Sup guys. My 88 k2500 with 350 always backfires when I rev it up in neutral. Does it a little when accelerating hard and letting off the gas. But if I rev it up high it won't do it. I found a small exhaust leak upstream (soot buildup). Welded it, no change. It has no cat, free flowing 3in to 2.5 single exhaust with flowmaster. Hooked my snap on brick to it and it shows its running good. It does have air injection and wonder if that's what's causing it.
Backfire out the exhaust
- Thread starter gric_az
- Start date
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Is your engine stock, any mods? Did anyone "tune" your ECM?
AIR injection shouldn't be the root cause of a backfire. To make sure, you can disconnect the vacuum line and plug it at the TB. Check your initial timing, s/b 0* with gray/black computer control wire disconnected. Shine a timing light (or flashlight) down your TB throat and look at the injectors' spray pattern. They should be an even cone without any drips.
AIR injection shouldn't be the root cause of a backfire. To make sure, you can disconnect the vacuum line and plug it at the TB. Check your initial timing, s/b 0* with gray/black computer control wire disconnected. Shine a timing light (or flashlight) down your TB throat and look at the injectors' spray pattern. They should be an even cone without any drips.
Unsure if it's been tuned, I would assume not. I cannot find the timing disconnect in the usual locations. I had a 94 before and it was by blower motor. Timing was one of my first thoughts, but it runs cool even in the summer (az). I'll try they vacuum deal after work
Backfiring is caused by unburned hydrocarbons collecting and igniting from exhaust temperatures. Your problem could be as easy as a new set of plugs are needed. It could be a working but slow moving O2 sensor if it has one. I would start with a full tuneup including a new coil.
evilunclegrimace
Does not always play well with others
Most of the time the EST Bypass wire is under the black plastic T shaped cover on the Passenger side fire wall( where the FP relay is). The other place to check is the loom that is next to the P/B vacuum booster
I've had the truck for 4 years. Did a full tune up when I bought it. Along with new brake everything, everything steering and ac. When my scanner is hooked up. It goes into closed loop after about 2 minutes from a cold start. Even with the free flowing exhaust. The scanner lights will go on and off indicating lean/ rich after going into closed loop
I would at least change the plugs to see if that fixed the backfiring problem. If they have been running in higher than normal fuel situation they may be giving a weak spark. That won't fix the excess fuel problem that may be causing the plugs to foul, but it will at least eliminate or point the finger to the problem.
A Lean/Rich indicator in closed loop leads me to believe its an O2 sensor like JackE said already.
The O2 sensor feedback resembles a SIN wave and as it ages that SIN wave gets longer and longer until it is bad enough to throw a Lean and/or Rich code. But before it gets to that point the Wavelength has been extended and unresponsive for quite some time causing the engine to be progressively less efficient. They are often overlooked until it gets bad enough to cause more severe problems. Thats why for me O2's are just another part of the 'general tune-up' just like plugs/wires/cap/rotor/etc.
I should have said 'full tuneup including coil and O2' I always change the O2 and coil with a tune up. My son had some really bad rattling in his V8 Jeep that was getting progressively worse. He finally gave in to my advice and replaced the coils with a high performance set and the rattling/knocking noise is now gone. I told him over a year ago to do a 'full' tuneup. He is finally listening.
