mlm0

1989 basically stock, auto, 53000 miles

i bought this car 4 years ago, if was in fairly good shape with 43000 miles. From the very first day, it has had a constant but erratic misfire. I notice it more at idle, but it could still be there at higher rpm, I just don't notice it. I have upped my idle to 685 rpm by making changes in Ecu, so I can live with it.
Any lower and I really notice misfire

if you were to look up my posts over the past years, you would see that I have adjusted, fixed, or replaced everything I can think of or what others had suggested. The only thing I can think of I haven't replaced is the distributor or the timing chain. I have another distributor, so when I feel better I will swap it out. Don't mind doing the work to replace timing chain, but I really don't want to drop oil pan

i have spent so much time and money on this car I love, but it has come to an end if I can't figure this out. I can't stand not having my corvette not running correctly, and I just cannot believe I can't fix it. There has to be an answer

please help me me keep my car, I need the wisdom of members on this forum to give me some ideas

Mike

Gibbles

Data logging would be very helpful.

Could be a bad sensor, computer picking up a knock etc.

I would start by disconnecting the spark advance, see if it idles better. If yes then it could be a knock, or even a simple bad knock sensor...

check sensors, look for corroded plug connections, and damaged sensor wires...

Common one is the cts sensor in the intake manifold, it was the first thing i replaced with the upgraded version, and newer style plug...
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mlm0

When say disconnect spark advance, are you talking about that same connection i unplug when I adjust timing.

I have not not done that but I will. I have replaced knock sensor once, the coolant temp sensor at least three times, and I have upped the speed and rpms in the ecru settings higher before the knock sensor would enable. Did not help

thanks for you response

Mike

RWDsmoke

If you want to drive it to San Antonio I would be happy to go through everything with you. I am retired and have the time. Larry

mlm0

i am retired also, and don't be surprised if I take you up on your offer if I can't figure this out

thanks for the response

Mike

dmaxx3500

you need to read ''live data'',using a scanner,you need to see which cyl is miss-firing,but there could be 20+ other things to make this happen,you need to get somebody that has worked on cars 20+ years ago to read this and ''NOT'' just throw parts at it,,have you ever done a compression test or a cyl leak down test?

cv67

RWD thats what its about, helping others

Dont have time to go through all your posts... a weak plug or wire can do that but when injectors are going bad even if they test good it will do that. Assuming head gaskets sound usually youll have symptoms with coolant levels even if it never smokes.
The injector thing fooled me drove me crazy.
set of bosch 2 later ran like it should
MAF sensor ok?

mlm0

i dont think hunk it is just one certain cylinder misfiring. I have not done leak down, but did compression test, about 180 in all cylinders I have tuner pro on computer and I have mt2500 but I guess I don't know how to use them to tell me if a cylinder is misfiring All plugs look great. Just blew head gasket, so had heads surfaced and valve job, but didn't change misfire. I have done several vacuum leak test, but found none. I did notice, if it helps that with engine running and vacuum gage connectted, the gage needle would slightly jump almost in time with misfire. Does not use any oil at all

thanks for your response and help

Mike

billschroeder5842

My quick thought is that your have a problem in your distributor.

The the fuel flow is good, compression good, valves are working then you have erratic spark issue.

If your plugs are good and wires are good, you could have a skip in the dizzy. Advanced timing, dirt, cracks in the electronics, slight internal angles, loose bushings.... ect... could cause a miss at idle.

BTW... you would, notice a misfire at higher rpms. That could mean a rounded cam lobe but my money is on a jacked up distributor.

mlm0

I replaced plugs and wires several times, replaced MAF last year. 3 years ago replaced injectors with Bosch 3, and about 3 months ago when I replaced head gaskets because I was losing coolant and white smoke out exhaust, I sent injectors to FIC to to be inspected and cleaned

mlm0

i have been thinking distributor myself myself and I have another used one to put in. I will be doing that in next few days. Not feeling very good last few days. It has roller cam I believe, so I doubt and hope it's not cam. I have also been thinking about timing chain? But with only 53000 miles, I wouldn't think so. The car, including interior and paint is like new

thanks for your response

Mike

BTW, if I figure this out, I promise to post answer to misfire

mlm0

[QUOTE=mlm0;1594180251]i have been thinking distributor myself myself and I have another used one to put in. I will be doing that in next few days. Not feeling very good last few days. It has roller cam I believe, so I doubt and hope it's not cam. I have also been thinking about timing chain? But with only 53000 miles, I wouldn't think so. What about a bad ground where it connects to block under oil filter. I noticed when idling the
Gauge panel lights dimming and then going brighter, back and forth The car, including interior and paint is like new

thanks for your response

Mike

BTW, if I figure this out, I promise to post answer to misfire. Won't leave thread hanging

RWDsmoke

I just sent you a private message.

mlm0

got it and responded. Thanks

C409

..... Hook your timing light to each plug wire til you find out if its a spark problem causing your mis-fire .....

ghoastrider1

Hope you get it fixed.. its a nice looking car.

ex-x-fire

It could be egr related, the egr shouldn't allow anything past it at idle/low speeds becauseit'll run rough or stall.

mlm0

me too. It's in such good condition except for misfire problem. I feel I can fix anything if I could just find out why it does it

mlm0

i thought that too, so I replaced it at first, then deleted egr by removing it, adding block off plates, reprogrammed ecu so it does not even switch on unless temp is above 300, which is not going to happen

Tom400CFI

Video of the rough idle?

Does it miss under heavy load too?