Low Compression on Cylinder 5 - Dry and Wet (5.3 LS Content)

I recently wrapped up an AFM/DOD delete on my 2007 Silverado.  Got the truck back together and it seemed to run decent and then I noticed a misfire at idle.  Pulled the codes and I have a cylinder 5 misfire (P0305).  

Let me backup.  The truck collapsed a number 4 lifter back in April and gave a clattered mess.  Pulled the heads off the get to the lifters and yep, #4 was collapsed.  Before the #4 failure, I had never had a misfire on cylinder 5.  Cylinder 5 is a non AFM cylinder.  

Last night I ran a compression test on cylinder 5 wet/dry and the compression was 60 psi cold.  Adjacent cylinders were 150 psi dry cold.  That leads me to think something is wrong in the head.  Let me backup again.  When I pulled the heads off, I took them to a machine shop that was close to my old job because I was working 70 hours a week and didn't have time to go to my trusted machine shop that has banker hours.  This was a huge mistake.  The guy held my heads hostage for over 6 weeks and ended up doing the work in a couple of days after I gave him crap.  He surfaced the heads did a "valve job" which I assume he just cleaned up the valves and valve seats.  Also did valve stem seals as well.  I'm really thinking that something is wrong with a valve on cylinder 5.  

Tonight I'm about to yank the head (which I really don't want to do) and hopefully get it to my preferred machine shop tomorrow morning for a look over.  Before I pull the head tonight, is there something else I should do/test/inspect?  I did pull the valve cover and the rockers/push rods seem alright.  Am I missing anything else? 

Get a leakdown tester (they rent them at auto parts stores), and figure out where the air is coming out.  If you hear the air coming out the intake or exhaust, it's a valve.  If you hear air coming out the oil cap, it's a ring or melted piston.

If you can't find a leak down tester make one.  Pull both rocker arms roll motor so cyl 5 is at BDC.  Take used spark plug grind off the crimp to the insulator above the hex, grind off the ground strap (on the motor side of the head) then push out the white ceramic insulator. Weld a 1/4 NPT nipple to the plug body, screw on a air fitting.  install newly made fitting in to cyl #5 hole and attach compressed air.  where do you feel air coming out?  stretch rubber glove over tail pipes, did it inflate or blow off? if so exhaust valve  you get the picture. I'm sure.

While you have the heads off, I'd pull that lifter and take a peek down at the camshaft lobe. The 5.3L has a bit of a rep for soft camshafts.

Could be something as simple as a failure to lap the valves after cutting the seats.  It may eventually work itself out as the parts get better acquainted but maybe not.

In reply to 44Dwarf :

Yeah, I guess he doesn't need a full on leakdown tester as he already knows something is awry with that cylinder.  Anything that can stuff compressed air in the cylinder will do.

Some compression testers already have a fitting that will come apart and let you stuff compressed air into the cylinder. Check the end of yours and see if it has a "spark plug" thread to 1/4" NPT fitting. If so, disconnect that from the gauge and use it. Make sure you disconnect the one-way valve too.

 Take used spark plug grind off the crimp to the insulator above the hex, grind off the ground strap (on the motor side of the head) then push out the white ceramic insulator. Weld a 1/4 NPT nipple to the plug body, screw on a air fitting.  install newly made fitting in to cyl #5 hole and attach compressed air. 

I am totally not against making things to get out of a pinch and i have done a similar thing to what you're suggesting to make an adapter for a 3v Triton engine, but...

If you're already doing a compression test, the hose you're using is one removed schrader valve away from doing a cylinder leakage test. So i recommend that!  

Was in a rush to get the head off last night so I just took it off before "leak down" testing it.  I really wanted to get it to the machine shop this morning.  Nothing seemed out of wack really. Lifters were good and not boogered up.  

I took it to the machine shop early this morning and the owner pulled a vacuum with Venturi vacuum and #5 was holding air.  Number 3 was losing pressure though?  Weird.  He's going to further inspect but I'm not so sure anything is wrong.  

After talking this through a bit with my Dad, we're leaning towards possibly a faulty lifter (all 16 are new) or even a bent push rod.  When I put the engine back together, I may have switched the right bank and left bank push rods.  I guess it is possible that I had a bent push rod that I never identified? 

At a loss really.  No issues on this cylinder before the #4 collapsed a lifter on the right bank.  

Well, all i can say is that's exactly why a tech wouldn't tear it down without knowing what they're looking for. At this point I think you will end up reassembling it back to runnable shape just to finish diagnosing it and there's a decent chance you will unknowingly fix whatever the original problem was in the process and never really know what happened. But, here's hoping you figure it out so you get some peace of mind!

Look for a broken or stuck ring.

Update. The machine shop had the head rebuilt on Friday and I put the head back on Saturday morning. So far it’s running great. No miss, no CELs. 

The problem turned out to be a poor job with the exhaust valves and distorted intake valves.  Kind of scary that the other side was done by the same crappy machine shop but it seems to be okay.

Oh, and it runs so good without issue that I’m almost afraid to check compression on #5. I’m sure I’ll get around to it this weekend, but the root problem seems fixed so I’ll keep my head buried for now. 

Thanks for the update. Still a mystery how it sealed up during a vacuum test but somehow leaked air so fast on the engine that it affected the compression reading. The root cause seems to be the machine shop(s) in both cases, which is frustrating. 

Even though they fixed it, at this point i hope you just don't need any machine shop's "help" again for a good long time!