Alright, long story short, I had a stock rocker bust and spit the needle bearings all over the engine. Pulled and diassembled the engine (left shortblock/timing components/oil pump/etc. in tact) got all the bearings out of the motor. Used a pushrod length checker to determine pushrod length, ended up going w/ a .025 longer pushrod (7.425) and installed a new set of Yella Terra rockers w/o using the shim as I determined I didn't need it (Note: I determined pushrod length using stock rockers, not the YT's, so perhaps that is a factor)......Car ran fine before the rocker arm went, dialed in on the tune and ran strong. Now that I have everything reassembled and back in the car, it keeps mis-firing so I check all the wiring, put new crank sensor/cam sensor/plugs/wires all in, checked all the igition coils as well as all the relays, I also added two more grounds directly to the block. All sensors seem to be reading fine and not throwing any codes. Fuel pressure is great, it will start and stay running, just very roughly. My question is, could there somehow be enough difference between the YT rockers and the stock rockers to cause a consistent miss like this? The only things changed since it last ran a few weeks ago were the YT Rockers, 7.425 pushrods instead of 7.400 and an LS6 valley cover. What gives? Any help is greatly appreciated. Thanks!

~Paul

 

BUMP! Really need help figuring this out. I've exhausted most of my ideas.

 

A bent valve??

 

You need to chack how far the lifter plungers are being compressed.

 

Geezer: I just had the heads off a few days ago and everything checked out on them. Unless the rocker/pushrod combo did damage....

Andereck: Thanks for the tip, I will look into that.

 

So is it agreed that this problem could be directly related to something in the valvetrain geometry?

 

If the slightly longer pushrod and different rockers make enough difference to hold to valve open a bit then that could cause your miss fire. But I highly doubt that there is THAT much difference.

 

Have you run a compression test on all eight cylinders? If your compression is good on all eight cylincers, are you throwing any diagnostic trouble codes (DTC's) and if so, which ones?

Steve

 

No codes whatsoever, not even showing any on the laptop while tuning. Compression test/leak down tests are the next thing I am going to try.....Hypothetically speaking, the compression checks out fine in all cylinders and the valvetrain geometry is spot on, what comes next? Is it possible that the computer may need a reflash or is that out of the question? Thanks for all the help!

~Paul

 

If the compression checks out OK, then you can check the ignition by pulling and replacing one spark plug wire at a time (use insulated plug boot puller pliers) while the engine is running and see if one of the pulled plugs results in no change. You could also have a plugged injector.

Steve