iceback

Several months ago my C55 developed a slightly rough idle. A month ago shortly after starting up to drive home after work, it developed a mis-fire which was immediately corrected by restarting the car. It threw the CEL and codes P2050 and P2052 (misfire on cylinder 5). Took the car to the dealership and after swapping wires and coil packs between cylinders 1 & 5 they couldn't reproduce the problem. They said they were baffled and suggested a compression test. I held off on the comp test and took the car back.

The rough idle only appears once the engine is completely warm (operating at ~82C). Things are smooth until I come to a stop and the engine idles down to ~510rpm. Occasionally the rpm's bounce up a little during a bigger vibration. If I hold the brake and apply a touch of throttle and raise the revs a bit to say 600+rpm then things smooth out. If I put the car in neutral things are hardly noticeable. I have run fuel system cleaner, pulled and cleaned the MAF sensor, and reset the ECU will no real improvements.

Any thoughts or ideas would be greatly appreciated.

mikekuriger

every time my idle gets rough, I clean my MAF to fix it.

iceback

I was really hoping that cleaning the MAF sensor would help, but no luck for me. I used the CRC cleaner from Autozone that others have recommended.

Wondering about the motor mounts. Can anyone comment on how the motor behaves when the motor mounts are deteriorating? I would guess the engine is always rough if they were going out but can't say from personal experience. Since my idle is only rough when warm and idling around 500 rpm's I'm doubtful it is the mounts.

mikekuriger

the way you describe it, it definitely could be motor mounts. however, if you get the cheapo motor mounts on ebay, they transfer a lot of engine vibration. I'm about to change mine again with good OEM mounts.

nladam12

mounts will deff help the vibration, but I have an odd feeling that its the MAS, mounts wont cause the car to correct the idle level... that almost always has to deal with the fuel air mixture..

JonMBZ

I had a similar issue I changed the plugs wires mounts and still had the issue. The tech ended up raising the idle which helped. Not sure if it's a perfect fix but at least it doesn't shake as much.

CerBErusM113

Hate to piggyback but my engine's been doing the exact same thing, it seems to go away if I put it in Neutral or Park. But definitely shakes pretty rough when fully warm and in Drive.

It shifts a bit hard too, I know the 5speed seems to have a mind of its own sometimes but it literally throws me forward if I'm slowing down rolling through a neighborhood, I just seriously noticed it today. Anyway could it be transmission related or does it sound like I have to different problems at once?

nladam12

This seems to be a common problem with our cars... engine mounts, MAS sensor, torque converters go bad... If I had to guess, the trans is unlikely unless your beat hard on it all the time it shouldnt give up b4 100k... I would check the throttle body (for carbon build up), and Mass Air Flow sensor (MAS)... usually that is what causes the whole rough idle... the trans seems to just act up because the engine is acting up... oh and if those options are wrong, check hoses for vacuum leaks (or the little red dipstick cover/cap)

Quicktwinturbo

Sounds like motor /tranny mount issue

CerBErusM113

Thanks for the reply nladam12 I'll take a look at the MAF and the TB, and hopefully Ill be able to get under into the engine bay and look at all the vacuum hoses. I will add that the idle tends to fluctuate about 20rpms when the engines fully warm. But once again, appreciate the replies.

riverace

Im far from an expert but have you looked at your EGR valve? I had a similar problem and replacing it fixed it.

MBfinatic

I have the same problem.

Now I have heard this problem can be related to a bad vacum line? Does anyone think that could be the problem?

Quicktwinturbo

Check your mounts (motor n tranny).. If you see a leakage, it's time to change it... I had same issues with my C55.
Your wires / plugs should last at least 100k miles..

dgrig

is it a stumble?

I think my slk55 might be doing the same thing, the c55 i had also did something similar at times. Brought the SLK in to the dealer and they wanted me to swap wires. The mounts have been replaced, valve covers gaskets replaced on both sides which seams to be a common problem. Part of me really think its the MAF going but no signs pointing directly towards it.

Going to have my shop check if there's a vacuum leak. Anyone have info on the EGR valve?

MBfinatic

What is the MAF sensor? How do I get to it? Im taking my car to the shop today and would like to get it done.

Quickvr4

If this problem comes and goes it is not a motor or transmission mount....it would do it all of the time.

nladam12

I agree, I have this problem every once in awhile, It seems to be that cleaning or swapping related parts seems to fix the situation... the MAF always gets blamed... and is best to just swap out instead of cleaning it... this problem came down to the fact that I put K&N filters on it... and bits of oil over time built up... cleaner doesnt fully clean it... it still leaves a residue... the best way to properly clean it.. is to take it out and with a Qtip gently rub some isotropic 90% alcohol on it...

BUT I will say when I replaced the EGR, the MAF, solving a few vacuum leaks, cleaning the throttle body, motor and trans mounts AND replacing the plugs and wires... it ran much better... but a slight vibration still occurs... after all of these replacements I have come down to my last suggestion... injector cleaner... I put 2 cans of total gas system cleaner mixed it with 1/4 tank gas... and am now driving it hard... hopefully it will clear it up... Ill let all you know how it goes... but this seems to be a very common problem with our cars... all the above listed suggestions worked... but only for a few months... But I will say do not keep replacing the MAF if the code comes up... one swap per 75-100k is fine...

nladam12

And for Tim, the MAF is the sensor that tells the ECU how much air is going into the engine... it is right down in the circle screen on the intake... after you take off the cover and airbox... it looks like a very expensive lightbulb... to clean it you will need to remove it... with safety torx wrenches... the ones with a hole in the center... it is a ***** to take out... so it is best to take out the surrounding hoses and parts... and just label them to put them back in the right place

CerBErusM113

So after taking my C55 to the Service Center they found a few problems relating to my rough idle and slight power loss. First off the engine mounts were going so thank you Quicktwinturbo for that heads up. They second part I believe was a recall item or maybe just a typical issue with our cars. The valve cover gasket was leaking oil and needed to be replaced. Issues not directly related to the engine were the Seat Heater Bottoms, Passenger Side HID Ballast and Driver's Front Side Suspension Bushing replacement. After all of thats changed it'll still come out to an easily bearable $200 :-) Thanks to the aftermarket warranty.

The technician also mentioned new spark plugs and wires and the Service Center was even willing to cut me a deal on labor, from $2000 to $1500 8-0 Not that Ima cheap a** but $1500 is still too much to drop on simple speculation. Either way just wanted to fill you guys in and thank you for the input:-)

nladam12

of course! I hope this problem that we all face is fixed by someone soon! Its just not making any sense as of yet... and i hear ya on the warranty... best 2500 bucks i ever spent... just last service it came to 900 bucks LOL... only 100 bucks for me lol

kent426

Seriously??? Shifting hard and down shifting is what the base programing of the car is supposed to. Its a sports car!!! Put the tranny in "C" if you don't like it.

As for the lumpy idle, the car should have a little bump to it due to camshafts etc. Its not the family but a hopped up motor. When the car fully warms up, the oil is pushing the valves to full open based on the cam profile which gives it the lump, when its cold, the lifters are sluggish.

Spark plugs changes can be done at home by anyone with some common skills, it's a 1 hr job.

mgiannetto

Was the issue ultimately fixed? If so, any intelligence to share for your fellow forum members?

redC55

Just had a rough idle too, seemed to only be bad when in drive and at idle. Park, neutral, or revving smoothed it out. I changed plugs and wires myself, which was not a one hour job (maybe only 2 hrs next time I do it). Our cars have 16 spark plugs. It seems to have smoothed out after driving a few miles.

kent426

Yup, figured out the cause of my rough idle and 1-2 shift, broken motor and transmission mount.................
Pretty poor for 50K, does anyone make polyurethane mounts?

MarcusF

One commonly overlooked cause of bizarre idle issues is a normal wear part – primary oxygen sensors. O2 sensors are used at idle and at cruise. When they die, and they all die eventually, they commonly produce low voltage. The ECU views that as a lean condition and enriches the mixture. Since the sensor is dead (or dying), it still reads low, causing the ECU to enrich the mixture even more. Aside from causing the cats to overheat, this causes a bad idle, and the fuel economy at cruise goes down the toilet.