Hey guys I just got my motor back from the shop and it misfires from idle-2500rpm and anytime after at >60% throttle.

The usual DTCs; P0300, P0301, P0302, P0303, P0304 and P1399.

Leak down is about 8% on a fresh hone (should see about 6% within 300 miles).

Compression is was retested at 240 accross the board.

The plugs after 200 miles look like the timing is advanced a little but mechanically it is correct.

All of the valves, seats, guides, keepers were replaced with ferrea parts so this just doesn't add up. My mechanic suggests that if the chain tensioner were to be going bad (although it doesn't sound like it) that could cause the timing to jump around.

I will check fuel pressure tomorrow and post that up, the injectors are clean and new.

I meausered the resistance accross the injectors and they were all around 18ohm.

Still waiting on valve clearances from the mechanic.
Any thoughts?

 

The chain tensioner will definitely NOT cause a misfire.

I'd start with the fuel trim, and see if it's rich or lean.

 

Personally, sounds like tight valve clearance.

 

Valve clearance is too tight? They are 1mm oversize undercut swirl and dished. Do you think that plays into lower compression on the test (160)?

I will get the freeze pulled tomorrow. The stealership is the only place that can do that right?

 

I had higher compression than that with a 3MM headgasket

 

Is it missing one just one cylinder? I don't know those codes (beings I work on gm's all day).

 

Well you should, P0300-P0304 are the exact codes that can come up in a GM, and every other 1996 and newer car in the USA

165 psi of compression sucks in my book. The lowest I've seen first hand was 190 or so. There are many variables, the test must be done right.

Find out where the clearances were measured, and what they were set at.

 

Still waiting on the blueprints from the build but I have some freeze data...

Three different pulls gave me values of -3.4%, -27.2%, -14.5% for short term fuel trim 1, and -100.6% for short term 3 for all of them. The only long term registered is .8%. I have a hard time believing that all 4 coil packs are bad, and the plugs are newer than 200 miles. Does this support a low compression = misfire theory?

I will post the clearances asap.