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.

[bradslt5]

have yo tried cking the vaccume of the secondary system. follow the plastic line from the vaccume pump in the right front . right about the oil filter there is a connector hook up your vaccume tester and see if it hold a vaccume if it doesnt then you have a leak under the plenum. is your fuel pressure with the motor running or just with the ignition on . with just the ignition on it should be between 48-55 with it running 8lbs less . hope this helps . also there are some good zr1 guys in your area that might be able to help.

[Paul Workman]

Originally Posted by bradslt5

have yo tried cking the vaccume of the secondary system. follow the plastic line from the vaccume pump in the right front . right about the oil filter there is a connector hook up your vaccume tester and see if it hold a vaccume if it doesnt then you have a leak under the plenum. is your fuel pressure with the motor running or just with the ignition on . with just the ignition on it should be between 48-55 with it running 8lbs less . hope this helps . also there are some good zr1 guys in your area that might be able to help.

Thanks for the tips.

I'll need to round up a vacume tester, per your suggestion.

Fuel pressure was 46# with the switch on, and not running, and pressure dropped to 43# w/ engine at idle. (So, if it had been 48-8 or 40# at idle, running 43# at idle doesn't strike me as an issue. Course who knows what it is doing at WOT - not that I even know at the moment what it should read at WOT, for that matter...)

I did "ohm out" the injectors, and they were all about 12 Ω Â± a few tenths. So, mebby the injectors were replaced at some time. In any case the injector solinoids check out OKay.

I guess I'm continuing to circle around the plenum removal possiblilty, but there may be a few more items to look at before that happens.

Thanks again for the suggestions.

P.

[Pete]

Try new wires and double check plugs and gap.
Wires are about $50 from GM

Pete

[Paul Workman]

Originally Posted by Pete

Try new wires and double check plugs and gap.
Wires are about $50 from GM

Pete

Hello, Pete!

They're on the list too. BTW, are the replacement wires stenciled with the LT5 lettering???

P.

No they are not. They are stenciled with AC delco.
For original wires you are looking at about 400.00

[Pete]

Originally Posted by Paul Workman

Hello, Pete!

They're on the list too. BTW, are the replacement wires stenciled with the LT5 lettering???

P.

Paul at this time you want your Z to run good,$50 wires could do it.
Just save your old ones or buy OE's which will be used at a later time,i would not pay $300-$400 for used wires.

Pete

Pete, the 400 bucks is for new original wires. Mainly for the NCRS guys. For a normal car the delco replacements are just fine.

[Pete]

Originally Posted by Jeffvette

Pete, the 400 bucks is for new original wires. Mainly for the NCRS guys. For a normal car the delco replacements are just fine.

My Mistake.

Pete

[tomtom72]

Paul, this is for sure JMHO.....and it's from the peanut gallery no less!

Plugs are clean. They always seem to be, the system must be that good. When my 1* injs went out on the rt bank the left bank richened up to compensate and there was barely a difference left to right on the electrodes and porcelins. Ohm the wires if you haven't already, with the plenum in place you can read a coil + a pair of wires at a time. Somewhere here is the ohm figure, about 25 to 28 K ohms?

Fuel filter & pumps & delivery. at idle and also the KOEO test there is no "demand" for fuel. Pumps are okay by your testing. If a partial restriction existed in the filter, then that only shows up under severe demand??? JMHO. I don't consider the filter a PIA to do but some people do think of it that way. My car had it's original filter in for 15 yrs! I do mine every yr.

Vacuum leaks in 2* port system. Usually if it's big enough it sets a code, or at least gives an intermitant SES light on the DIC, but sets no code. Do what Brad said. Also see that the pump works properly while you're do that test, might as well. If that checks out then it is possible that the actuators are at issue or/and the linkage for the 2* butterflies may have a binding issue. I don't have too much faith in that, my OE actuators were on upside down & still passed the FSM test. Check the PCV valves too....yea mine were 15 yrs old too! Oh, yea I still had FR2LS plugs in the car!

Injectors. I apologize in advance for this statement! You did the ohm test cold and hot? Sorry...I should be smarter than that. Just as an aside. When I had to replace my injs I found most of the plenum bolts loose, and even quite a few of the I/H bolts not real tight....no "snap" when I put a torx on them? Oh, while we're on the loose things subject, all the rubber in my PCV system eventually got r & r'ed due to poor fit after 17 yrs. That's grabbing at straws, I know...add up all the slightly leaking connections?...I don't know if it means anything?

Sorry to be this wordy!
I'd think along the lines of why your issue happens when the 2*'s are called....common denominator type thing...unless this happens with the key off also up in the rpm range?....anyway fuel, spark, & air (yea my a/f was 15 yrs old too, car only had 7600 miles on it) right?

Oh, finally my base line logic is: the car looks new but remember it's 17 yrs old & so is a lot of the stuff under the hood....everytime I have a problem I start my diagnostics with that in mind....eventually I won't have to do that if I keep it long enough!


Tom