brapril714

iTrader: (1)

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.

LS1_TA_2002

Did you recently buy it? Has it always done this? Have you done anything with the timing? Chain, sprockets, oil pump etc.?

brapril714

iTrader: (1)

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.

LS1_TA_2002

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.

brapril714

iTrader: (1)

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

LS1_TA_2002

Wait. It NEVER comes on? It should at least come on, for a few seconds, when you first start the car. If it doesn't and you say 'no codes' then something ain't addin' up. You get my drift? Do you have tuning software?
I'll keep an eye on this thread. I wanna know what's going on with your car. Keep posting updates.

brapril714

iTrader: (1)

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.

LS1_TA_2002

Unfortunately, if the codes have been tuned out your buddies diagnostic machine will not help. Tuning software effectively removes the PCM's ability to monitor certain things once they've been 'tuned' out. So scanners and the like will not find any codes. You did not answer my question. Does the MIL come on at least once when you first start the car? It should. If not the bulb is bad or there might be a problem with the wiring.

brapril714

iTrader: (1)

What do you mean by MIL light?

LS1_TA_2002

MIL=Malfunction Indicator Light or check engine light.

brapril714

iTrader: (1)

Yes it does come on for a few seconds when first started

LS1_TA_2002

The stock exhaust rusts quickly and rarely loosens. This leads me to believe the previous owner tinkered/modified the car. He might have returned the car to near factory condition. Things like exhaust systems, headers, lowering springs, and aftermarket rims are easily seen so they're generally removed and sold. Internal engine components, HOWEVER, are sometimes not worth the time and effort to remove and replace with stockers. I still hold to my belief that you have an aftermarket cam with incorrect idle speed tuning. It's a gut feeling, but everything you describe leads me to believe this. The only sure way to find out is to have a race shop inspect and "degree" your cam. Post the results so we can compare them to stock specs. The number of misfires you mention (1 to 2 /cycle) are not excessive. If the you believe the cam to be the stock cam, then the misses could be attributed to worn valvetrain components, bad gas(water), or anything else really. When you say, "it could be any cylinder". Do you mean it misses in every cylinder according to your scanner? Or do you mean, "I don't know which one, it could be any cylinder." Sorry for such a long post, but with a tiny problem like this and no hard concrete diagnostic info it's really hard to nail down these sort of problems. This is kinda like calling your dentist and telling him you have a toothache, but don't know which tooth hurts.

brapril714

iTrader: (1)

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.

LS1_TA_2002

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.

brapril714

iTrader: (1)

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

jmdale1984

iTrader: (6)

Mine did this and it was the EGR valve... but it threw an SES code.

Optimus_Prime

iTrader: (4)

Have you tried fuel injector cleaner? My car was doing the same thing and some seafoam in the gas tank fixed it.

brapril714

iTrader: (1)

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.

getthesand0ut

iTrader: (1)

its a coil pack for sure

brapril714

iTrader: (1)

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.