Random Misfire at low RPM
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around 600 - 1000 rpm my car has a random misfire. No SES light. Hook it up to the shops computer and it will show a misfire every cycle or two. It could be any cylinder. We changed plugs and wires and it still does it. Car is hesitant off the line, but once it winds up it runs good. Any Ideas.
Car Info.
2000 TA A4 3.42 gears. Stock except for SLP LID and Magnaflow exhaust
I can also here a slight rattle at 800 rpm, It's coming from the passenger side around the Cat.
Car Info.
2000 TA A4 3.42 gears. Stock except for SLP LID and Magnaflow exhaust
I can also here a slight rattle at 800 rpm, It's coming from the passenger side around the Cat.
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Sounds like it could be an aftermarket cam or incorrect cam timing. The misfires down low could be from high overlap/lsa. Powerband gets stronger with higher rpm. All sounds like an aftermarket cam or retarded cam timing. It could be a number of other things, but your going to have to do some more diagnosing. Random misfires generally rule out things like one bad injector or ignition coil. Do you have a scanner? If you do hook it up and see if you have any stored codes. These codes may not always turn on your (CEL)check engine light. Next, does the CEL turn on when you first start the car? Sometimes the bulb goes out and we assume it's not on. Lastly, these cars are always getting codes 'tuned' out. So you may have a code, but your PCM ignores it. Hope this helps. If you haven't done all of the PM(preventive maintenance) I would do that first. New air filter, fuel filter, oil change are a start. Good luck.
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I'm just about done with all of the pm. New air filter, oil change, seafoam, rebuild rear-end, change plugs and wires. Today I cleaned the MAF and changed the fuel filter, it still had the original GM filter after 75000 miles.
They're are no codes to be found with the computer. SES light never comes on. Back to square one.
It does run alot better now than it did when I first bought it.
Thanks
They're are no codes to be found with the computer. SES light never comes on. Back to square one.
It does run alot better now than it did when I first bought it.
Thanks
#6
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Originally Posted by brapril714
SES light never comes on.
I'll keep an eye on this thread. I wanna know what's going on with your car. Keep posting updates.
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No tuning software. My buddy owns a shop and has the large diagnostic machine.
The car will shake a little while idling. When it does that, watching the computer it will show a misfire. It could be any cylinder. If you get the rpm's past 1000 it doesn't shake or misfire at all. I'm at a loss myself.
The car will shake a little while idling. When it does that, watching the computer it will show a misfire. It could be any cylinder. If you get the rpm's past 1000 it doesn't shake or misfire at all. I'm at a loss myself.
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#8
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Originally Posted by brapril714
No tuning software. My buddy owns a shop and has the large diagnostic machine.
Last edited by LS1_TA_2002; 05-13-2007 at 06:35 PM.
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Originally Posted by brapril714
Bought it 2 months ago. Done it since I bought it. Haven't touched any of the stuff mentioned. The rattle was from a loose bolt on the passenger side exhaust.
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The misfire will show up on #1 then maybe 3 then 5 then 7 and so on. It might skip a cycle or two without showing a misfire.
I got the RPO on the car and it showed that a Cat was replaced at 35,000 miles. Looking under the car it looks like the passenger was replaced. The collector after the cat looks to be smashed sideways. One bolt was loose so we loosened it and tried to realign it the best we could, and the noise went away most of the way.
I really appreciate the help, thanks.
I got the RPO on the car and it showed that a Cat was replaced at 35,000 miles. Looking under the car it looks like the passenger was replaced. The collector after the cat looks to be smashed sideways. One bolt was loose so we loosened it and tried to realign it the best we could, and the noise went away most of the way.
I really appreciate the help, thanks.
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Ok, now we're getting somewhere! The four cylinders you mentioned just so happen to be on the SAME side of the engine. So, now we can narrow it down to things that only affect the driver's side cylinder bank or Bank 1 for short. All four bank 1 injectors AND coils are powered by ONE fuse. All bank 1 coils share one ground. A loose connection from power or to ground may cause your poblem. Have your friend wiggle all of the harnesses near the coils and injectors while you view the misfires on the scanner. Check for a lazy driver's side o2 sensor. Or try switching the rear driver's side o2 sensor with the front one from the same side and reset the PCM. Post back.
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Sorry so long on the update. I put it back on the computer and it does show misfires on both sides. I pulled all plugs and wires and everything looked fine, since it's only been 2000 miles since I changed them. Car is up on Jack stands now trying to find the problem.
The rattle at 800 rpm is from the connection between the cat and y-pipe on the driverside, not from passenger side as prevouisly state. GM replace cat at 40000 miles and really fubar the connection there. Long Tube headers are on the way, along with some new O2's and a tune. Hopefully I can figure this out soon.
Thanks for the help
The rattle at 800 rpm is from the connection between the cat and y-pipe on the driverside, not from passenger side as prevouisly state. GM replace cat at 40000 miles and really fubar the connection there. Long Tube headers are on the way, along with some new O2's and a tune. Hopefully I can figure this out soon.
Thanks for the help
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I used sea foam and injector cleaner. I've notice some oil in my intake, so I'm doing the catch can mod today to see if that may be causing it. There is a little smoke on hard takeoff's.
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but which one? It never throws a ses light.
When I'm coming to a stop the rpm's surge at 10mph. It makes the car jump and then stop again. It happens at exactly 10mph everytime, the rmps jump from 850 to 1200 and then back down to normal.
I did install the oil catch can, and after 30 miles there is a good tablespoon of oil in there.
When I'm coming to a stop the rpm's surge at 10mph. It makes the car jump and then stop again. It happens at exactly 10mph everytime, the rmps jump from 850 to 1200 and then back down to normal.
I did install the oil catch can, and after 30 miles there is a good tablespoon of oil in there.
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Last edited by brapril714; 07-11-2007 at 09:37 PM.