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Engine Timing Question, and Help with Miss.

jeepie

Well-known member
Joined
Feb 6, 2002
Messages
162
Location
Dunmore, Pa.
Corvette
1991 Coupe Blk/Blk
I have been having a problem with a slight miss at idle and a slight pinging under load for a while now. Neither is bad and the car runs well, but both of these things have been bothering me. Since it is winter here I would like to try to fix both. The car is a 91 auto with about 45K on it, I really don't use it much so the following things I have replaced over the last couple years are still like new. I changed the plugs, wires, cap, rotor, egr valve and solenoid, injectors, fuel filter, removed the two pre-cats and installed a new main cat, O2 sensor, and knock sensor. I had the chip reprogrammed to have the fans come on earlier and I believe the rest is stock. I noticed that when I set the timing (with the connector disconnected) that the timing stays right on the mark (6 deg), when I turn it off and plug the connector back in and check the timing it goes off the scale ( around 15 deg before) but it is not steady and jumps around 4 or 5 degs. Is this normal ? I also noticed when I unplug the connector the idle is not steady it goes higer and lower, but with the connector pluged in it is pretty steady, except for the slight miss, is this normal ? Can someone tell me why when the connector is plugged back in (computer takes over) the timing goes up that much ? It is the same idle condition, I would think the timing should be about the same, since the engine is just at idle. I also have cleaned the IAC and TB which did not help the miss at idle, anyone have any suggestions, the car has done both things since I got it, but like I said these are not bad, but bother me.
Thanks.
 
A lot of things might cause this, but I think I'd look for a bad plug or wire first. Have you run the car in the dark and looked for an arcing wire? What octane gas are you running?
 
If you have an after market chip I would see what the advance is set at .
When you have a chip installed all they like to do is jack up the timing and charge you a couple of hundred dollars.
I think your car has a baro sensor? not sure.
 
Yup, I have run the car in the dark and found no arching, I run 93 octane gas. Maybe I will try swapping another plug wire, one wire at a time and see if I find anything. The chip is the original with the temps changed, that was the only change in the programming.
The only other thing to note is the person who had the car before me must have had the battery leak at one time, the battery cables and a few of the frame ground connections were corroded. I changed both battery cables and cleaned and replaced the ground wires. The single wire connector comming off the pos battery cable was destroyed and corroded and not doing anything, I cut and soldered in a new connector. Could this have caused a problem with the computer ?
Thanks.
 
Hi jeepie,
just a suggestion. have you tried removing the chip from the computer (if it is a "Piggyback") and see if this corrected the problem? I had a '91 a few years back and had a bad chip. It did the very same thing. Also if the chip is installed wrong it will cause the engine to "Lope".
One other thing...in the HEI there is a ignition module. This could cause a problem too. Let me know if I can help you any further. I have a "book" with certain criteria for the '91 as I had bought the book when I had the '91. Just e-mail me at
scalestim@msn.com
God Luck! Tim
 
I have a 90 L98 with the same conditions but mine is all stock except I replaced all the ignition parts with Acccel items. It misses slightly at idle and has a slight knock when I start off lightly. Looking at the parts you have changed the only things left would be a vacuum leak, intake gaskets. When you solve it let me know it's somthing I need to do in the spring
 
Just my o2 but if you have good luck with Acccel ok, but I never have they put about 2 degree advance in there moduels in the dist. I would check your timing with the est wire disconected . Take the rpm up to about 3000 and see what the timing reads.Your vacum advance could also be bad ,or check the springs under the plate in the distributor the previous owner could have changed them to make the car advance quicker.
 
I bought the car in May of this year. The ignition system was bone stock with 84K on it. The distributer cap was so bad that the carbon rotor button was held in place with just the spring tension and the plastic that was suposed to hold it in was burnt away. But it ran ok with it that way. It runs great with the new Accel parts except for the miss at idle and the minor knock.
 
I believe that your year distributor does not have a vacuum advance, nor advance springs (my 1990 L98 has neither one). I had a similar problem and replaced the ignition module in the distributor, cleaning the existing wire connectors before installing. This corrected my problem.
 

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