I'm looking at a 99 Ls1 m6. When driving it, it seemed to have a stumble in the lower RPM range. Almost similar to the infamous opti problem... someone help me out here as i am an LT1 guy and know little about LS1's...

The stumble was most pronounced between 1500 and 2500 RPM, and it seemed almost throttle position dependant. Above 3000 there was no stumble.

Any thoughts?

Ricky

 

Could be a spark plug loose, computer messed up, wires broken, ignition module faulty, etc...

Ask them if you can take it to a mechanic to get it checked out first before buying, or ask them to known some money off for it.

 

I have an 2002 SS with a similar hesitation(3.23 gears). If I stomp it all is normal, but if I take it easy then it seems to be weak till I get over 2krpm's. I also have this hesitation at 70 mph, the shop(dealer) told me the car should be faster. I am concerned about these hesitations as they seem to be similar to the original poster, but I also believe it may be catalytic converter related(had all plugs and wires replaced last year) and will resolve when I have my QTP headers and catted Ypipe put on the car(yeehaw!). Hopefully some others will post and get some more ideas, as this issue is not fatal but a little disconcerting.

 

could be an o2 sensor.

 

car has no cats and has 2 out of 4 o2 sims. the 2 o2 sensors have been bad once before and replaced. There are no codes being thrown. I would geuss something simple could be wrong though.

the more responses the merrier(spelling?)

 

Same problem, '02 SS. I brought it in to the dealer under warranty. The mechanic took the car out and flogged it on the freeway, 7 miles worth. Brought it back, told me I had a great imagination, but no miss. I stewed about this for a while, and contacted the dealer again. Gonna give them another shot at it Tuesday. No codes, a rather subtle misfire (or surge), which will result in some mighty fine bucking with the M6 at about 15-20 MPH in 1st.

 

Check your knock retard if you can get access to a scanner (ie. predator, hptuners, efilive, etc.). I noticed with my stock spark table the PCM was pulling timing at lower RPMs/moderate engine loads. It mainly happened in 4th when the torque converter was locked and I put my foot into it, but not enough to make it downshift. I had to back out 1-2* of timing there in my tune. 1600-2400 RPMs @ .48-.56grams/cyl.

 

I think speed demons idea has some merit, all though I will be adressing that after the headers are on my car and is tuned(hopefully this will be taken care of during the process), as for bert02SS- every time I had a burnt plug wire(on my 96 V6 f body 3.8 and this car) you could feel the bucking in the lower rpms (heaviest load/ gets better @50 mph +) and hear the miss in the idle. Hopefully you can check your wires somehow, allthough this ignition system is pretty sweet it seems at least in my humble opinion to be prone to burning the stock delco packard 7mm wires, and i will be upgrading to msd super 8.5's when I have the chance.

 

Daves02SS, thanks for the advice. I hope it is something as simple as a fried wire. I forgot to mention that it idles kinda lumpy at about 950, assuming the tach is accurate. That seems high for an M6. Shouldn't it be lower? The beast is totally stock.

 

That idle (rpmssounds high for a non-cammed car, factory idle is 650-750 rpms(maybe manual is a hair higher). I believe that the idle is set from the factory and can change) only from tuning, this is for emmisions and drivability I believe(could be wrong though). Does your car still idle this high after a good warm up?

 

Cold idle is higher, maybe 1K, or slightly more. Hot idle is about 950, way too high, I agree. Convincing the dealer service writer that there is a problem may be difficult, though. I have a shot at it again tomorrow (Tue). BTW, no tuning, bone stock. So, what should I say to him??

 

Have then check your cars ecm for idle speed, as it is factory set, that should be the first item to look for.

 

also intake, from lid to throttle body for leaks, test idle air control sensor, and throttle position sensor for proper voltage, and maybe a vacuum leak.