On my 2001 C320 w 3.2L V6, the left bank (passengers side) of cylinders are all misfiring curiously enough.

Borrowed the scan tool at Autozone to find this out.

Symptoms:
1. Normal driving does not produce any isues
2. When accelerating hard, engine retards timing resulting in slugglish accel.
3. At full gas, when the tach hits 5000rpm, I hear a rattle from the engine bay and then the check engine light comes on, car idles rough and it will only limp home.
4. Curiously once I clear the codes from the Autozone diagnostic tool, everything goes back to normal.

What i've done so far:
1. Change out all 12 spark plugs with Denso Iridirum
2. Swap out coil packs from left to right as a precautionary measure.

Car has 70,000 miles. Still the same damn problem. Any idea on what the heck could be causing it? Can't see any visual oil leaks on the top end gaskets.

Any help is much appreciated.

I'll take a stab at it... using a "Scan Tool" for engine misfire diagnosis is a smart move, although if misfire is random and is moving around from cylinder to cylinder. The cause here would likely be something that upsets the engine's air/fuel mixture, such as a major vacuum leak, leaky EGR valve or unusually low fuel pressure (weak pump or faulty pressure regulator).

There's really no magic bullet for finding misfires. It takes a certain amount of detective work to isolate the fault and determine the underlying cause.

THANK YOU for a response. Help is much appreciated! What about the mechanical grinding noise at 5000 RPM? That scares me the most.

thanks,
ben

THANK YOU for a response. Help is much appreciated! What about the mechanical grinding noise at 5000 RPM? That scares me the most.

Curiously, the misfire(s) are not random. Only happening on one bank of the vee, and not the other.

thanks,
ben

I don't have a 320, but this response is pretty accurate with what it can be based on my experiences as well. I had a vacuum leak on one of my pipes and it caused my car to go in "limp" or "safe" mode when I accelerated past 5000 (keep in mind, my car is tuned, so redline is a little higher than normal also).

What it ended up being was one vacuum line and I also ended up having a slight exhaust leak. Once those were cured, car could accelerate without problems.

^^^ Possible culprit... If you've recently upgraded the exhaust system, if not then I'd suggest to have a leak down or compression test.

pnoye2, you wouldn't happen to actually be Pinoy, would you?...LOL...No reason for asking. Just curious.

Lol drex, my SN gave it away? hehe... but not to be off track here, YES I am one.

lol...I figurd. so many of us on here and we r alwys spnding ungodly amounts of money n time with our cars! damn pinoys! lol good stuff.

plugd cats

exact same problem on my coupe .. i ahve a private benz mechaninc .. he ordered me parts .. and he is gonna install them to fix the problem ..

i run in to that many time when im on the FKING HIGHWAY ... lol next thing u know ur going 70 km and hour..

u can turn off u car and turn it bak on ... it work normally after that..

my car mis fire on 3 -4 gears ..

Thanks guys for all the input. Muchos gracias. Here's what happened.

Brought it to an independent shop. Little Car Garage in Waltham MA. Damn good shop. They replaced one vacuum hose and diagnosed it as a plugged cat as one member already said.

Brought it to the dealer. Armed with the knowledge from the independent shop, the Service consultant said $1800 worth of goods with a 10% coupon would solve the problem:
Specifically:
MAF sensor $800
Spark Plug Wires $700
Air Filters $120

and some minor stuff. I brought in the sheet from the independent shop to save the dealer some time. After 3 phone calls each time me insisting on them replicating the problem, they finally found the problem. Apparently the left cat was plugged. Wow, all they had to do was listen to a "Little Car Garage" a fully competent and great shop and they could have saved themselves a days worth of work.

So they ordered a left Cat overnight. Next day while installing, apparently the flange or something cracked on the right side so they replaced both left and right. I was SO RELIEVED AND HAPPY. I was worried about having one Cat with 68000 miles and the other with 0 miles on it.

I threw them a bone by letting them change my air filters asking them to save the old one's making up a bull**** story about how they fit my home Central A/C unit. It was a face saving move since I did the air filters not a few months before so I knew they were in top shape. Coincidentally they forgot to keep them and I guess they got thrown out. My original idea was to just put them back in and save my new $120 air filters for when they were necessary.

Props to the Little Car Garage and the dealer for replacing both left and right Cats. While this job falls under the 7/70000 yr/mile emissions warranty, they could have given me a hard time but surprising did not.

Brought the car home, ordered a MAF sensor for $110 bucks, put it in, now no codes from the ODBII scanner, but the engine stuffers. Tried resetting the throtttle to no avail. I'm going to post another thread about the engine stuffer/hesitation.

Thanks ladies and gentleman. I love this car, but not the problems. 2 new cats baby!

I think you need some stuff reset by the dealer (star diag) if you put in a new MAF, otherwise the car will run weird - goofy excelleration and what nots.