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Discussion Starter #1
Greetings,

I've been trying to sort out some rough idle problems with girlfriend's car. Had P0170 P0173 DTC Codes. Replaced air mass sensor, and these codes cleared, and the engine runs much smoother, and accelerates much better.

Car idles at about 800 rpm, with transmission in Park, and idle compensates when AC compressor comes on. However, shifting transmission into Drive causes idle RPM to drop significantly, and idle becomes rough. Car stalls occasionally in stop and go traffic, etc. because the idle speed is too low.

I suspect that there is some signal missing between the transmission control and the fuel injection control (I am guessing, this is an electrical fault, or is it vacuum related?)

Looking for some guidance... Any ideas or replies are greatly appreciated!
 

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Updated information - still need advice

OK... I am still having rough idle, but also stalling in certain situations.

Here's some additional information:
Since replacing the air mass sensor (with a Bosch replacement not eBay) all codes are clear (except BAS, which I think is the brake pedal switch problem). I also replaced the crank position sensor, and had no change in the idle steadiness, nor has it cured the stalling.

Stalling occurs in Drive, mostly after coming to a stop, when the idle speed drops to about 500 rpm. This usually happens 5 - 10 seconds after coming to a stop. Stalling also occurs when travelling below 10 mph, coasting with near zero load on the engine and transmission. When coasting or pulling away from a breif stop, there is no throttle response, the pedal feels "dead" as if the throttle is not responding at all. Engine stumbles below 500 rpm and stalls. Engine restarts immediately.

Also tried resetting the throttle/transmission link by doing the key on, WOT, key off, closed throttle, wait 3 minutes for the ping in the ignition switch routine... but this did not help either.

Other things that have been replaced recently are: Fuel filter, air filters, oil filter and oil. Plug and wires are old but engine accelerates well without misfiring.

It seems that when the additional load of the transmission is applied to the engine when Drive is engaged, there is no compensation to the idle speed and it falls from about 800 in Park to 500 or below in Drive when the idle becomes rough and irregular.

I am nearing my wits end with this problem. Please help! Ideas are welcome.
 

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im having the same exact issue with my 02 C320, I changed plugs, CPS,fuel filter, cleaned the MAF and nothing. coils are next..
 

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Sorry/Happy to hear that you are having the same problem. I have avoided replacing the coil packs on speculation that they might be involved, because of the expense.

But if you are doing that, maybe I can benefit from your experience. Please share your results.
 

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Im getting a code for multiple misfire now... not sure what to do.. were you able to solve this problem?
 

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Hey uptown... I don't have any misfire issues here.

The problems are partially cured.

The stalling turned out to be fuel starvation when the fuel tank was less than half full. The tank is shaped such that it has 2 "wells" on either side of the car. When the tank is full, gas can slosh from side to side and the level in each side stays equalized, but when the fuel is low, the fuel pump is on the driver's side and there is a fuel recirculation pickup tube on the opposite side that keeps the fuel levels equalized on each side of the tank. This tube on the passenger side of the tank seems to operate on a venturi "jet" principle, and this part had failed. When that was replaced, the stalling issue was cured.

The vibration when in Drive still exists, but seems to be worn out engine mounts that are failing to dampen the vibrations. When those are replaced, I think the vibration will be reduced or eliminated.
 

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You two most likely have a cracked or disconnected vacuum hose. Few weeks ago one of the vacuum tubes towards the rear of my engine managed to completely disconnect itself. You need to take the covers, and intake off your engine, and look it over thoroughly for any problems with vacuum hoses. My C32 didn't even get a check engine light, but there was a pending code for P0105 (Manifold Absolute Pressure). So either the MAP sensor was bad, or there was a vacuum leak.
 

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thanks for the reply.. i will check for any leaks this weekend weather permitting. btw the car is getting worse.. as soon as I get to a stop it start to idle rough to a point that the engine is getting ready to shutdown, i have to shift to neutral to mantain the idle above 500, it also loses power when i try to give gas making a turn, specially left turns.
 

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same issue still.. it idles rough and it sputters / misfires if I try flooring at launch.I tried disconnecting the MAF but it still idles rough so thats out of the question. A friend of mine looked at it and he said it might be the throttle sensor / actuator.. what bugs me the most is that its not throwing a cel.

I have only done a visual check for leaks, I still need to remove the intake and check for leaks underneath it. Does someone have a vacuum diagram for the V6 engine?
 

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I had a rough idle problem and hard start that turned out to be the fuel pump on my 02 c320
thanks for the reply.. my car starts ok with no hesitation at all.. what I did notice its that the car idles a bit better after a gas fill up. This car is driving me crazy!! Im thinkin about trading it in and buy a Volvo or Audi. no joke
 

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Remove your intake system all the way back to the y-splitter on rear of engine, and remove the y-splitter. There is a vacuum line that runs to the engine, and is just pressure sealed. My c32 blew this hose off completely after raising boost. I would check that one just to be sure.
 

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Hey uptown... I don't have any misfire issues here.

The problems are partially cured.

The stalling turned out to be fuel starvation when the fuel tank was less than half full. The tank is shaped such that it has 2 "wells" on either side of the car. When the tank is full, gas can slosh from side to side and the level in each side stays equalized, but when the fuel is low, the fuel pump is on the driver's side and there is a fuel recirculation pickup tube on the opposite side that keeps the fuel levels equalized on each side of the tank. This tube on the passenger side of the tank seems to operate on a venturi "jet" principle, and this part had failed. When that was replaced, the stalling issue was cured.

The vibration when in Drive still exists, but seems to be worn out engine mounts that are failing to dampen the vibrations. When those are replaced, I think the vibration will be reduced or eliminated.
how was this problem "cured"?

I have the identical problem.

did you replace the fuel pump?
did you replace the fuel sending unit?

did you replace something else?
 
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