Constant Backfiring and Flames?!

[Michael88STX]

1988 2.9L, Pacesetter headers, gutted cats, 2.5in straight pipe from cats back. Just recently started backfiring (sounds like a 12 gauge shotgun going off) and spitting flames like crazy whenever I down shift to slow down or even just let off the throttle in 5th gear on the highway at 70mph ever since it broke down on me about a month or so ago. It cut off on me and i couldn't get it back started again. Ended up being I just needed to replace the battery cables because they were going bad and didn't have a good enough connection. Anyways, also whenever I rev the throttle up all the way to about 4,750-5,000 rpm and let off it backfires incredibly loud every time. Anyone have any ideas as to what could be causing this issue? Also I've had the headers and custom exhaust on this truck for about a year now since I put the new motor in so I don't see how either of these could be the issue.

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[BrutalPanda]

Put a muffler on it and make sure you have the distributor on the right spot

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[adsm08]

I agree, first things first, check the timing. That sounds like it is sparking off while the exhaust valve is open.

 

[Michael88STX]

Put a muffler on it and make sure you have the distributor on the right spot

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Muffler is on it's way, ordered it earlier this week. Will update this weekend once I get it on there

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[Michael88STX]

I agree, first things first, check the timing. That sounds like it is sparking off while the exhaust valve is open.

Good idea, don't know why I didn't think of that (duhh). I'll take a look at that asap and post an update

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[BrutalPanda]

I've showed how Ive built my rangers motor in a post on here, similar to yours. Just got another 2.9 last week I have to swap parts over to. Muffler alone made my popping go away, but when it did pop it sounded healthy and nasty. I pulled up to a Corvette who was obviously playing around and let off at 3500 rpm and he wanted nothing to do with my old truck lol. He would've waxed me no doubt,but with good pipes that motor sounds like far more of a sleeper than it is. But a good muffler tones it down to almost a quiet V8 rumble at idle and still loud at WOT cause it's a sweet German, flat plane motor

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[Bgunner]

Years ago I had a 83 chevy S-10 with the 2.8 built with a high lift cam. It had no cat and I put a thrush tubular muffler on and I would get popping with flames but this was due to being carbarated. When lifting off unburnt fuel would build up in the exhaust then be ignited by the hot muffler causing the popping and cool blue flames out the tail pipe.

I agree with checking the timing but if not off this is the most likely culprit.

 

[dirtymike]

timing

 

[BrutalPanda]

Don't play with the timing until you add a muffler, my muffler retarded my timing and I had to reset it again, but if you are comfortable moving the distributor it'll only take you a few minutes

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[Michael88STX]

UPDATE 11/23: By this point I've added a muffler and checked timing, still nothing. Thought maybe distributor was going back so I swapped it out with a good one, still popping and spitting flames like crazy.

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[Paulos]

It's been a long time, but I had an '86 with a 2.9 that back-fired out of nowhere and blew up the muffler. I think that I replaced the ignition coil (and the muffler) but I'm not certain.
You replaced the distributer without any change, so it's not the pickup coil. Was there a different ignition module, cap, and rotor on the distributer you swapped out? Have you checked the ignition coil? How are your plugs? Plugs that are worn too much can cause unburned fuel to detonate inside your exhaust system. Check your plug wires also.
Does the engine idle and operate normally at low RPMs?

I would first check the ignition coil. You should have 0.3-1.0 ohms on the primary side (+ and -), and 8000-11500 ohms on the secondary side (coil wire terminal to either the + or - terminals).

 

[Bgunner]

Also can't hurt to make sure your not running the fuel on the rich side since you say the timing is correct. As mentioned before the build up of fuel in the exhaust will cause this to happen. I used to do it just to scare my friends standing next to the exhaust outlet.

 

[Michael88STX]

Also can't hurt to make sure your not running the fuel on the rich side since you say the timing is correct. As mentioned before the build up of fuel in the exhaust will cause this to happen. I used to do it just to scare my friends standing next to the exhaust outlet.

I had an issue with it running really rich in the past but it isn't as bad as it used to be but it definitely still runs a little rich. I get about 10-12 mpg and you can smell it's rich

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[Bgunner]

OK before replacing anymore parts I highly recommend adjusting the fuel/air mixture to get the proper ratio then see if the issue, popping and flames, is reduced. I had my S-10 when I was 16 and my brother, a mechanic at a local shop, adjusted my carb to get through inspection and reset it properly after. I still got the popping and flames but as he said at the time it wont hurt it but it was caused by the fuel building up in the exhaust because it didn't have a cat on it to remove a bunch of the unburnt fuel from the exhaust.

 

[Paulos]

How do you suggest he adjusts the air/fuel mixture on an EFI engine? I don't think you can do this without a chip.