Mystery Backfire when not Under Load
Thread Starter
Staging Lane
Joined: Feb 2019
Posts: 79
Likes: 5
From: Houston, Texas
Mystery Backfire when not Under Load Hey guys, I've got a misfire/backfire on my 94 Firebird Formula that has me completely stumped. It first started a few weeks ago after I replaced all my spark plugs with new NGK TR55s. I noticed that occasionally at idle, and almost constantly when revved up around 3k rpm when in neutral, I'd get several loud pops in the exhaust. I disabled my fuel injectors individually with EEHack and discovered that it was being caused by cylinder 2. Come to find out, I'd been sloppy when putting the plug wires back on and the header had burned through the insulator. I wrapped it in electrical tape and all seemed well for a couple of days. I replaced my header gasket a few days later, and the backfire was back, behaving the same as before. I've replaced the plug in that cylinder with another NGK TR55 with no improvement, as well as replacing the plug wire with a new Taylor 8mm one with only minor improvement. As of now I can't think of anything else I could add or do to further diagnose the issue. Every component of the ignition is pretty new and high quality (either MSD or Taylor) except for the optispark itself, but I don't think it's likely to be the issue as only Cylinder 2 isn't functioning as it should. If the #2 injector is disabled the backfire immediately stops. Anyone have any ideas?
TECH Resident
Joined: Jun 2013
Posts: 853
Likes: 32
From: Kernersville, NC
If you haven't replaced the burnt plug wire with a new one, I'd do that first.
Edit:Oops, missed that you did that.....
How does it look on a compression check? Have you tried swapping the injector from one cylinder to another?
Opti is always a concern on misfires. Scoping it is a good thing to do. You can see a lot by looking at the firing pulse of the plug. Relative compression check with an amp clamp and scope can tell you a lot too.
Have a look at these...
Edit:Oops, missed that you did that.....
How does it look on a compression check? Have you tried swapping the injector from one cylinder to another?
Opti is always a concern on misfires. Scoping it is a good thing to do. You can see a lot by looking at the firing pulse of the plug. Relative compression check with an amp clamp and scope can tell you a lot too.
Have a look at these...
Last edited by ACE1252; Apr 6, 2019 at 02:45 PM.
Thread Starter
Staging Lane
Joined: Feb 2019
Posts: 79
Likes: 5
From: Houston, Texas
I swapped the injectors between cylinders 2 and 4 and it made no difference at all, the backfire still quits when cylinder 2 is disabled. I also used the EEHack test utility because I don't have the proper gauges for a compression test, and it ironically showed that cylinder 2 is the strongest out of all of them. I'm also completely positive the plugs aren't backwards, I have wire retainers over the valve covers making it impossible for the wrong plug wire to reach a different cylinder.
