mcheung

I have a 2003 C320. The engine goes rough from time to time at idle. It happens when coming to a stop at a traffic light. When it happens, I feel like the engine is shaking and that it's going to stall. It never stalls though. The rpm doesn't really move up/down much, staying at about 500-550 rpm. It seems low, but it's like that since the car was new. When the engine is shaking, there's no response when pressing the gas pedal. The shake will go on for about 10 seconds or so, and then everything is back to normal. If I am pressing the gas pedal when the engine stops shaking, the car will dart forward. The Check Engine light never comes up and there's no error code thrown.

I have the following replaced:
- new spark plugs
- new spark plug wires
- two of the O2 sensors went bad in the mean time and were replaced
- 1 of the 6 cell of the battery died in the mean time and I put in a new battery

It's completely random, full/empty tank, engine cold/warmed up, sunny/rainy weather, warm/cold (summer/below freezing), A/C on/off. It started about half a year ago. For a one-hour drive, I might experience it once every few days. It seems to be happening more often now, perhaps once/twice a day. I tried putting in a bottle of SAP fuel injector cleaner in and the shaking seem to be less severe, but now it's shaking like before again.

What could be the problem possibly?

Thanks.

5by5

What happens if you put the gearshift into neutral while it’s occurring? Any change?
5

mcheung

There is no difference putting it in Neutral, still shake, and no throttle response, until the shake/vibration stops.

5by5

Ok, so not an issue with transmission or torque converter lockup. I had a similar issue on my 2003 C230. It would intermittently shudder and lose rpms at stoplights. Taking it out of gear would smooth it out. I replaced the torque converter lockup solenoid.

Have you considered a clogged or blocked cat? If the catalyst is moving around and intermittently blocking flow you could see rpm drops and power problems.

Do you notice any drivability problems under higher rpms or acceleration?
5

mcheung

There’s no driving problem at high rpm or under load. It only happens when stopping at traffic light or accelerate when turning at intersection when almost coming to a complete stop.

5by5

Still wonder about catalytic converter.

Tap or shake it while running see if the idle changes of if you hear a rattle. Can also measure back pressure before and after through O2 ports or check to see if there’s a temp increase before and after. If it’s functioning you should see a temp increase on the output tube.
5

jaGTAR KAHLON

Lookup 12 dollar fix. No it will not throw a code

mcheung

I'll have to ask the mechanics to do that. Changing the air filter is as complicated as I can go When you say measuring the back pressure before and after, it is before and after of what?

5by5

Catalytic converter. Can tell you if the cat is plugged. Temp increase after cat can tell you if it’s working.

Harbor Freight laser infrared thermometer would work. $25. Pretty handy too for lots of stuff. https://www.harborfreight.com/121-in..._q=thermometer

If you wait I can do it on my daughter’s 2005 C240. (Same M112 motor, smaller displacement)

The reason I’m wondering about your cat, is because I’ve seen several long, involved diagnosis threads where people have replaced a lot of parts before settling on cat problems. Cat issues also seem to resist throwing codes that can help diagnosis.

Don’t tap or shake the cat while it’s hot though. Btw, I think you have two—right and left. Also do you have a code scanner with data logging? If so try to capture data when the idle drops.

mcheung

Thanks. I wouldn't be surprised if it's the cat converter is beginning to go bad. It was replaced in 2011.

I have an infrared thermometer. Do I measure the temperature of the pipe going into the cat converter and the one coming out of cat converter?

5by5

https://axleaddict.com/auto-repair/H...ytic-Converter

Cats don’t wear out though under normal operation. You mentioned replacing O2 sensors, have you had this problem since replacing the sensors? If so, you may have burned up your cat (s) possibly by flooding them with too much unburned HCs.

Could still be other things though. If you’re not going to fix this yourself, I’d bring it to a good indy, describe the behavior and let them consider the possible causes.
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mcheung

I had this problem before replacing the sensors. Let's consider intermittent engine shake as the starting state. I realized it's time to replace spark plugs. At the same time, I flushed and replaced the brake fluid, transmission oil and the real differential oil. These shouldn't have anything to do with the problem. New spark plugs didn't fix the problem. Then I began to have starting problem. If I didn't use the car over the weekend, I couldn't start it on Monday. The starter would crank but seemed like no fuel, so the car wouldn't start. After a few trail, it might start. This was also the time I began smelling some gasoline in the garage after the car was sitting in it overnight. Then I replaced the spark plug wires, which were never replaced, and I put in a new fuel filter. Cranking problem and engine shake intermittently at idle were still there. Cranking problem was getting worse. Mechanics found two dead O2 sensors. Cranking problem then changed from no fuel to weak cranking. Battery only charged to 10.4V. Mechanics found 1 of the 6 cells in the battery died. Replaced battery. Now the car is back to the starting state. And I still smell some gasoline, although little.

Wilkinda65

I'd look for a vacuum leak. These cars are known for vacuum leak issues. You could pull the vacuum line off the brake booster and plug that line. If it smooths out idle thats it. I have talked o several shops about the rough idle on my sons C230 and its always the first or second suggestion to try.

mcheung

The tricky part is that the idle goes rough only once or twice a day, while driving. If the idle is rough all the time, the mechanics can use this test. Are there other ways to find vacuum leak and where the leak could be?

mcheung

I think the problem is fixed. My next guess was dirty throttle body. There is a thread about how to get to it and clean it, but I was stuck at one of the steps. I took it to my mechanics and had it clean, together with the MAF sensor since it had to come off first to get to the throttle body anyway. The throttle body was indeed very dirty. After it is cleaned, my engine felt like new. It was amazing. It's been a few weeks and I've never experienced the engine shake anymore. Total cost: $20 for the MAF and throttle body cleaners, $100 labour.

5by5

Great news! Thanks for coming back with the solution.

jggarcia.vc

I just did this last night cleaned the Throttle Body which was extremely dirty and the MAF. It fixed the rough idle at stops while in drive and reverse for sure! But I am thinking my MAF may need to replace it looked really dirty even after giving it a nice spray with the MAF cleaner. My car has about 180K miles on it and the MAF is the original one so I think it should be replaced... Also this is the first time i've ever cleaned the throttle body and I am the original owner..

VenomX

how do you get to the throttle body ? Engine is 112 v6 c240
i can get to the maf but the throttle body seems tricky to get to