1991 C4 internal backfire
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6th Gear
Joined: May 2024
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1991 C4 internal backfire
Hi all. I’m new here and have a 1991 C4 in mint condition. Runs like a dream but when I reach 3500 rpm, it has internal backfiring and I cannot figure out why. Any suggestions? I’ve ruled out spark plugs and fuel. Thanks!
Melting Slicks
Joined: May 2014
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From: New Port Richey FL
So it sounds like you did the this Ain't right so you changed the plugs, which isn't a bad thing. So where they dirty, Tired?
Back firing is usually a timing problem, cracked/dirty cap cross arching, Knock sensor problem, or a incorrectly set timing (someone didn't disconnect the advance wire setting) so do a little looking
You also said it's Mint, so is a Carpet Garage Queen that never saw a full throttle, because your heads and valves could be Carboned up.
Back firing is usually a timing problem, cracked/dirty cap cross arching, Knock sensor problem, or a incorrectly set timing (someone didn't disconnect the advance wire setting) so do a little looking
You also said it's Mint, so is a Carpet Garage Queen that never saw a full throttle, because your heads and valves could be Carboned up.
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6th Gear
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@s carter correct 😂 I still have yet to check the distributor. I did check the pvc valves and one was completely plugged so I cleaned it and put it back on. No difference. I’ll shoot for the distributor itself. I only put premium gas in it and I always stabilize it before storing in the basement in the winter. Thanks a lot for your feedback! Much appreciated!
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@s carter 2/2 Plus, while I am certainly far from a garage queen (but I know what you mean!) I mostly deal with heavy equipment mechanics. I figured after checking all of the obvious things, maybe some folks here who have owned these C4’s longer would have some insight to other things I didn’t think of 👍🏼 Thanks again!
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6th Gear
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One or both pre-cats have disintegrated and plugged the main cat.
The primary clue is that the "breakup" doesn't occur until 3,500 RPM, and that it occurs consistently at a specific RPM. Another clue that it is exhaust restriction is that you changed the spark plugs. Restricted exhaust mimics "bad spark plugs" or other ignition maladies exactly.
Put a vacuum gauge taped to the windshield on it and go for a drive. If you see manifold vacuum drop markedly, or to zero at only partial throttle and loading, the exhaust is restricted.
No one thinks of "restricted exhaust" because it so closely mimics fuel or ignition maladies. This comes up on here from time to time on the 86-91 L98 that still have their now 30+ year old pre-cats on them.
The primary clue is that the "breakup" doesn't occur until 3,500 RPM, and that it occurs consistently at a specific RPM. Another clue that it is exhaust restriction is that you changed the spark plugs. Restricted exhaust mimics "bad spark plugs" or other ignition maladies exactly.
Put a vacuum gauge taped to the windshield on it and go for a drive. If you see manifold vacuum drop markedly, or to zero at only partial throttle and loading, the exhaust is restricted.
No one thinks of "restricted exhaust" because it so closely mimics fuel or ignition maladies. This comes up on here from time to time on the 86-91 L98 that still have their now 30+ year old pre-cats on them.
hey thanks so much! I recently had some exhaust work done so I wonder if some debris has gotten into the system. Spark plugs were all clean. I appreciate your time and input. 🙌🏼
Thread Starter
6th Gear
Joined: May 2024
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One or both pre-cats have disintegrated and plugged the main cat.
The primary clue is that the "breakup" doesn't occur until 3,500 RPM, and that it occurs consistently at a specific RPM. Another clue that it is exhaust restriction is that you changed the spark plugs. Restricted exhaust mimics "bad spark plugs" or other ignition maladies exactly.
Put a vacuum gauge taped to the windshield on it and go for a drive. If you see manifold vacuum drop markedly, or to zero at only partial throttle and loading, the exhaust is restricted.
No one thinks of "restricted exhaust" because it so closely mimics fuel or ignition maladies. This comes up on here from time to time on the 86-91 L98 that still have their now 30+ year old pre-cats on them.
The primary clue is that the "breakup" doesn't occur until 3,500 RPM, and that it occurs consistently at a specific RPM. Another clue that it is exhaust restriction is that you changed the spark plugs. Restricted exhaust mimics "bad spark plugs" or other ignition maladies exactly.
Put a vacuum gauge taped to the windshield on it and go for a drive. If you see manifold vacuum drop markedly, or to zero at only partial throttle and loading, the exhaust is restricted.
No one thinks of "restricted exhaust" because it so closely mimics fuel or ignition maladies. This comes up on here from time to time on the 86-91 L98 that still have their now 30+ year old pre-cats on them.
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6th Gear
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Could chunks from the main cat have made it into the mufflers?
Do you have a scan tool? Drive it while monitoring MAP voltage. If MAP voltage rises rapidly with partial throttle and maxes out at mid throttle, the exhaust is restricted.
Have you inspected the center-button in the distributor cap? I've seen the center button completely erode away. That can cause misfire...
Do you have a scan tool? Drive it while monitoring MAP voltage. If MAP voltage rises rapidly with partial throttle and maxes out at mid throttle, the exhaust is restricted.
Have you inspected the center-button in the distributor cap? I've seen the center button completely erode away. That can cause misfire...
