Hi all!
My beautiful car is driving me crazy. I posted late last year about a backfire problem and got some great advice but no cure yet. I have had the following done to my 5.7 Z28-
New Fuel Pump and filter, New Plugs, Injector, fuel rail and throttle body clean, Still Backfiring!
Now have gone back to mechanic and he has replaced the Optispark (Ouch) but mine was in terrible condition (cracked), replaced plugs and tested everything. Still Backfiring, but now even worse. You can gently accelerate and it will pop 8-15 times and really struggles.
Went back this morning and he replaced coil and module and the car is as bad or even worse.
It's such a strange problem, the car starts and idles beautifully, pulls away from a stop quite well till it gets to around 1500rpm and then backfires all the way thru 1500-2200 and steadys out. If you forget to be gentle and just boot it, it does a little or no backfire. It then accelerates like a rocket, breaks traction and flies with no apparent miss or problem with high revving, it just keeps on pulling to very high speeds, but you can't drive it like that in the wet.
My mechanic did suggest that it could have been modified some time before I bought it, as he felt it went a lot harder than others he had driven. He thought possibly a warmer cam could have been fitted incorrectly which would explain the bad rev range.
Any ideas out there. I am desperate and he is also highly perplexed. Anything anyone has found causes this problem is something we could look at.
Many thanks
Petra.

Hi all!
My beautiful car is driving me crazy. I posted late last year about a backfire problem and got some great advice but no cure yet. I have had the following done to my 5.7 Z28-
New Fuel Pump and filter, New Plugs, Injector, fuel rail and throttle body clean, Still Backfiring!
Now have gone back to mechanic and he has replaced the Optispark (Ouch) but mine was in terrible condition (cracked), replaced plugs and tested everything. Still Backfiring, but now even worse. You can gently accelerate and it will pop 8-15 times and really struggles.
Went back this morning and he replaced coil and module and the car is as bad or even worse.
It's such a strange problem, the car starts and idles beautifully, pulls away from a stop quite well till it gets to around 1500rpm and then backfires all the way thru 1500-2200 and steadys out. If you forget to be gentle and just boot it, it does a little or no backfire. It then accelerates like a rocket, breaks traction and flies with no apparent miss or problem with high revving, it just keeps on pulling to very high speeds, but you can't drive it like that in the wet.
My mechanic did suggest that it could have been modified some time before I bought it, as he felt it went a lot harder than others he had driven. He thought possibly a warmer cam could have been fitted incorrectly which would explain the bad rev range.
Any ideas out there. I am desperate and he is also highly perplexed. Anything anyone has found causes this problem is something we could look at.
Many thanks
Petra.

I would have said it was the fuel pump but you've already changed that. Try and post this on LS1tech.com and see if you get a good answer.

Check the connector that plugs into the optispark. It will be on the passenger side of the throttle body, check and make sure the pins are clean and not GREEN, we used to have problems with this back then causing bunches of problems. Also disconnect the vacuum hose going to the egr and see if that changes things. There used to be a recall on the egr's and if the spring is week in it that can also cause this problem. The solution is going to be a simple one not a major one.

i agree with tumper on the egr...it causes a lean condition if allowed to open early...

Hi, thanks for the ideas, I will get him to check the egr (whatever that is) and someone also suggested MAF sensor or O2 sensors. What do you think. Anything else that may cause this problem?

If the maf is causing a problem you can disconnect it and see if the problem continues to happen, but your problem doesn't sound like a maf problem. The o2 won't cause a problem like you have either, I have seen them go to 0mv and cause idle issues but other than that no, but there is always a possibility though. Check the egr like I said first, the way it works on a lt1 is it allows exhaust gases to flow from the passenger side exhaust so the lean condition would be on only that bank, the reading would be scewed.

I'm going through the same problems with my 94 Z28, Check the Cat, I'm in the process of hollowing it out right now and I will see what happens, I've also heard that it might be the MAF Sensor or possibly the whole computer but the O2 sensor is before the Cat and it won't show anything wrong because the problem happens after the sensors. Do you get a check engine light? I don't. also look into when the last time your distributer cap/roter were changed, its a big job but might be worth it. Anyway I'll get back to you after I reinstall the CAT.

It is not the CAT. I had that problem and there was a general lack of power and the car sounded quieter... like a vacuume cleaner a bit...

The car runs well when he really gets into it... so a midrange misfire... the EGR is a really good place to look.

I worry about the mechanic a bit... no disrespect intended.. you have thrown a lot of expensive parts at the problem..

sometimes the GM dealer is not a bad place to go with these complex issues... they solved an OPTI issue for me way back in 96 when they were just beginning to fail, and solved my plugged CAT.

good luck