1fast97gsx

I just got a 1995 Z28 as a weekend toy. The car is bone stock, but has very high miles. The engine starts up right away, no smoke, no leaks or anything, but it has a minor idle surge and has a really bad shutter / misfire at low rpms. It clears up after about 2000 rpm or so and it does it when free reving as well. I haven't determined if it's a harmonics problem or ignition, but since it does it in p/n when free reving you would think it's more of a harmonics issue right? Balancer feels very tight though. Previous owner says he has not done a tune up in 2 years on it so I suppose plugs / wires would be a good start. Any other ideas?

ThoR294

iTrader: (6)

id look at the throttle body and IAC passage for build up first. very easy to check

96slow6

Try fuel filter and o2 sensors first.

1fast97gsx

Wouldn't a fuel filter cause problems under load at higher rpms, and not have it clear up? O2 sensors maybe, but the car is in open loop when started and it already has the problem then.

96slow6

Depends on the condition of the filter. As the throttle comes up and the vaccuum goes down the pressure at the regulator goes up. The bump in pressure may be what is smoothing it out. Also as to the o2 reference, you are right in open loop they are not used, however your pcm's fuel and timing curves for idle are learned durning pre operating temp with the o2's warm and normal driving conditions. So if they are getting less acurate during regular driving then those characteristics carry over to cold start no load conditions. Also more info on your cars current set-up would be very helpful. Also weather or not it has this same issue once warmed up.

1fast97gsx

I appreciate the info. The car is a bone stock 1995 Z28. It has around 190k on it, but the engine runs great besides this issue. Once the car is warmed up to operating temp it still has the exact same issue. There is no cel on.

1fast97gsx

Did plugs and wires .. issue still there. Reset the compuer and the problem still happens right at startup. That would elminate O2 sensors as a possibility right?

1fast97gsx

Just took the car for a drive and really payed attention to it. The miss is really bad around 1500 or so and actually doesn't clear up until the car upshifts and the throttle is barely being touched. If you push the gas down it will miss and if you push really hard it will hesitate and then miss badly.

What are the signs of a bad or going bad opti? If I take a timing light and test each wire just to see if they are firing they all miss every 3-4 sparks. It did the same thing with the plugs and wires that were in it before.

1fast97gsx

Anyone have an update on testing the opti? All data shows how to chck the vent system, but no way to actually diagnose the opti itself.

96slow6

A constant miss is very opti sounding. How many miles do you have on it? Are you sure the wires are not arching?

bobdec

If you have access and knowledge to use a scope you can check the low voltage side of the ignition for consistent (no missing or erratic) signals from the Opti to the PCM and from the PMC to the coil, ref wire diagram ' http://shbox.com/1/95_ign_system_schematic.jpg ' .

1fast97gsx

Car has 190k. Wires are def. not arching.

1fast97gsx

I have knowlegde, but no access as of right now. Thanks for the heads up.

jasonp31

When you did the plug wires, did you replace the wire from the opti to the ignition coil? Did you use plenty of dielectric grease on the plugs wires?

Clean MAF.
Replace fuel filter.
Replace air filter.
Remove and fully clean throttle body.

Opti is last resort in my opinion.

1fast97gsx

Yea I did replace that wire and I did all the other things other than clean the throttle body. When opening up the tb and looking inside from the front it looks relatively clean though.

Whitez406

iTrader: (14)

This is the same exact problem i have with my 97 rite now. I think its my pcm tho. Cause my fans dont work and some other stupid stuff. I have an extended start, it misses at low rpms clears up wiht minor throttle after 2500 or so and high rpms it backfires throgh the intake but i also get the vibration as well as u stated. Its really ackward. Im puzzled. Im taking it to gm to get it scanned caseu basic scanners didnt tell me anything. I also did plugs, wires, new opti, fuel pump and filter and it didnt change. I think its either a sensor somewheres or the pcm is just junk. Not sure yet when i get it scanned i will post up what it is or what they found.

jasonp31

Hmm, well I doubt its your throttle body at this point, but I would still take it off and clean it. The area after the butterfly is the part that gets dirty, as it is opened up to the intake, which gets very dirty.

jasonp31

Measure your fuel pressure at idle and WOT.

Whitez406

iTrader: (14)

This is one of the things i thought of. Didnt have time yet. Gotta get down the shop and get the damn gauge. Still say more of an ignition type problem. Its really jsut a weird problem.

Whitez406

iTrader: (14)

I know its not a fuel problem. When i check spark at the coil it sometimes stops sparking and it varies everytime. Is there anyway a bad or failing crank sensor could do it? Anyone think it could be a PCM?