* 2003 STR, 48,000 miles.


My STR started running slightly rough 2-3 weeks ago—almost like it had bad gas in it. I’ve gone through three tanks of gas and only use premium so I don’t think it’s that. Symptoms include a gentle intermittent rattle at on engine idle and especially if the engine is cold (goes away after a few miles even if I stop and let it idle)

At very high RPM (fast acceleration in sports mode), it feels a bit rough and the check engine light will blink maybe 10 times, and then everything will go back to normal. No codes are showing (amazing for this car!). MAF? Spark plugs? Any thoughts would be welcome.

 

Flashing MIL = misfires so terrible they're damaging the cat(s).

Will also be why it's rough. Probably an air leak - check fuel trims. If not, try the coils & look in plug wells.

 

Thanks. Is the car safe to drive gently while I figure it out?

 

You may have oil fouling the spark plugs and coil packs causing the misfire.

Have the cam cover gaskets and spark plug well seals been replaces recently?

 

I wouldn't drive it till fixed since I wouldn't want to be replacing cat(s)

 

Thanks for the post. No, they have not been in the ~3 years I have owned the vehicle (at 20K, now 48K miles).

 

I finally got a couple of codes today--misfire codes: P1316 and P0308 (cylinder #8). Question for you: coincidentally, the supercharger started sounding more 'whiny' the last 2-3 months before the recent slightly rough idle started, and I swear the gears seem not to be up-shifting until slightly higher RPM (hence maybe the more prominent SC sound). You think this is related?

Will report back.

 

Why worry, just fix it

 

After the vehicle cold-soaked, remove the coil from the no. 8 cylinder and see if there is any oil contamination in the spark plug well. If there is, the cam cover gasket and spark plug seals must be renewed.

If you find no evidence of oil contamination, swap the coil unit with another cylinder and then see if a DTC sets for that cylinder. If it does, replace the coil unit.

Under no circumstances is the vehicle to be driven if the MIL is flashing as catalyst damage is occurring.

 

Thanks for taking the time to post! To be fair, the MIL flashed only a couple of times when I gunned it...did not come on since and never with normal driving. Will not be crazy until it's fixed, and I'll let you know.

 

Then let us both hope you do not see P0420 or P0430 as a DTCs.

 

Cylinder 8 was clearly misfiring. All four coils on the side where that cylinder is were replaced. No longer running rough, no flashing MIL (I've had it up to high RPMs since) and, in case anyone is interested (since it bothered me and I did not see anyone comment on it), the whiny sound from the supercharger has now dissipated. I'd love to see an explanation for that since that sound was the first symptom I noticed about two months ago before the rough idle. Hopefully it will help someone else...

 

What were the fuel trims?