1991 Zr-1 Stumbling off idle - ZR-1 Net Registry Forums

Enginerd101 · 02-08-2014

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!

Dynomite · 02-08-2014

I always do compression test first thing to make sure rings,valves are in order.

Then in your case plugs and vacuum leaks. Then from there all sorts of oppurtunities.

Check my signature first link especially ENGINE post/page in that link

ZZZZZR1 · 02-08-2014

Welcome to the forum and congrats on your purchase!

Original injectors are likely part of the cause, but your likely in need of a few things.

Injectors (get stainless steel ones from Fuel injector connection)
Coil packs (4 needed, I prefer OEM)
Spark plug wires (Jerrys Gaskets)
Fuel Pumps (Chevy Suburban's have the same)
Actuators (2 needed, these can rust and have rubber inside) Good idea to replace
plenum Gaskets (Jerrys)

Please post pictures of your car! Any plans?

Headers? Exhaust?

If you need more info on parts, I can post up or PM you.

Cheers

David

We Gone · 02-08-2014

Not much to add at this point But Welcome. Post up some pics we love pics.

Enginerd101 · 02-08-2014

Dynomite - I checked all plugs before digging further. They appeared normal, but may replace them anyway. Wires have been replaced at some point. They are the red wires made by Packard. Vacuum seems to be holding fine, pump comes on at start up and stays of.

David - hope to eventually get to headers, ported plenum, Haibeck chip, and maybe a set of 4:10's. It has a B&B exhaust on it already. I've ordered a few things from Jerry already.

scottfab · 02-08-2014

Quote:

Originally Posted by Enginerd101

...snip...
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.
...snip...

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

Thank you!

Here would be my approach.
1. I would not shot gun replace parts.
2. I would check first to see if any error codes are set.
3. In the fuel vs ignition approach I'd focus on ignition based on what you've written. Specifically measuring the coils and perhaps replacing the HV wires.
4. above all first check to see if the MAP sensor hose blew off. (between engine and firewall) It is a common event when a backfire occurs. And see if the hose is filled with oil. (if it gurgles in there is causes grief)

USAZR1 · 02-08-2014

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

What part of Colorado are you located?

gbrtng · 02-09-2014

It could be something simple as a defective or misadjusted throttle position sensor - but you would be better off to find someone with a TECH 1 scanner to check all the ECM parameters - assuming there are no error codes. Does the Check Engine indicator light during the bulb check before you turn the ignition switch from run to start?

Paul Workman · 02-09-2014

Congratz on your Z, and welcome to the Brotherhood of the Beast!

I agree w/ Scott & Glenn; taking a logical approach - and it sounds like you are, so far.

A Tech 1 approach is a good place to start, OR, have it scanned at one of the many auto parts stores that can do OBD-1 scans

The Throttle Position Sensor (TPS) is located on the left side of the throttle body. I would want to pull the connector and read the resistance through its entire range, making sure resistance change throughout is smooth and unbroken.

Both fuel pumps come on initially for a few seconds, and then the secondary pump shuts off until load conditions are met for it to be switched back on. This will result in fuel pressure reading, more or less "normal" when performed w/o starting/running the motor. But, under actual driving conditions, a failing primary pump scenario matches your symptoms "to a tee" (as the secondary pump comes on to "rescue" the primary pump as load increases).

Dynamic fuel pressure test... Taping a fuel pressure gauge to the windshield and going for a drive puts a finger on a delivery issue very effectively.

If you don't already have one, for sure you'll want to get the Factory Service Manual (aka FSM). AND, I'll say that my scanner has paid for itself many times over. (I'd kill for a TECH I, but even my little 'ol AutoXray has saved the day many times. Beit a TECH I or a Snap-On or whatever, I highly recommend everyone have one!

)

P.

ZZZZZR1 · 02-09-2014

Quote:

Originally Posted by Enginerd101

Dynomite - I checked all plugs before digging further. They appeared normal, but may replace them anyway. Wires have been replaced at some point. They are the red wires made by Packard. Vacuum seems to be holding fine, pump comes on at start up and stays of.

David - hope to eventually get to headers, ported plenum, Haibeck chip, and maybe a set of 4:10's. It has a B&B exhaust on it already. I've ordered a few things from Jerry already.

If you are "in there", we typically change out PVC hoses and the valves. Very inexpensive and better than OEM. (Jerry's gaskets again)

The original PVC hose are old and usually very soft. Good idea to replace

Nice looking Z!!! Highly recommend 4:10's! One of the best upgrades!!

I'd also recommend a short throw shifter. (Hurst or bill Boudreau).

You will LOVE the addition of headers!!! Great sound and 20 rwhp!!

David

02-08-2014	#1
Enginerd101 Join Date: Dec 2013 Location: Colorado Posts: 21	1991 Zr-1 Stumbling off idle 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!

02-08-2014	#2
Dynomite Join Date: Aug 2009 Location: South Dakota/California Posts: 3,788	Re: 1991 Zr-1 Stumbling off idle I always do compression test first thing to make sure rings,valves are in order. Then in your case plugs and vacuum leaks. Then from there all sorts of oppurtunities. Check my signature first link especially ENGINE post/page in that link __________________ Left Clickable links -Solutions- LT5 Modifications/Rebuild Tricks Low Mileage ZR-1 Restoration 1990 Corvette (L98) Modifications LT5 Eliminated Systems LT5 Added Systems LT5/ZR-1 Fluids 1995 LT5 SPECIFIC TOP END REBUILD TRICKS Last edited by Dynomite; 02-08-2014 at 07:37 PM.

02-08-2014	#3
ZZZZZR1 Banned BANNED Join Date: Oct 2006 Location: MD Posts: 3,690	Re: 1991 Zr-1 Stumbling off idle Welcome to the forum and congrats on your purchase! Original injectors are likely part of the cause, but your likely in need of a few things. Injectors (get stainless steel ones from Fuel injector connection) Coil packs (4 needed, I prefer OEM) Spark plug wires (Jerrys Gaskets) Fuel Pumps (Chevy Suburban's have the same) Actuators (2 needed, these can rust and have rubber inside) Good idea to replace plenum Gaskets (Jerrys) Please post pictures of your car! Any plans? Headers? Exhaust? If you need more info on parts, I can post up or PM you. Cheers David

02-08-2014	#7
USAZR1 Join Date: Oct 2013 Location: Wichita Falls,Tx Posts: 604	Re: 1991 Zr-1 Stumbling off idle Welcome! Nice to see another White Z on the site. What part of Colorado are you located? __________________ Clint & Sherry 1994 ZR-1 White/Red #152 (sold) 1991 ZR-1 Callaway Aerobody #1332 (sold) 1990 ZR-1 Red/Red #952 (sold) 2002 Z06 Torch Red/Mod Red widebody

02-09-2014	#8
gbrtng Join Date: Feb 2006 Location: San Antonio area Posts: 1,179	Re: 1991 Zr-1 Stumbling off idle It could be something simple as a defective or misadjusted throttle position sensor - but you would be better off to find someone with a TECH 1 scanner to check all the ECM parameters - assuming there are no error codes. Does the Check Engine indicator light during the bulb check before you turn the ignition switch from run to start? __________________ “Buy the best example of what you want, and pay whatever it takes. That way, you cry only once.” ~ Bruce Meyer

02-08-2014	#4
We Gone Join Date: Apr 2004 Location: ATL. GA. Posts: 2,162	Re: 1991 Zr-1 Stumbling off idle Not much to add at this point But Welcome. Post up some pics we love pics. __________________ Steve 1990 Steel Blue/Black #2355 1990 Red/Red #1473 1991 Quasar /Black #118 Sold 1991 Turquoise/Black #766 Parted Out 1993 Yellow/White #179 Sold 1990 Black/Gray #1361 Headers/4:10s Sold

02-09-2014	#9
Paul Workman Join Date: Nov 2007 Location: Squires (near Ava MO in the Mark Twain N'tl Forest) - Missouri Posts: 6,493	Re: 1991 Zr-1 Stumbling off idle Congratz on your Z, and welcome to the *Brotherhood of the Beast!* I agree w/ Scott & Glenn; taking a logical approach - and it sounds like you are, so far. A Tech 1 approach is a good place to start, OR, have it scanned at one of the many auto parts stores that can do OBD-1 scans The Throttle Position Sensor (TPS) is located on the left side of the throttle body. I would want to pull the connector and read the resistance through its entire range, making sure resistance change throughout is smooth and unbroken. Both fuel pumps come on initially for a few seconds, and then the secondary pump shuts off until load conditions are met for it to be switched back on. This will result in fuel pressure reading, more or less "normal" when performed w/o starting/running the motor. But, under actual driving conditions, a failing primary pump scenario matches your symptoms "to a tee" (as the secondary pump comes on to "rescue" the primary pump as load increases). Dynamic fuel pressure test... Taping a fuel pressure gauge to the windshield and going for a drive puts a finger on a delivery issue very effectively. If you don't already have one, for sure you'll want to get the Factory Service Manual (aka FSM). AND, I'll say that my scanner has paid for itself many times over. (I'd kill for a TECH I, but even my little 'ol AutoXray has saved the day many times. Beit a TECH I or a Snap-On or whatever, I highly recommend everyone have one!) P. __________________ Good carz, good food, good friendz = the best of timez! 90 #1202 "FBI" top end ported & relieved Cam timing by "Pete the Greek" Sans secondaries Chip & dyno tuning by Haibeck Automotive SW headers, X-pipe, MF muffs Former Secretary, ZR-1 Net Registry Last edited by Paul Workman; 02-09-2014 at 06:15 AM.