts of varnish or debris may prevent the plunger from traveling freely within its bore.
To prevent this possible condition, many AERA members collapse each lifter before installing the cylinder head. Other solutions include disassembling, cleaning and reassembling used lifters, or installing new lifters.
If new lifters are used, they should be installed as received. That is, with only the amount of oil in them that the manufacturer assembled them with and cam assembly lube on the wear points. New lifters will prime themselves while engine oil pressure is established. Some hydraulic lifters may take up to a half an hour to completely bleed trapped air, allowing the lifter to become primed. According to manufacturers, some valve train noise may be noticed
during this time period and is considered normal.
The AERA Technical Committee
AERA Senior Technican
500 Coverntry Lane
Crystal Lake, IL 60014
Honest to goodness, Dave, I don't remember reading that TB (although I'm sure I did at one point, I read ALL of them).
When I do valve jobs on OHC heads that have hydraulic adjusters I always bleed them down, although I DO remember a TB about (I think) some Volvo's where the lifters should NOT be bled down. Many times, with the hydraulic bucket-type versions, I also disassemble them and clean them.
I still have to tell many customers who by cam and lifter kits to NOT pump them up. There have been more than a few Hot Rod-magazine style articles published that advise pumping them up........
Edited to add: It's the 3.2 Isuzu OHC that I was thinking of. Refer to TB1388.
Last edited by machine shop tom; 03-05-2009 at 05:16 PM.