slow-vette

It sounds like you have done a lot of things to this car. Live data logging would be the next best diagnostic tool. But, if you wanted to try one last thing before data logging or swapping out the dizzy I would suggest swapping in another coil and make sure that the coil grounds are clean and good as well as the main car grounds. I would also check the wire harness leading up to the dizzy and make sure that there are no pinches or bare wire showing. Coils, thin wire insulation, and grounds can all cause all sorts of weird things to happen.

cv67

bet youre right
A wires with some broken strands pinched wire, goofy connection etc can cause this
Shame to get it all sorted except that sell it only to hear a week later it was some simple thing he just hadnt found yet.

mlm0

I replaced coil, but I will check over wires just incase I missed something The one I really to double check is the main engine ground down by oil filter. Do not notice misfire at higher rpm or under load

thanks for your response

Mike

whalepirot

Been throught his sort of thing a few times. Usually, a time out helps me, plus the encouragement of pals!

Why not follow your feel and check or replace the distr.? Tiiming chain and all the rest of the multi-replacements make way less sense to me, especially at such low mikes, when a crack or carbon trace inside is quite likely. I am personally acquainted with the iffy quality of Chinese replacement parts, so some duplication of effort...
All the advanced checks will NEVER replace the simple basics, which are every bit as important to diagnose an issue as data, etc. are.

Best of luck with what ails ya, and the Vette!

mike

mlm0

i don't notice misfire under load. I can do video, but it might be hard to notice it. I can feel misfire thru steering wheel and seat of car even though I upped idle speed from factory setting to 675 rpms. It's a constant miss but not any pattern I can nail down

cv67

Did you ever put the LT1 throttle body on?
How is your IAC?

When they are running "right" they are fairly smooth but not like silk due to the banks firing one side at a time
Sure its a miss and not a motor mount or something?
Remember when they were new guys would complain about it

Hope you find it soon sounds like you have the ability just be patient
Once ironed out they are pretty good cars.

mlm0

im sure it's a miss. I have checked engine mounts. I ordered brand new 89 in 1989. Been so long ago I can't remember how it idled. I was still living in Alaska so I could only drive it so much. I was much younger and insurance was killing me so I sold it with on 1500 miles on it and bought 4-wheel drive pick up. It's was metallic red and she was a looker

mlm0

im making a list of all suggestions so I can go right down the list checking off things to check as soon as I get a break. If I'm able to fix this car because of someone on this forum, I'm buying Everyone a beer or Their choice of drink.

RWDsmoke

Does it run better/worse cold in open loop? Which injectors do you have? Has anything been changed in the tune? Do you have software/ALDL cable to view or record data?

I just sent you a PM.

mlm0

Answered by pm

slow-vette

mlm0 I appreciate the offer of a beer but I expect that everyone here will agree that fixing your car and making you a happy C4 owner again will be thanks enough. I don't think that we are going to let you get rid of that beautiful car. Owning an old car is a joy and a challenge. You will be successful if you keep at it and this forum will be there the whole way.

1993C4LT1

Good luck man. If the distributor is easy to get to, take it out and inspect it. I feel the same as you, except I'm having a hard starting problem when sitting for a few minutes/hours.

VikingTrad3r

mlmo i remember when you did your head gaskets.

sounds spark related, or, fuel delivery related. i think i read you replaced injectors with no change.

it could be electrical/fuel related in the harness. i had a wire in my injector harness coming loose and it did this until i found it after it came all the way out and the car was running on 7 cyls. i discovered one wire missing out of the injector harness plug on cyl 3 right at the plug. i peeled back the rubber boot and there was only one wire! it was discovered using the stethescope. i could hear that it was different sounding than the rest of the injectors. im sure if i had done this excercise while it was coming loose, i would have heard the odd miss.

the other suggestion i saw was using the timing light. "check your timing" (point it at something) and keep watching until you find the cylinder that is missing. if it is in the dizzy, it will show up this way.

the other thing it could be is the ESC, electronic spark module. aka "knock module". a bad knock sensor is only part of the system when checking for knock related issues.
check out agent 86's scan on it here: http://members.shaw.ca/corvette86/El...%20Control.pdf

then there is the components of the dizzy. there is a lot of stuff in the dizzy and it could be any of them. thinking ignition control module, capacitor, pickup coil, main coil. i had a major miss on my 87 that got worse and worse and then i finally took of the cap of the dizzy and it was melted inside. the spark was gapping from the coil nubbin to the point. the strap that was supposed to be contacting the nubbin was gone (found it mangled) and the spark arched all the way from the underside of the coil to the point!!

i am really doubting anything related to an actual plug or injector as im guessing that you have replaced them all. but im guessing its an electrical signal issue.

i've been there with crap that is hard to solve. all i can say is keep going, keep your chin up.

mlm0

Thanks guys. I am getting some really good input to try. I am making a list of things suggested that I have not done.

mlm0

Guys, today I hooked up timing light, disconnect esc plug, and checked timing. Timing was right on. Mark did not move around. When I increased rpm, the mark would advance forward, but I could still see mark and it held steady. I also hooked timing light to each plug wire and none of them seemed out of whack when compared to each other, but not sure if I would notice it if misfire is so small. I rented set of injector noid lights and will check injector for firing voltage tomorrow

just going down the list of things suggested to try by this forum

Mike

billschroeder5842

Was the miss still present when you were going from plug, to plug?

Can you throw up a video of the stumble? I'm having a real hard time visualizing the miss as you keep going down your list.

mlm0

i tried video, but you can't tell there is a miss from video. You really have to be behind the wheel in the seat to feel the miss. I can stand by engine and notice it because I have been working on it so long. No change when I went from wire to wire with timing light

wikdwizard

Power balance test......unplug one injector at a time and see what happens to see if you can pinpoint one cyl causing less RPM drop than the others.

How consistent is your vacuum signal? A broken valvespring will cause your rough idle and a consistently inconsistent vacuum reading.

Tom400CFI

mlm0

thanks, I will try that test tomorrow. Just had valve jop done when head gasket blew, so valve spring should not be broken. This same miss has been there from day one, 4 years ago. It seems to get worse the longer it idles When I first stop at light, idle not to bad, but starts getting worse the longer I set