2002 C320 Wagon 88,000mi

I've had my '02 C320 for 6 1/2 years and it's been flawless with the exception of a slight rough running near idle (not in gear), especially when the engine is cold. Get it into gear under even slight load, or get the rpm and speed up, and it's fine. To be honest I couldn't say if it did this when I originally acquired the car.

I figured it was just V6 firing order roughness compared to the 4, 5, 6 inline and V8 MBs I've owned in the past until driving a C240 recently.

It's not misfiring, mind you- it's more like a bit of a "shiver", or a subjective feeling, especially when not in gear. No visible engine shake, no MIL light, or any codes ever thrown, but it's just not silky smooth. The exhaust note is smooth with no evidence of misfire even when the chassis has the shiver.

I was shocked at how a C-240 wagon I drove recently was how absolutely silky smooth that V6 engine felt compared to my C320, so I decided to change my plugs a bit early, thinking maybe that was it. All 12 plugs came out looking absolutely beautiful. No deposits at all, nice color, though the original (I assume) Bosch platinums were opened up to about 0.060". I also changed the air cleaners, serpentine belt, and the tensioner, which was making a small bit of noise (though off the car the it feels fine).

Unfortunately no difference.

I know that in light aircraft, where the cylinders each have 2 plugs like the C320, switching off one set causes a slight RPM drop and some roughness, but I wouldn't think that all the C320 plugs would look so uniform if some plugs were not firing.

I don't know if this engine will throw a code if only one plug/coil in a cylinder isn't firing-and there are no codes (generic, proprietary, or pending) on my Autel DS-708 MaxiDAS scantool.

I didn't do a compression check, but in gear it's smooth even stationary at idle, and the fuel consumption is quite acceptable (25/26 around town in suburban driving and 31/32 Interstate driving at legal speeds) so I'm thinking no burned valves

Ideas?

 

Maybe a bad MAF sensor.

 

Or dirty MAF sensor. Or small vacuum leak. My 02 c320 does it too, worse when cold than hot, minor enough that I can mostly ignore it. I haven't had a chance to dig into it.

 

Thanks, guys.

I was thinking of spraying off the hot-wire w/ the MAF sensor cleaner I have, but didn't.

But today I took the car on a proper drive of a about 30mi, and by the end of the drive, it was like a new car again. I cannot remember when it was so smooth.

I did have a minor concern after a 5 mile initial drive when I had a couple of misfires just off-idle, them upon returning found some "latte" deposits on the bottom of the oil filler cap (fearing an imperfect seal of the gasket on the water pump replacement that could admit coolant on one of the through-bolts that go through the timing chain case into the block), but I took it for a longer test drive (25mi), and everything was OK.

I disturbed a lot of things with the water pump, transmission service, and plug replacement, and there's always a chance of violating the Hippocratic oath of "do no harm." The old saying, "If it ain't broke, don't fix it" comes to mind as well since the car had no "real" problems that forced me to do the work. I just was doing preventative maintenance and addressing minor annoyances.

 

Although probably not the issue here, I’ve also had a problem with shaky idle and hesitation at start up that was caused because I didn’t reseat the air cleaner correctly.

 

Thanks, Tim.

Actually, you are right on. I've had that happen twice now. The first time I was caught unaware after installing new air filters, and didn't have a clue why it hesitated/stumbled off he line, and even had a strange downshift 3/2 slowing for a turn.

It just happened today again after I removed the air cleaner assembly to top off the trans fluid using a dipstick (after a drain/refill). It "feels" seated, but not quite, and that's enough to I guess screw up the air flow pattern. At first I feared I hadn't fully seated the plug wire connectors onto one or more of the new spark plugs I installed, causing a misfire, but remembered an earlier time I'd mis-installed the air filter housing and had a similar symptom.

I think I've now got the drill. I leave the two front cold air air intakes in place in the radiator core support, lower the air cleaner assy. (tilted maybe 15 degrees up at the rear) down and toward the front engaging the air tubes, then lower the assembly spring clips onto the two front rubber retainers (easy since they are visible, unlike the rear ones that are out of sight), then lower the rear onto the rubbers, while feeling for the rear air tube to settle onto the air flow meter as I wiggle the air cleaner housing L/R a bit and down) and engage it as the rear spring clips engage.

I'll see if it's really a foolproof technique the next time I do it!

I've tried doing it different ways, and had trouble having the rear springs go over the rubber retainers. I, in fact, slightly bent one spring clip a bit trying, then decided to rub a tiny amount of silicone grease onto the rubber pieces just to make them slippery. Now the air cleaner pops on with incredible ease, but as I said, even today I mis-seated the rear air tube onto the AFM flange, but at least today when I got the off-idle miss, I knew immediately what I'd done wrong and fixed it. (the first time I had no idea what was causing the stumble/hesitation)

Thanks for making me feel better knowing I am not the only one to have misaligned the air cleaner assembly!

Bob.

 

Thanks Bob for sharing the technique for aligning the air cleaners. I’ll need them for the next time I replace the filters.

Tim

 

Thank YOU for confirming my hypothesis that a small misalignment of the cleaner assembly could cause such noticeable symptoms.