1991 Zr-1 Stumbling off idle

[Schrade]

I've been investigating this since Oct.

Marc H (an authority), says it's normal.

I think time and mileage contribute to exacerbation of the 'normal' issue.

Here's what I've found on mine '90, 32k; Throttle Body secondary butterflys leak air, which made elimination of problem variables more difficult. I have them temporarily blocked. This made for a rock steady idle - something necessary for diagnosis.

I put in an 8mm hex bolt in the throttle stop screw hole, to make MINUTE increases in throttle, and do DataMaster readings of all parameters (I'm still learning DM navigation).

Have you seen this vid? http://www.youtube.com/watch?v=ahqv0vs1Y1o It shows increases in throttle using the throttle stop bolt. Note RPM's @ 2:50, in the graph box.

APPARENTLY, a transfer of control of one of the operational parameters is at fault.

I'm still working on it...

I suggest plugging in DataMaster, temporarily blocking the sec butterflys, and replacing the throttle stop screw, and seeing if you can duplicate DM readout.

Originally Posted by Enginerd101

Hi - I'm not new to Corvettes (89 coupe, 1994 coupe, 1999 FRC, 2001 Z06), but have recently had the opportunity to purchase a 1991 Zr-1. It is a car that I have always been intrigued by and wanted.

I bought a 1991 Zr-1 with just under 60k miles (#407). It is arctic white. I bought it sight unseen from a dealer who surprisingly enough said that it runs great with no issues. HAHAHA... It will start and run, but seems to have a major stumble off idle. It almost seems like it is choking out when you give abrupt throttle from idle up to a little over 1,200 rpm. It idles reasonably smooth (probably not as smooth as the LT5 should though) and runs very rough in this range. Above 2k it seems like it runs ok. It had original multec injectors, which I wanted to blame for the issue, so went to work changing those out. Thanks to all of the tips and "how to's" here from all of the great forum members, I was able to do this job reasonably easy.

Finished it up this afternoon and fired it up. Much to my dismay, it is still exhibiting the same symptoms. I checked fuel pressure before and after i did the job, and is seemed to be adequate, ranging 40-50 psi.

I have no service history on the car and it does have a remanufactured ecu installed. Also has a Vortex air intake and B&B exhaust. I haven't really been able to road test, as I'm in the middle of a deep freeze in the Colorado front range.

Any thoughts or suggestions? I'm a bit stumped at this point.

Thank you!

[Enginerd101]

Originally Posted by USAZR1

Welcome! Nice to see another White Z on the site.

What part of Colorado are you located?

Longmont

[Enginerd101]

Thank you all for all of the suggestions. May have a lead - car threw the check engine light today. Looks to be a code 16 (grounded pins a to b and counted flashes). Started with code 12 3x and then code 16 3x, then back to code 12. All I can find on this code is ignition 3 fuse circuit open. All fuses in kick panel on passenger side are good. Anyone have experience with this code?

[Dynomite]

Originally Posted by Enginerd101

Thank you all for all of the suggestions. May have a lead - car threw the check engine light today. Looks to be a code 16 (grounded pins a to b and counted flashes). Started with code 12 3x and then code 16 3x, then back to code 12. All I can find on this code is ignition 3 fuse circuit open. All fuses in kick panel on passenger side are good. Anyone have experience with this code?

Me thinks that is Code 16 or DIS system fault......

[dredgeguy]

When I purchased my 92 it had about 52,000 miles on it, it ran rough. Found out the fuel filter was full of crap. Changed the filter, injectors, coil packs and new wires and it was a new car.

[Enginerd101]

Originally Posted by Dynomite

Me thinks that is Code 16 or DIS system fault......

I'm not sure what that means. Coils, wires, & or plugs maybe?

[Dynomite]

Originally Posted by Enginerd101

I'm not sure what that means.

Distributorless Ignition System or Ignition Control Module

Which DIS Module is Located Under the Plenum

[Schrade]

Originally Posted by Enginerd101

Thank you all for all of the suggestions. May have a lead - car threw the check engine light today. Looks to be a code 16 (grounded pins a to b and counted flashes). Started with code 12 3x and then code 16 3x, then back to code 12. All I can find on this code is ignition 3 fuse circuit open. All fuses in kick panel on passenger side are good. Anyone have experience with this code?

Is it DTC 16 module 1, 4, 7, 9, or DERM? (some do not apply)

Is it H History, or C current?


ed.:
Maybe clear the code, run it to Closed Loop, and see if it sets live (Current).

Do you have FSM?

[4-cam]

Welcome to the forum. Its great to have another ZR-1 owner in Colorado. Im sure you will find the issue with the knowlege base avalible here . Once it warms up we will have to get a Colorado meeting together.

[Franke]

Originally Posted by 4-cam

Welcome to the forum. Its great to have another ZR-1 owner in Colorado. Im sure you will find the issue with the knowlege base avalible here . Once it warms up we will have to get a Colorado meeting together.

Sure. Why not. I now know of 4 out here.

Enginerd101. Be sure and check that fuel pressure from both pumps. Remove secondary fuel pump fuse. Ignition on pressure has to be about 48-55psi. Should hold steady when pump stops. Lower than 48 would be suspect. Replace fuse, repeat test. Also with engine running pressure will drop anywhere from 3-10 PSI at normal operating temps.