Need Advice: Miss, low rpm stumbling

[EWest]

Originally Posted by scottfab

It would be nice to know if this same behavior is exhibited with the power key off. And what does the fuel pressure drop to when under load? I'm concerned that all the effort is going to spark.

Power key off = same behavior. I never got a chance to check the fuel pressure under load, but would that cause a problem in 2 specific cylinders, and only under 3000RPM?

[scottfab]

Originally Posted by EWest

Power key off = same behavior. I never got a chance to check the fuel pressure under load, but would that cause a problem in 2 specific cylinders, and only under 3000RPM?

absolutely. If you get to a low pressure situation one cyl is going to go lean before the rest. The "miss" has to start somewhere.
I assume the wires were checked out? Length on a few are the same so they can be swapped. AND if one plug was cracked I'd not rule out more damage to others. These aren't those funky Bosh plugs are they? I've seen multiple problems with those.

[GSJoe]

You're getting lots of good advice here from people with lots more mechanical "savvy" than I have, but I need to suggest something you may have already done, and if not, you must do it whether or not it's the cause of your miss issue. Let me explain. Last year my young friend Matt who owns a '91 experienced a similar miss issue. We did just about everything--plugs, coils, wires...tons of other checks. We chased our tails in vain for months. In August at Carlisle, however, Matt talked to the smartest ZR-1 guy who exists in mho, Marc Haibeck. He gave Matt the trick we implemented when we returned home to Charlotte, and the problem went away. Here's what you must do if you haven't done so already. First, take out those old plugs--I don't care if they're new--and throw them away. Get yourself a brand new set of AC plugs of the right heat range and install them. Next--and here's the trick Marc Haibeck gave Matt--get yourself a 3oz can of Permatex Dielectric Grease (item #81153) at any Advance or AutoZone parts store. (I'm sure other versions of dielectric grease will work, but this one dispenses easily from a narrow nozzle because the can is pressurized.) Put the nozzle into each spark plug boot AND COMPLETELY FILL IT with the grease before putting the boot on the new plug and do the same with the remaining seven. OK, why does it work? According to Mark, the extremely high intensity discharge from that ignition system causes air trapped in the spark plug boot to ionize. When it does, the spark then takes an easier path down the outside of the plug rather than through it. If you look at your existing plugs you'll probably see several with a very fine burn line on the insulator where the spark has been tracking. That's why you must replace the plugs again when you do this. As I said, if you've already completely filled those boots with dielectric grease and the problem still exists, I apologize for the verbosity of this post. If you did not, however, do so. It's a must, and even if your problem does not go away it will prevent another common and similar one from occurring. I sincerely hope this helps you.

[Funracer]

Originally Posted by GSJoe

You're getting lots of good advice here from people with lots more mechanical "savvy" than I have, but I need to suggest something you may have already done, and if not, you must do it whether or not it's the cause of your miss issue. Let me explain. Last year my young friend Matt who owns a '91 experienced a similar miss issue. We did just about everything--plugs, coils, wires...tons of other checks. We chased our tails in vain for months. In August at Carlisle, however, Matt talked to the smartest ZR-1 guy who exists in mho, Marc Haibeck. He gave Matt the trick we implemented when we returned home to Charlotte, and the problem went away. Here's what you must do if you haven't done so already. First, take out those old plugs--I don't care if they're new--and throw them away. Get yourself a brand new set of AC plugs of the right heat range and install them. Next--and here's the trick Marc Haibeck gave Matt--get yourself a 3oz can of Permatex Dielectric Grease (item #81153) at any Advance or AutoZone parts store. (I'm sure other versions of dielectric grease will work, but this one dispenses easily from a narrow nozzle because the can is pressurized.) Put the nozzle into each spark plug boot AND COMPLETELY FILL IT with the grease before putting the boot on the new plug and do the same with the remaining seven. OK, why does it work? According to Mark, the extremely high intensity discharge from that ignition system causes air trapped in the spark plug boot to ionize. When it does, the spark then takes an easier path down the outside of the plug rather than through it. If you look at your existing plugs you'll probably see several with a very fine burn line on the insulator where the spark has been tracking. That's why you must replace the plugs again when you do this. As I said, if you've already completely filled those boots with dielectric grease and the problem still exists, I apologize for the verbosity of this post. If you did not, however, do so. It's a must, and even if your problem does not go away it will prevent another common and similar one from occurring. I sincerely hope this helps you.

Never heard of this before but I too am chasing a stumble and am willing to try this to see if it helps. Anyone else tried it or know if it works?

[scottfab]

Originally Posted by Funracer

Never heard of this before but I too am chasing a stumble and am willing to try this to see if it helps. Anyone else tried it or know if it works?

I've never heard of this causing a stumble either.
A simple one to try is cleaning the IAC and the tunnel it sits in.
Two screws and you can inspect it for carbon build up.

[Pete]

Just a couple of questions.

What spark plugs you using? hope not any of those pricey crappy plugs just regular resistor plugs work just fine.

Where did you gap them? .035 is best.

Did you buy new injectors or rebuilds?

Injectors can Ohm out good and still be bad.

Did this start after replacing injectors,if so borrow a good set and replace them.

Pete

[Starman]

It will be plugs or injectors. Injectors must have 14 ohms, any less and they are on their way out - replace the entire set when needed - trust me it will save a lot of frustration down the road.

The guys are also right about the plugs, use only the correct AC plugs with the stock coils - fancy, multiprong plugs can cause misses. Always seal your plug boots with dielectic grease.

Good luck