It's like hunting a ghost...HELP!

[PhillyZ28]

I'm new to the board and this is my first post. My name is Marc and I have a '94 Z28. If anyone has any info, I’ll be really grateful. This is the situation. My Z has 117, 000+ miles and it was running great up until yesterday. I've owned the car since 14, 000 miles so i know what it's been through. I leave for work, the Z runs perfectly. On the way home it starts to run a bit funny. It starts to stumble under acceleration (almost like it's missing or dropping Cylinders), just a bit at first and progressively gets worse (the stumbling starts when the car is plenty wormed up) like tonight where I thought I wasn't going to get home. In my garage i shut the car off for about 2 minutes and started it again and it was a bit better....weird. I let the car sit there for about 2 to 3 hours and BINGO it's fine again. This has to be a sensor or maybe the EGR or something. If anyone has a starting point, it would be great. I'm no stranger around cars so technical info is not a problem.

[Z Ya Later]

What rpm range does it stumble? Is it around the 1300-1700 range? Is it fine above 2k? Any problems at idle? When it does stumble, does the tach jump around or does it stay where it should?

[Mr Nice Guy]

I'm thinking the Opti may have had it. Is it the original?

[Benm109]

Did the check engine light (SES) come on at all?

[PhillyZ28]

I'm not sure what RPM it hits at.....probabpy between 1,500 and 2,000. If i jumped on it, it got worse. In my garage if i pushed the peddle, the tac just hung around 3k with the motor. The tac toally hung with the motor revs. And i replaced the opti spark once at about 75k. Sorry for the choppyness of this post...i'm trying to get everything in....thanks for the quick responce.

[PhillyZ28]

no check engine light at all...that was a shock.

[Z Ya Later]

If you get the chance, test the rpm band above 2k rpm. I had similar symptoms around that range, but was fine from 2k rpm to redline...turns out it was the EGR valve. If you have a stretch, test 2k rpm to redline and see if you notice anything. Also, make it stumble again and watch the tach. My guess is that if it does NOT jump around, the opti is fine. Here's the thread with my problem, Craig 94 TA GT started it for me.

http://www.f-body.com/forum/showthread.php?t=8037

Some of the info is for the 93 only, but it should be helpful. Let me know if what the results are from the 2 tests above.

[Trans Am]

might be the ignition coil, easy and cheap to replce

[Joe 1320]

Known causes of this condition with these cars are:
1. Optispark beginning to degrade
2. Plugs and wires
3. Coil Driver Module
4. IAC valve carbon fouled/stuck
5. EGR carbon fouled or stuck open
6. leaking/partially plugged injector
7. MAF sensor degrading/bad/contaminated
8. Lazy 02 sensor(s)

Without a scanner, it's going to be guesswork. However, usually the first three are what will cause the symptoms you describe with no codes present.
the fourth, usually affects idle only (stalling). 5 will cause it to run poorly once it warms up but should be consistantly bad. 6 should make it start hard and run poor, 7... disconnect the maf and see if it runs better. The computer will run it on different tables.

[PhillyZ28]

Thanks for the help...i'm going to get started.

I don't ythink it's plugs or wires because it would be more consistant but i bought new plugs so that's going to be step one....i'll probablt do the EGR as well since it's been a while. I'll post me results and let you know how i make out.

[PhillyZ28]

Plugs are in and the EGR is on order. I'm still having the came problems. I did notice that when the stumble occures the tac is at about 3k and does bounce a bit.

[Z Ya Later]

I hate to say it, but its sounding like the Opti to me. If it is, its either a weekend job or about $1200 depending on the shop.

[PhillyZ28]

Yeah i know the drill. I had the Opti replaced a little bit back. I'm replacing the EGR this week and i'll (once again) post my results. Thanks for all the great info.

[PhillyZ28]

UGH........

Wednesday afternoon I finally got a check engine light and i limped the car to a friends shop. We put it on the computer and low and behold it's the optispark....ugh. Sorry to doubt you "Mr Nice Guy". My friend sold me the part for his cost and I'll be putting it in on Friday. Thanks for the help and thanks to Backyard Buddy or i wouldn't even attempt this job. Thanks again guys and happy thanksgiving!

[PhillyZ28]

Success

New Opti is in and the Z is running great. This is kind of a tough job but do-able. It took me all day saturday and about 2 hours sunday (just bolting on the last few pieces). If you attempting this job, keep in mind that you'll need to remove the water pump and the lower pully...and you'll need a pully puller for that job....other than that, patience. Shoot me an e-mail if you have any questions and thanks to everyone that offered some advice.

Marc