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Old 12-10-2004, 10:38 PM
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350 TBI backfire

Hey, new guy here. my 88 chev k2500 has a slight problem, whenever I accelerate (moderatly or heavy) i hear a poping sound. Whenever it pops the engine cuts out for that split second. It does this while giving it steady throttle down the highway also, only not as often. Also, the tach needle bounces, just sharp jumps about 200 rpms up, the back down again. It stops bouncing at about 3000rpms. I can't hear any increase or decrease in actual engine speed, so im assuming it must be electrical. I was thinking it could be a faulty pickup coil, or a bad pickup signal wire. Got any ideas? It didn't start this until recently, and its a real annoyance.

Also,
I rebuilt the engine about 2 years ago, ran perfect up until now. All stock except I put flat top pistons in.

Thanks

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Old 12-10-2004, 11:44 PM
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Backfire

Based on what information you gave if you brought the vehicle into the Dealership the first thing I would do is make sure of the condition of your ignition system. I'm assuming this is a factory stock ignition.
Check the basic's, plug's & wire's, cap, rotor. Look for corosion or carbon arching traces. Most of the time a pick-up or module are either working or not, however I have encountered intermittent problems with pick-up's but very, very few. If your's has the remote mounted coil remove the wire and check the coil tower, rusted looking or heavy powder on it and it's time to be replaced & the wire also.
If that check's okay I would scan the on board computer paying attention to TPS voltage signal on light throttle application.
Look also for any fault codes that might affect this problem.
Also check the charging system and make sure you are not getting an overcharge above 15v that is creating a power surge. I have had that many times. And it can damage the ECM. Check all grounds also and the ECM power wire coming off the positive battery cable for being corroded and breaking loose.
The only other problem I can remember causing a quick intermittent stumble was a EGR valve. I spent about 2 days on one tearing the secondary apart looking for this split second action that seemed like a miss because it did stumble the tach.
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Old 12-11-2004, 07:41 AM
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alright thanks, ill check out the things you mentioned today. also i forgot to mention that ive installed a new coil, cap & rotor.

thanks again
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Old 12-11-2004, 10:40 AM
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Also a lean condition can cause a backfire. But you should notice a drop in power if the fuel press was dropping under a load. TBI needs 9 to 13 psi to operate properly driving the vehicle with a fuel pressure gauge taped to your winshield would be the next step. But I would do Restore60's tips first Its probably is a Ign problem.
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Old 12-11-2004, 05:03 PM
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ok, last year i put new wires, cap & rotor on. and about 2 months ago i installed a new coil. i tested the pickup coil, 840 ohms, (spec say 500-1500 ohms). i checked the signal the coil produced. it was good. everything else seems to be ok. later today i gunned it (from a stop) and it started poping real bad, it almost killed the engine completly. i can also get it to pop if i try doing a breakstand.
while i had the scanner on it today, i was watching the O2 sensor data, it switching rich to lean as often as other vehicles ive seen. it stayed lean for about 10-15sec. then rich for no more than a second. then back to lean again, and so on. any other suggestions?

thanks
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Old 12-11-2004, 08:24 PM
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I had a customer with a 1982 305 that would pop.It turned out to have a flat lobe on the cam,and it also had 260,000miles on it.
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Old 12-12-2004, 12:10 AM
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To get an accurate reading on a O2 sensor you need to run around 2000 rpm to get it hot enough for a proper reading and that still isnt an indicator of a bad fuel pump. You must use a fuel pressure gauge. Another possibility is a clogged cat converter. Remove your o2 senser and run it this would relive enough pressure to tell an improvment. Also check your base timing should be @0 deg with the bypass unhooked. Also try to get a spark tester like a st125 this will tell you if there is a problem with the spark strength running with weak spark will give problems.
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Old 12-12-2004, 11:49 AM
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ok, ill check my fuel pressure tommorow, i already replaced pump last year. and i havn't got a cat any more.
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Old 12-13-2004, 10:24 PM
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fuel pressure is good. what the hell is ****ed on my truck? tons of power when its cold, but once it warms up, sounds like a damn popcorn machine.

DEAR LORD PLEASE SHED SOME LIGHT!
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Old 12-14-2004, 07:00 PM
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TAke a look at you scan data if you have a scanner and dee if the coolant temp sensor is staying cold.
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Old 12-14-2004, 08:26 PM
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I say I see a new set of wires in your future...?
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Old 12-15-2004, 10:52 PM
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thanks for the suggestions guys, but i just replaced the temp sensor (for ECM) last week, and the wires arn't even a year old. replaced them with the cap & rotor.

just put some new plugs in last night, and a new 02 sensor. still poped. but strangly enough, this morning and all day the tach wasn't bouncing and i couldn't get it to pop... until later this evening, the tach started bouncing again

i thought maybe the EST module was fried so i disconnected the timing connector (disable the EST), and it still poped just as bad with it connected (this was last night)

once again guys thanks for all the input, its giving me different ideas as to what the problem could be...
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Old 12-16-2004, 12:09 AM
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I am currently having the same problem. I am sending the truck into the shop. It would be either A. Cracked Heads B. Blown Head gasket. I know you really dont want to hear that, but my 91 blazer does the same thing and I am burning coolant and also the plugs i pulled out had a green tint. It's something to consider, I know you dont want to hear that but neither did I.
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Old 12-16-2004, 10:02 AM
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it pops like pre ignition or it hesitates like you have to convince it that it wants to go
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Old 12-16-2004, 11:57 AM
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cracked heads huh, yikes! blown head gasket wouln't be so bad.

it hesitates like i have to convince it to go.

and now this morning the tach didn't bounce, and it had lots of power. i left it running for a while to warm it up and when i got back in the needle was bouncing. but it didn't pop, only once when i gunned from a stop, then i took off.
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