Looking for some help diagnosing a recent problem on my 2004 E320 Wagon. About a week ago on the commute home from work, I took a right hand turn at speed, which then proceeded up a moderate hill, and the car threw a CEL. It also began to stumble significantly. At the next light, it felt as if it was running on only 4 or 5 cylinders. I was able to continue and pulled into a parking lot. I shut the car off, let it sit for a minute or two, then restarted. It ran fine the rest of the way home, which was another 20 miles or so. I scanned and got misfire codes on cylinders 4 and 5. I cleared the codes, took the car for a brief drive and had no issues. Then this past Monday I drove the car into work. 25 mile commute - no issues at all. On the way home in the heat of the afternoon (85 degrees and humid), I took an exit which has a large right hand curve that goes slightly uphill at moderate speed. After straightening out, the car began to stumble again and I got the CEL. Couldn't accelerate past 45 mph. Pulled off and the car died. Shut the ignition off, waiting a minute or so and she started back up, although still running rough. Drove a couple of miles and exited to be safe - died again. Pulled into a parking lot, sat for five minutes, fired up and drove fine. Next light - shuts off. Restart and this time it's fine for 20 miles - no issues running. Decided to chance it into work yesterday and had no issues. Clear, cool morning with low humidity. At the end of the day when it reached mid 80's I headed out and it started acting up almost immediately. After several shut offs and ignition recycling (over the span of about 8 to 10 miles) it finally smoothed out and got me home.

A friend who is a master tech seems to think that it's the fuel pump, based on the details above. He's not a Mercedes tech, but he knows his business pretty well. Anyone here concur with that diagnosis? Does it sound like fuel pump or something else? Car has 152K miles. Any advice is greatly appreciated.

 

As I was reading the first paragraph, and before I reached the second, fuel pump began to emerge on my list of possibilities. So, I concur. However, there could be others or combinations of these
Fuel filter
Fuel pressure regulator
Bad gas (possible water in gas) especially possible if you're on the East coast where this pipeline went down.
More on my list possibly, but not enough info.

Question - you report misfires, I assume a P0304, P0305, after you cleared those, and had subsequent issues on other days, did the CEL set again? Same codes? Others.

 

Thanks for the reply. P0304, P0305, etc. are the codes I saw, and yes...after clearing I got the same codes when the issue returned.

 

If you keep getting the same two misfires, then I'd have to place a few more items on my list as check items. I am not familiar with this V6 per se, so these are just general observations along with online diagnosis so there is an "error rate" to weigh.

First for me would be to eliminate one item on my list - bad fuel. Only you know whether all that you described above is on the same tank of fuel? Or different fill ups? If you can eliminate that, then my next step would be to get a fuel pressure gauge on the fuel rail. You need to see what fuel pressure you're seeing at the rail, but especially under different load conditions as this is when you experience the failures - under load. I personally use a fuel pressure gauge that has a length of fuel line, so I can thread it onto the end fitting of a fuel rail, then feed it through so the gauge sits in the cowl space by the wipers. Then I can actually drive and monitor fuel pressure under different conditions.

If fuel pressure proves to be the problem then you need to focus on - Fuel Pump, fuel filter, pressure regulator.

If those are OK and you do not have a fuel problem, then I would begin to focus on cylinders 4 & 5. First, would be the fuel injectors, then coils, then spark plugs. Since cyls 4 and 5 are on different banks it eliminates a lot of possibilities too.

 

Thanks again. I don't think it's the fuel, but anything is possible. I actually have a fuel pressure gauge on my desk as I type. Will be checking that tomorrow since I have some down time. Sadly I don't have enough hose to situate the gauge by the cowl, but if I can replicate the issue and get it to misfire and sit at idle I should be able to see it on the gauge (if it's a pressure issue). On the misfires, I've actually gotten them on all cylinders, now, except for #1. I'm no mechanic, but it sure does seem like a fuel starvation issue. Hopefully the pressure test will narrow it down.

 

Yes, that's a whole different issue if you're now seeing most/all cylinder misfires. So, forget the cylinder diagnosis, now more than before, a fueling problem.