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!

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 ;)

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

Re: 1991 Zr-1 Stumbling off idle
 

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

Re: 1991 Zr-1 Stumbling off idle
 

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.

Re: 1991 Zr-1 Stumbling off idle
 

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)

Re: 1991 Zr-1 Stumbling off idle
 

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

What part of Colorado are you located?

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?

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.

http://i185.photobucket.com/albums/x...PtestLarge.jpg

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.

Re: 1991 Zr-1 Stumbling off idle
 

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!!


:cheers:

David

Re: 1991 Zr-1 Stumbling off idle
 

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.

Re: 1991 Zr-1 Stumbling off idle
 

Longmont

Re: 1991 Zr-1 Stumbling off idle
 

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?

Re: 1991 Zr-1 Stumbling off idle
 

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

Re: 1991 Zr-1 Stumbling off idle
 

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.

Re: 1991 Zr-1 Stumbling off idle
 

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

Re: 1991 Zr-1 Stumbling off idle
 

Distributorless Ignition System or Ignition Control Module

Which DIS Module is Located Under the Plenum

Re: 1991 Zr-1 Stumbling off idle
 

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?

Re: 1991 Zr-1 Stumbling off idle
 

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.

Re: 1991 Zr-1 Stumbling off idle
 

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.

Re: 1991 Zr-1 Stumbling off idle
 

I like the DM approach, but I'm not sure what the OP is experiencing is that same as yours. His sounds more severe, and all the way from 1200 up to 2000 rpm...and throwing a DIS code is a "horse of a different color", i.e., not "normal".

Re: 1991 Zr-1 Stumbling off idle
 

Be sure you're on page 6E3-A-38 for code 16.
It is easy to get skewed onto a wrong page in this FSM

Re: 1991 Zr-1 Stumbling off idle
 

Lets just post what Code 16 is.....I do not own an FSM ;)

And post the FIX..........as well when we are at it.....just in case I get a Code 16 :thumbsup:

What does the FSM look like? May have one and not know it......Is there one for 1990 and 1991 (are there separate FSM by year)?

Re: 1991 Zr-1 Stumbling off idle
 

If you pulled the DIS connector, make sure you didn't fold a pin on the ignition module.

Re: 1991 Zr-1 Stumbling off idle
 

That's a good point. If he pulled the plenum and disconnected the DIS connectors maybe one did not get all the way on or bent a pin. I did that once.

Re: 1991 Zr-1 Stumbling off idle
 

Where is OP anyway here?

If he had folded an IgMod pin over, wouldn't that have given him a 16 light right away? He didn't have it at first...

OR... is 16 only lit up in CL?

Irrespective of the 16 code, I'm willin' to wager lunch that he has EXACTLY the same problem, that Marc ID's as a normal glitch, BASED ON HOW HE DESCRIBES THE PROBLEM:

"It idles reasonably smooth..."

"stumble [coming] off [of] idle." .............. not the first time I ever heard of sagging coming off of idle, huh Franke!!!

"It almost seems like it is choking out when you give abrupt throttle from idle up to a little over 1,200 rpm." .............. I don't like the 'abrupt' descriptor...

"Above 2k it seems like it runs ok." ...................... and what about from 1,200 - 2,000 there OP - smooth in that range too - right?

Re: 1991 Zr-1 Stumbling off idle
 

Chuck,
To be honest, I think it may be a little different than your situation in that yours acts well until your slowly reach about 950 RPM if I remember right then starts RPM hunting until above about 1200 RPM or so.
What really bothers me here is he indicates a reman ECU and I've seen defective ones from the auto parts counters. I don't really believe they actually test all functions of the ECM on the simulator bench over all RPM, temp, MAP, TPS, IAT, IAC ranges etc, etc . I'd like to swap in a known good one to eliminate that possibility. I wonder if that is why the car was traded in?
Like others above stated, I think the code 16 was due in part to the plenum removal as it occurred right after that maintenance. Could even be a loose connection or dirt at the DIS.

Re: 1991 Zr-1 Stumbling off idle
 

[QUOTE=Schrade;195389]Where is OP anyway here?

If he had folded an IgMod pin over, wouldn't that have given him a 16 light right away? He didn't have it at first...

OR... is 16 only lit up in CL?

...snip...
/QUOTE]

He probably is at that thing called "work" :)

If it were snapped all the way off sure but bent has a chance of making mechanically intermittent contact. And on that back one if the screw is not tightened who knows?:rolleyes:
Hard to say. Results of traversing the troubleshooting tree for a #16 would seem to be prudent.

Re: 1991 Zr-1 Stumbling off idle
 

Xfire - you were exactly right. Folded a pin on the DIS. No more check engine light.

Re: 1991 Zr-1 Stumbling off idle
 

Good for you and excellent information to remember there XfireZ51.....appreciate that....and that applies to all plugs having multiple pins :thumbsup:

Some plugs actually get lose also having the original plastic tabs break off that are suppose to keep the plug together.

I like it when someone (XfireZ51) NAILS IT and in one little ole sentence.......:D

Re: 1991 Zr-1 Stumbling off idle
 

Schrade - It seems to smooth out a bit in the 1,200-2,000 range. By abrupt, i was trying to convey that when you ease into the throttle as if there is a cracked egg under it and you don't want to crack it more it is pretty smooth. Stumbling starts by applying the throttle any more quickly than this i.e. - letting the clutch out and applying throttle to start rolling. I'm not talking about ripping off any 0-60 runs by any means.

CEL is corrected thanks to suggestions on here - folded pin on dis.

Re: 1991 Zr-1 Stumbling off idle
 

Ha - I'll take one off your hands if you have one leveling the table in your shop. Thank you for the wealth of knowledge you provide on this forum.

Re: 1991 Zr-1 Stumbling off idle
 

If slow gas pedal (as opposed to quick), reveals no irregularities, I have no ideas about the original issue - the stumble that you have...

Re: 1991 Zr-1 Stumbling off idle
 

Enginerd,

If the car had 60k on it w original injectors, I would suggest changing the in-line fuel filter, if you haven't done so already, and then check the fuel pumps for a few things. 1) the plastic clamps connecting the hoses in the tank should be replaced, 2) change the pump "socks". Frankly, I'd swap out the pumps to Bosch rotaries while I was at it or the Suburban pumps if original. Also, you need to check fuel pressure under load so a setup like Paul's will help doing that.

Re: 1991 Zr-1 Stumbling off idle
 

Glad you fixed it. Been there done that. Here is the first time I had heard of it. Don't feel like the Lone Ranger.
This post was made right after I had seen the same issue.

***********************************************
Date: Thu, 20 May 1999 14:48:16 -0500
Reply-To: paul.smith@AMD.COM
Sender: "ZR-1 Corvette enthusiast list." <ZR1NET@HOME.EASE.LSOFT.COM>
From: paul.smith@AMD.COM
Subject: Re: [ZR1] Plenum Removed
To: ZR1NET@HOME.EASE.LSOFT.COM
Content-Length: 3819
Status: RO
X-Status: $$$T
X-UID: 0000001160

Be very careful plugging in this connector with the 1/4" bolt. Do not force
it into the socket!!! You can bend the pins on the ignition module very
easily. I found a bent pin on one that I took apart recently.

It helps to have very small hands :-)

Paul Smith
'94 #207 Admiral Blue/Black KUL ZR1