Greetings all,

My LT1 runs fine normally, but when it goes into overdrive and I give it a moderate amount of gas it starts misfiring badly. If I go ahead and give it enough pedal that it downshifts it runs fine. It also does this sometimes at very low speeds, and again if I punch it on down it runs fine.

I do not smell any gas when it starts misfiring.

The plugs, opti and plug wires have about 25K on them. The coil is new.

I have had it scanned twice for any trouble codes and there are none. Any ideas?

 

EXACTLY the same symptoms as my 95. When it first happened a couple of years ago, dealer replaced opti and either plugs or wires (not both). The problem remained and a couple of months later, the dealer again replaced the opti and whatever he hadn't replaced the first time (plugs or wires). Problem was solved for a couple of years but has slowly come back--miss at idle and miss or backfire at low rpm under load

I just replace the coil and the car is definitely better but not 100%. I noticed a bunch of corrosion at the opti-to-coil wire and cleaned up the end that plugs into the coil (could not easily remove the other end to inspect that but suspect corrosion may be there as well)

You will find umpteen other forum members posting similar problems at various times. There does not seem to be a consistent solution. I recall that one guy changed everything but the kitchen sink and the problem was still there. Oftentimes it turns out to be something simple like a bad wire or plug.

Let us all know what you find.

 

and heres how it went for them. If they bring it and say, its the opti spark, replace it, and the water pump while your at it, I do what I'm told and do it....

Having said that, quite often that is not the cure. Quite often they over look a cheap part, the fuel filter....for $4.00 at Advanced auto it has cured more than a few of them when no codes are showing or even when some are and it just shuts down...

To weeks ago, a 93....a women friend of my wifes who likes to drive a vette in summer......it would start fine at first.......later it would start hard at times, other times, after she restarted it, took off it would backfire, shut off, then pick up again in x amount of seconds run fine then maybe do it again, maybe not......Next time it might start skipping, backfireing right away after it was warm.......

Now I guess I could have said, ok, opti spark, but for $4,00, 3 open end wrenches and 30 minutes of your time if you can get under it, I have a lift its a cheap and fast change. It looked to have been on there along time...Filter was dirty enough that I could barely blow air thru it....I let it dry out, then kept tapping it and rust from her tank or fuel lines was falling out of it.... Maybe it was the original filter, I don't know.......but I know this.....It stopped doing what was actting just like and opti spark problem..... I'm not pointing fingers but its a well know problem with opti sparks in 92 thru 94 cars.... Do you think someone might exploit the problem on some? Its looks hard to change with the cat in the way, but on a lift and can be done with the cat on.... The front bolts are the same as the back ones so the wrenches will work fine...You don't need to see that connection, its like the rear one. Loosen the one nut holding the fuel lines under the hood on the right side and wala, its off.......put new one on, make sure its tight,as its right over the cat and thats it.......

If it don't fix it,well, the filter is clean and your only out $4.00 and 1/2 hour of your time if your used to working on cars....a little more if your not.....

Why start at the most expensive part if no codes are showing? Thats my 2 cents on it......That and another $1.48 will get ya a coffee.....

Good luck in your pursuit of the problem.....

I say the above because I've had a lot of women friends of my wifes, and guys, who know ZERO about any care, come in and say, oh, its the OPTI......they heard it said enough that it must be so.......

If the codes are showing that, by all means do it......but otherwise.....us Swamp Yankees go for the cheap repairs first that in the end of wrong will only hlep it keep running better...

 

Before replacing the OptiSpark - change the plugs and wires first. I had the same symptoms, bought a new opti, but changed the plugs and wires first and symptoms went away before U got the chance to replace the opti. Could save you a couple bucks.

 

This exact thing happened to my '95. Diagnosis was easy using a HEI spark tester and the flow chart in the service manual. First I tested the coil output, it was fine. Then I tested the output at each plug wire, no problem there either. No OPTI codes were set. Service manual said to replace the plugs. I yanked the plugs and found the platinum "puck" was missing from all plugs (they were original factory A/C plugs). The gap was opened up to 0.070" or so. I replaced them with a set of NGK platinum, gapped properly, and I haven't had a misfire since.

In your case where you recently changed your wires and plugs, maybe it's the coil. Get yourself an HEI tester (?ST-45? is what GM recommends) and check the coil output, then plug wires as I did. The ST-45 is relatively cheap. I just borrowed one from a mechanic buddy.

I'd bet good money the OPTI is fine.

 

I like coil... I love coil wire (they have a tendency to chaffe a bit) and the plugs - the AC Delco Platinums are JUNK.

BTW, the gap grows .065 when the puck falls away.

 

My 93 was doing the same thing. I didn't have the time to play with it so I took it to the dealer. Coil was going bad and the coil wire was shorting out. That seemed to take care of the problem for a bit but it started back up again only not as bad as before. When I bought the car the owner gave me a work order/receipt for a major tune up done just a couple months before. New plugs, filter changes, etc. The miss always seemed like a bad plug miss, not a problem with the ignition system. Discounted it because it had just had a tune up. Finally I pulled all the plugs and found out that the shop had changed the plugs on the passenger side bank but left in what looked to be the original plugs on the driver side. They had a gap big enough to drive the car through. No wonder it missed under a load. The shop had ripped him off big time.

Who replaced the plugs, you or a shop? I know it's a pain but you might want to pull them again and check. Could be a bad wire too. Under normal load it will work fine until you get on it hard and then it will arc and miss. Again, double check that coil wire.

 

Thanks for the replies.

The fuel filter is new (2 weeks old), as is the coil. The local dealer replaced the opti, plugs and plug wires. I think the only other parts that aren't fairly new are the EGRs and the O2 sensors, might go ahead and replace those too, and I will check the coil wire. Would O2 sensors contribute to the misfires?

After reading the posts I am beginning to suspect the plugs, as they are the AC Delco platinums that a lot of people complain about.

Should I use an anti seize compound on new plugs? I've dealt with Chevy engines for 30 years. Isn't the LT1 block aluminum?

 

Use anti-seize compound and buy a set of NGK TR55 spark plugs. I've been running them since May and they are excellent!

 

I will try those plugs.

thanks everyone.