I'm re-reading this and I realized none of us have mentioned exhaust leaks. If you have an exhaust leak ahead of your O2 sensor, you'll get a bad mixture, and it'll also cause the backfiring.
91 350 tbi extreme exhaust backfiring
- Thread starter ChevyGuy572
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ChevyGuy572
Newbie
I re-tightened the headers multiple times, I'm guess the exhaust gasket is shot. I might try replacing them to see if that helps at all. But it seems like each cylinder backfires every other exhaust stroke even if it's cold.
ChevyGuy572
Newbie
*I'm guessing the exhaust gaskets are shot*
CrustyJunker
Is STILL Here?
Think I know the issue. Your PROM isn't compatible with the computer it's installed in!
Your PROM sticker (ARJW) shows a calibration for a 1991 TBI 350, manual trans (M20 option code, heavy duty?) and it looks like it's supposed to have broadcast number 16147496. An 1228747 ECM with C2 architecture.
From your pictures, that ECM appears to be remanufactured as a 1227747, which requires C1 architecture. Externally, they'll look the same and even plug into each other, but those little PROMs don't interchange and won't talk to the computer correctly.
Might want to find an old 1228747 ECM, or find a suitable remanufactured replacement for that exact ECM number - no substitutes!
That code 51 will disappear after that.
Your PROM sticker (ARJW) shows a calibration for a 1991 TBI 350, manual trans (M20 option code, heavy duty?) and it looks like it's supposed to have broadcast number 16147496. An 1228747 ECM with C2 architecture.
From your pictures, that ECM appears to be remanufactured as a 1227747, which requires C1 architecture. Externally, they'll look the same and even plug into each other, but those little PROMs don't interchange and won't talk to the computer correctly.
Might want to find an old 1228747 ECM, or find a suitable remanufactured replacement for that exact ECM number - no substitutes!
That code 51 will disappear after that.
CrustyJunker
Is STILL Here?
Question 2, I was looking at that M20 option code. Do you have the M20 option code in your glovebox, or MG5? Half ton stepside in '91 I'm not sure was optioned with the M20 trans.
Stepside 92
Newbie
Drunk
drunkCanuck is corrrct on the ecu. If they didn’t replace it with 5.7 ecu it will do stupid ****. Experienced this with a 305 to 5.7 swap of my own. Replaced with 5.7 ecu from donor motor truck. Worked for me.
Also check the timing . Mine would back fire with deceleration and during upshift also. Stock timing is 0 degrees. I advanced mine to 5. Truck came alive. Remember to check and set the timing with the brown disconnected. Then plug wire back in after adjustments.
ChevyGuy572
Newbie
I will find a ecu for a 5.7 and see if that helps. I've always just timed it by ear since I think the rotor might be off by a tooth or two. Would that effect the way a timing light works? It always sounded like it runs super rough at tdc.
Yes, get that fixed 1st!
Yes. One wonders if being installed in the wrong computer would damage the PROM.
Mistake.
A tooth, or two, or eight, or whatever...makes no difference at all provided the distributor housing is rotated; and/or the spark plug wires are rearranged to suit.
No. They flash when the inductive pickup detects the voltage pulse of the spark.
Fix what's wrong. Which in this case, may be basic incompatibility of PROM and computer. But who knows? There could be more to it than that. Or not.
ChevyGuy572
Newbie
Nevermind, I don't think it would mess with the timing light, the cap would just be turned slightly more than normal. Idk I'll set it a tdc and actually drive it to see what happens.
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