Damn it - I've been fighting this battle for a month now, and it needs to be over... I have a '91 Z-28, 5.7L TPI, 700R4, and I've had a combination of vibration and supposedly misfiring for about a month. The problem first occured out of the blue - leaving a store, started it up and damn - there it is. As soon as I put it in gear, it started vibrating and I'm like WTF? Supposedly the torque converter was bad, pulled the tranny and replaced the converter and had the tranny rebuilt while it was out. Well, I tried the flexplate, too - not a chance. Last night I R&R'ed the dampner and that seemed to alleviate the majority of the vibration, but it appears to still have a misfire. The vibration problem was there without driving the car - hold the brake, put in gear and you got vibes. Eliminates the d/s, but I did put new u-joints in, too. As for ignition, LESS than 2000 miles ago, I put in a new dist/wires/coil (all MSD)/plugs (bosch)/timing chain. In the midst of all this, I took it to a friend of mine (mech at Prec Tune) and he diagnosed the plugs and wires. I went home and pulled all that out, found the #4 wire to be cooked and all the plugs wet with fuel along with carbon build-up. I cleaned and re-used the plugs, and put on a new set of wires and after changing the dampner last night, I rechecked the timing - it was still on the mark. I didn't want to go for the plugs and wires because remember, they're all new... but I'm trying to check everything. And to make this long story as short as possible, what the hell else can it be? I'm thinking I got some bad plugs, and they're the next thing to come out, but this is getting old. I've barely been able to drive the car this past month because it's in my garage more than on the street... I'm all ears, guys - try and help me out here, please...

 

Sounds liek you have an ignition problem (somewhere) and/or a mixture control problem. I'm assuming you have no error codes set, right??? If you have codes, fix them...

If no codes, I would put engine on diagnostic scope at a shop for 25-$50ish to have the ignition primary/secondary fully scoped-out.

Got any codes???

 

When you did the tune up did you change the pickup coil in the distributor? This is an overlooked item sometimes and will produce some of the problems you have described.

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Hank
87'IROC Z28

 

Pickup coil or ignition module???

 

NO error codes... Distributor is new (less than 2k miles), which includes having a new pick-up coil and module in there.
To listen to the motor, you'd never know it was acting retarded. With the car in park, it seems to run perfect. Give it throttle, and it's got good response. The motor mounts seem to be good as well. When you put it in gear, rev or dr, you can feel the "surging", but again, it's slight now. When I get it on the road and cruising along, say about 40-45, and go for the gas, the "immediate" response is not there anymore, it seems to hesitate for a second, like it's bogging down, and then get it's act together and goes. When I accelerate from a dead stop, or even a soft "rolling stop", I can feel the misfiring/shudder until the engine seems to catch up. I hope I'm describing this enough for you... Once the car "planes out", it seems to run good...

 

I know this may be hard but I would beg/borrow/steal a scan tool to see what the Block Learn numbers are doing. Sounds like a lean condition that cleans up eventually, or perhaps rich that cleans up... BL numbers will tell you either way, then you can continue diag from there. Also, can monitor TPS/MAP and 02 sensor to see if weird fluctuations going on.

Also, new distributor can still be suspect...

 

I'm gonna do some more 'shooting this week - probably Thursday, soonest, but I'm gonna check TPS & IACV, too. If if comes down to it, I still have my old dist and coil and all that, so might have to throw 'em back in to see... I'm gonna double check for codes today, but I guess if it don't get fixed, then I'll have it diag'd. I just hate to hook up to a machine for about $50+ and the end result be "it's a $20 sensor or $2 plug".

 

if it "cleans up when it planes out" I would check the tps, I had one go on a tbi car and it would run rough, and flood at start up, didn't set a code for a while though, which made it real tough to track down