Low rpm intermittent shudder

[BlueDevlC7z]

Hello my Z brothers. Looking for opinions. My 93 Ruby with 22k started to intermittently shudder a few months ago. It would be most noticeable at loads under 2200 rpm. It?s not noticeable after that all the way to 7k. Last summer I replaced all 8 original plugs with AC Delco 41-913. Ran fine till this spring. I started digging after it and wound up replacing the #8 plug. Still had the shudder. I drove the car the last 2 days and it ran smooth as glass. I?m thinking it?s a sign of one of the coil packs wearing out. I don?t think it?s a fuel or injector issue as it pulls strong to redline and it probably wouldn?t if it were a bad injector or bad fuel pump(s). No mods to engine and has original wires and coils. Any help gladly appreciated.

[BlueDevlC7z]

Meant to post this in the technical section. Can a moderator please move it to there? Thanks

[Young1]

PM me to share phone number and we will talk.

[BlueDevlC7z]

Hey Steve, not sure if my message went through
Bill McLean
845-629-1763
Thanks brother

[DRM500RUBYZR-1]

Hello my Z brothers. Looking for opinions. My 93 Ruby with 22k started to intermittently shudder a few months ago. It would be most noticeable at loads under 2200 rpm. It?s not noticeable after that all the way to 7k. Last summer I replaced all 8 original plugs with AC Delco 41-913. Ran fine till this spring. I started digging after it and wound up replacing the #8 plug. Still had the shudder. I drove the car the last 2 days and it ran smooth as glass. I?m thinking it?s a sign of one of the coil packs wearing out. I don?t think it?s a fuel or injector issue as it pulls strong to redline and it probably wouldn?t if it were a bad injector or bad fuel pump(s). No mods to engine and has original wires and coils. Any help gladly appreciated.

Describe what the #8 plug looked like.
If an injector leaks, it may be fine off idle and upstairs, but at idle or just off of it, if it is dripping fuel that would cause a shudder / stumble.
Certainly also could be a coil pack going south, but that would have the effect on two cylinders / plugs, just one seems to suggest injector.
Keep us posted.
:cheers:
Marty

[XfireZ51]

Describe what the #8 plug looked like.
If an injector leaks, it may be fine off idle and upstairs, but at idle or just off of it, if it is dripping fuel that would cause a shudder / stumble.
Certainly also could be a coil pack going south, but that would have the effect on two cylinders / plugs, just one seems to suggest injector.
Keep us posted.
:cheers:
Marty

On the coil pack Marty, not necessarily. The casing on the coil packs can develop cracks allowing voltage leakage on one side, or the other or both. I had that situation occur on MSD coils

[DRM500RUBYZR-1]

On the coil pack Marty, not necessarily. The casing on the coil packs can develop cracks allowing voltage leakage on one side, or the other or both. I had that situation occur on MSD coils

Yes, you are indeed correct, good catch!
It is indeed possible, and should also be checked.
Normally, I would replace if suspect, but some replacements often are of poorer quality than the originals.
If you don't mind the labor of plenum removal, you can swap the one that is suspect with another and see if the problem moves to another cylinder or two. At least then you know,

Testing them often does not reveal an issue, but if you do, certainly test them both cold and hot.

Again, let us know what you find.
:cheers:
Marty

[XfireZ51]

:cheers:In my case, I was chasing knock at about 1800-2400rpm when cruising at light throttle. Changed plugs, injectors wires and made modifications to my timing and fueling. I pulled the coils and noticed what looked like carbon tracking. Closer examination showed cracking of the coil casing allowing the voltage leak on a particular cylinder. Another coil was starting to show similar issue. The MSD coils were red so the tracking showed up more readily. With stock black, u may need to see if u get carbon on fingers when handling coils.

[viper107]

If you pull the plenum,just automatically replace coils,they are a weak link since the were born

[BlueDevlC7z]

Update?..car is behaving perfectly know all the way to 7k. 200 miles and the issue hasn?t presented itself again. Thank you all for the help and especially Steve Young who walked me through several ZR-1 related things to look out for in the future and some tricks and tip on maintenance.
Bill