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Old 03-19-2007, 08:58 PM
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chevy 4.3l help-detonation, running rough

Just bought a 01 chevy express w/ a 4.3L. It ran good for a day then the check engine light. It is skipping and backfiring in the exhaust. I here the loudest detonation I think i've ever herd. and it stutters, lopes, etc. I took it to a shop, witch was a first for me. I just didn't have the time to diagnose and fix. Their scanner read a problem in the crankshaft sensor circuit, so they installed a new sensor.
After driving the van a few miles it started acting the same way.Engine light came back on, and more detonation. I then ran my own scan and got code p0335 - no signal from crank sensor for more than three seconds. I don't see how the engine could run w/ no signal for 3 seconds. It starts and idles fine.It's never stalled out on me.
Next I did a ignition system tune up to eliminate that. Same rough running.
I then got out the old timing light just to look. It's firing at a few degrees BTDC at Idle. oddly though it seems to be firing somewhat randomly, firing slow briefly then very fast briefly. All while at Idling at steady rpm's, and always a little before TDC. ??? Also, when I gave it gas, the timing dropped way advanced for a split second then way retarded. Is this the knock sensors in action trying to eliminate the detonation/preignition?

Is the computer is giving false reading on the crank sensor circuit. The engine won't run w/ no reference signal.Correct?

I was hoping that someone has run into the same problem.
Any advice would be much appreciated
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Old 03-19-2007, 09:18 PM
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did you reset your computer? it maybe still trying to read the crank sensor errors.... or the idiot machanics didnt change it and just reset your computer to make a quick buck. try reseting the computer and if i does the same thing a few miles around the block then they didnt change it or you have gotten a defective sensor.
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Old 03-20-2007, 12:20 PM
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I erased the trouble codes. Is this the same thing as resetting the comp? There is a new sensor installed, I checked. I also checked the connections. I also unplugged it to see what would happen, the engine stalled. I thought maybe the computer used a default signal in the event the sensor fails. Not the case.

The oddest thing is how its firing. Slow, then fast, then slow again, all at a steady idle.

Could the reluctor wheel be out of place or wobbling somehow? Could the coil be failing?
It has compression, and its firing a few degrees BTDC at idle.

Thanks for the input. I am thinking of taking it to a chevy dealer so they can
"perform a CKP system variation learn procedure" with their "proprietary scan tool" , or so my haynes manual says.
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Old 03-20-2007, 03:07 PM
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i beleive erasing the code is the same thing as reseting the computer um im not sure what it could be is it ran with a chain or time belt? if its a chain maybe it jumped a tooth and is causeing it to jump in a out of time but the best thing for you to do is take it to the gm dealer have them diagnose the problem and then maybe you can fix it yourself anyways good luck with that sorry if i was no help

Have a good one
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Old 03-20-2007, 07:14 PM
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Its a chain drive system. I'll post the solution when I find it.
Thanks for the input

Brian
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