stojakovic

The story started at one week ago when I feel poor performance of my 02 ML320 (hardly up 40kpm). I took it to dealer and they give me a quote around $3000 for mass air sensor, spark plugs and fuel filter. I then decide skip the feul flter and do others myself. However, when I pick up my truck at dealer, I found the check engine light on. It was off when I went there. The codes are P0300 and P0306, cylinder 6 misfire apparently. I guess it because of the faulty spark plugs.

Then I first replaced mass air sensor. The performance come back.

Then I change spark plugs. erase all codes. Everthing goes fine even the rough idle disappeared when I test it around my house.

Then I test it on the high way. It reaches 40-50kpm easily and smooth at first, and then the check engine light on and starts rough idle. I went back and read the codes, still P0300 and P0306. P0306 was deteced twice so the reader says 3 codes.

The next day I start the engine, the CEL is off. Then I try it again, the same thing happened.

I figure out if I keep slower around 40mph and turn down A/C, the CEL may stay off. Once I go faster and cooler A/C, the CEL will on soon, and the same codes read.

It looks like the same situation in here: http://www.benzworld.org/forums/w210...ifference.html

However, I switched coils/wires, the same codes are still there. I don't believe it is because of fuel injector as it works at low speed.

Any ideas for this? I searched a lot but no topic inspired me.

coop356

Did you ever find the problem. Mine is doing the same.

mihir_d

http://www.justanswer.com/mercedes/4...306-found.html

I have the exact P0300, P0303 & P0306 codes registered on my 2000 ML320....seems like somehow water gets into the fuel rail...

I cleared the codes and put the car through an Italian tune up (high rev./high speed highway drive)..codes haven't come back so far...

Question is, how does water manage to get into the fuel system?

Anybody, any guesses?

coop356

At first I thought it was the fuel filter. So I pulled off the drivers side rear. Removed the fender liner and there it was. The vaccume line at the regulator side was split. Cut it out and replaced rubber hose. Now the truck runs great.

Hope this helps.

mihir_d

Thanks for sharing..
It helps to get the fuel filter inspected/replaced every 5 years of so as per factory scheduled maintenance.
I still think, it is bad design water intrusion in the fuel rail should not occur under any circumstances.
I would have never guessed in my wildest dreams that p0303/p0306 pattern is a classic case of water intrusion in the fuel rail.

Maj. Dundee

How did you come to the conclusion that there is water in the fuel system?

mihir_d

Read this:
http://www.justanswer.com/mercedes/43gqb-1998-ml320-p0300-p0303-p0306-found.html

Maj. Dundee

I read it, but does that mean that because one person had water in his fuel system, that this is your issue? There are countless reasons/causes that produce a misfire and water is only one of them.

You even asked the naive question of how water gets into the fuel system. You still don't know?

"Question is, how does water manage to get into the fuel system?"

mihir_d

I agree about the countless reasons for cylinder misfires, however if the misfire pattern is specifically 03-06 (as in my case), the water explanation seems to make the most sense.

My spark plugs, coil packs, wires MAF etc. all have been replaced at recommended intervals & so have my injectors been cleaned.

As you can read from other posts, it is real easy to be misguided into expensive repairs on this cylinder misfire problem.

I was glad to have found an explanation specifically for the 03-06 pattern. I am even glad, I took the trouble to google specifically for the misfire pattern.

For people facing cylinder misfire issue, my post should help people know that water getting into the fuel rail is one additional possible root cause & someone was good enough to pin point the source of where that water can come from..

Maj. Dundee

Water in anyone's fuel system can only come from one of two sources. The fuel provider (Shell, Mobile, etc.) or the gas station drop plates are not sealed correctly and when it rains water enters the underground tanks. The auto manufacturer is never at fault.

The proper way to find a misfire is to switch the plug, wire, coil and injector, one at a time, from the problem cyl, to another one and see if the problem follows.

One thing that has been overlooked is your motor mounts and or trans. mount. If the engine bounces even slightly in the engine frame cradle, it will cause the Crank Position Sensor to miss its mark on the flywheel producing a misfire. Have them checked for breakage.

mihir_d

Maj. Dundee, your advice & insight, as usual are invaluable. I learnt more from your post. My motor mounts are in fact on their way out. No engine bouncing yet, but mild motor vibration can be felt on the steering wheel.

I was out of town 2 weeks & in my absence, wifey filled up at a few unfamiliar stations - reported the CEL light to me 3 days before I came back in town. And yes, California did get mild rain.

If I get the cylinder misfire again it is more than likely motor mounts, because I refilled the near empty tank that wifey had with full tank of fuel from a familiar station.

My CPS too has been replaced not too long ago.

I too felt a bit perplexed reading that the fuel supply system could be breached by water that easily. I am glad to know, that is not the case.

AlbertTimme

98 ML320 +/- 190,000 miles. Idle with brake on is about 900 RPM. Can it be slowed down so I do not have to hold the brake on?

coop356

Do not wait to long to replace motor mounts. I did and the movement cracked a weld on the drivers side cat at the top. I had to pull it out and have it rewelded.

Now I had the same problem with idle and codes.
I found a cracked vacuum line on the fuel pump. Jack up and remove the back tire. Remove the drivers side back wheel well. There is a hard plastic vacuum line with a rubber 90. The 90 was leaking.

Another big help was to look up cleaning the EGR valve and tube. Mine looked as solid as there pictures. I cleaned it out and now I get 19.5 city and 24 highway.

Hope this helps