Stumbling still...Thoughts?

[Paul Workman]

It is a situation that starts out rough and works it's way out(?), almost every time out. As many have said, their Zs like to be whipped like a rented mule, and mine is no exception to that. However, my idle is sometime irratic and sometimes "smoother". But, before it does smooth out, I usually get some stuttering an even a hard balk as it transistions to the secondaries. (In fact, the transition to secondaries is never seamless - ranging from a just perceptable change (normal, I guess) to a hard cut-out balk followed by considerable missing and stuttering. Usually, letting off the gas and then getting right back on it seems to "clear" the problem and it pulls hard again... (Hence, my recent interest in pulling those pesky secondary throttle plates and changing the chip to operate in "16" mode and forget about all that fussing around.)

Note: At normal idle (around 750 rpm) I have always noticed a slight throbing - not a miss, necessarily - just a throbbing apparently on one cylinder. I may pull one plug at a time and see if I can narrow it down to which cylinder. (That is a minor "niggle" at the moment, but I thought I'd mention it - putting a bookmark in the discussion, as it were.)

I've gone over the vacume hoses that I have access to w/o removing the plenum, and I re-torqued the plenum itself, followed by the propane bottle procedure to isolate air leaks. That seems to have helped the idle a little.

Per Jeffvette's suggestion I changed plugs. The dealer said the LT5 had been gone thru - including new injectors (as yet not confirmed), and for sure the plugs were new. But, as far as the plugs go, I've been reading plugs for many years, and I was surprised to see that the plugs that came out looked almost like they look out of the box. That is to say there was no brownish tint on the insulator, but only a trace of soot...IF anything at all. The insulators were mostly white. Keeping in mind I've put less than 1000 miles on what apparently were new plugs, perhaps what I was seeing was "normal" for the LT5s. I dunno, but I keep that in mind.

Stumbling, if not spark related, puts the focus on fuel (or at least it does to me). I measured fuel pressure with the switch on, but not running. I got 42#. Armed with my new FSM, I'll pull the pump fuses and jump them one at a time and make sure both both pumps are working. But, back to the fuel pressure for a moment...I thought I read that 48-55# was the normal range. And, for the record, the primary fan runs anytime the switch is on. The the old battery was a little weak anyway (replaced yesterday) and with the engine running (and the voltage in the normal range) it might change the fuel pressure figure at idle. (I'm not ruling out the fuel filter either, but since for now the fact that it runs better as it has been running a while leads me to dismiss the filter as an issue - at least for now.)

So...Like a buzzard circling a fresh kill, I'm circling around the plenum and getting curious about what I might find under there. Several have mentioned that changing the injectors fixed similar symptoms for them, but of course I'd like to avoid pulling the plenum if I don't have to. If someone pulled the plenum to change injectors, I'm curious as to why they didn't wipe off the fuel rails...They are grimy, and that makes me a bit skeptical w/ regard to the supposed injector change out... Wouldn't you or any self-respecting mechanic wipe off grimy fuel rails? If the rails weren't wiped off, what about grime and the mechanicals of the secondaries??? Hmmmmm....sez me. I guess I can always run a fresh dynamic scan in the mean time to see if there are any clues. (I'm not getting any codes, I might add, nor any SES lights either.)

Well, THATs my story and I'm sticking to it! I'm all ears. What say you?

P.