03MBML350

2003 ML350 with about 200k miles.

This year: New waterpump, tensioners and pulleys.
New NGK plugs, Bosch wires and Bosch MAF.

Last year: New aftermarket CPS, Brake Sensor

I have had the ML for a couple of years. Previous owner had the Cats changed at dealer. I am unsure if the previous owner changed the fuel filter or any O2 sensors.

Running rough at idle in Park, neutral and especially drive.

Idle fluctuates between 650, 700 and 730. Above 1200 rpm truck runs good. Not great. Slight shaking between 15-40mph similar to symptoms that come with center shaft bearing/support. Engine/trans mounts seem OK. No significant movement, clunking or thumping.

Fuel trims bounce around.
STFT bounces between -1.8 and +3.7 on average. Always fluctuating but usually on the + side.
LTFT has been pretty consistent at 3.7 and sometimes 0 and as high as 5 while driving it for a while.

Both O2sensor readings before cat bounce back/forth between high/low readings.

I did my best to check for any air leaks and i have cleaned the passenger side valve cover breather/vent. I did also spray throttle body with seafoam spray and have noticed these issues since the pass valve cover service and throttle body spray. Drivers side valve cover breather has not been done yet. I stopped after slightly disturbing the cracked breather hose.

I did also put regular seafoam in the fuel tank. Is it possible that either clogged up my fuel filter and/or injectors? I have a new OEM fuel filter, just seems like alot of work so I wanted to get your opinion on my next move. I have read how to remove IACV/Throttle to clean it correctly. I am just feeling burnt out dealing with this.

I have tried swapping the new wires and plugs with another cyl. with no change. I did the throttle service and seafoam additive after replacing the plugs, but before replacing the wires. I plan to change the plugs again after things are back to normal.

No CEL triggered until above 4k rpm. New Bosch coil on cyl 2. Misfire shows on that cylinder first usually. Sometimes to cyl 5,6 and random multiple if driven hard.

Power steering pulley has faint rattle and shake. I have the replacement pump and pulley, haven't replaced either yet. Prefer to address the misfire/rough idle first. (I am unsure if power steering pump would cause misfire)

Maj. Dundee

What "slightly cracked breather hose" are you referring to? A crack in any one of these hoses will cause a vacuum leak.

03MBML350

Thank you for replying. The hose was number #140. I replaced it yesterday. I haven't cleaned the breather yet on the driver's side because I wanted to have the replacement hoses 140 and 146 on hand first. 141 just came in today $6 OEM. I also removed and cleaned the MAF (Bosch 3 months old). It drove much better since the hose was replaced.

After a 40 mile drive yesterday I then removed the neg. battery cable for 1 hour and code reader confirmed all fuel trim setting reset then drove another 40 miles and still have the misfire issue above 4k rpm. First cyl misfire is always 2. Then mis on 2,3 and 6. Sometimes even random multiple as well.

I still have a rough idle that causes the steering wheel to shake. The power steering pulley is making more noise than it used to. Likely the power steering pump/pulley is about to give up. I was planning on replacing the pump and reservoir myself next but I prefer to address high rpm the misfire issues first.

What is the recommended next step? Diagnostic at MB or indi shop?
Replace fuel filter or steering pump?

I did notice some oil had made its way to the throttle body. Should I remove it then clean it again along with the IACV? Push something through the driver's side breather connection at hose to ensure they are not blocked? I wouldn't think that could cause a high RPM misfire.

This was the pump that I ordered last month for $120 and was planning to install on Friday:
When I pulled it up again today it says "doesnt fit my vehicle"

The following pump now shows as an exact fit for $67 :
At this point I am just trying to get the ML running correctly and I plan on replacing with an OEM steering pump. Until then I don't wish to spend $300 plus on a pump until I am sure the issues are resolved.

Maj. Dundee

Please, let's stick to one problem at a time, the misfire, and the misfire only.

Is this the #2 cyl. that you replaced the coil at?

Fuel trim values are supposed to fluctuate. If there was a problem P0170 or P0173 would be set.

If code P0300 is set, report that code number, don't simply state random misfire.

Stop disconnecting the battery thinking that will cure your problem. Also stop clearing the codes with the scanner. After you replace a part, take it for a drive and if the problem still exists, misfire codes will reappear. When you turn the eng. off, misfire codes are automatically cleared by the ECM until the next event.

03MBML350

Yes. The no. 2 cylinder coil.

Maj. Dundee

Have you confirmed #2 cyl. by opening the pdf? If you did then the only thing left is a faulty injector because you have replaced everything else.

03MBML350

Yes. At 5k rpm misfires p0302 0303 0306 and 0300 random multiple

Maj. Dundee

With the proper scanner you would be able to see how many misfires occur on each cylinder. There could be misfires on all the other cylinders but not enough to trigger a code for the remaining cyls.

Misfires are difficult to diagnose. It could be fuel delivery (filter, fuel pump, injectors, fuel pressure), electrical, clogged cat. converters, etc.

You would need to buy a few gauges and a tool (for the fuel filter). Where do you want to start?

03MBML350

I recent purchased a fuel pressure gauge but the hose is not the right diameter nor does it have the correct fitting for the schrader valve. I guess I could remove it. What other equipment/tools?


I was mistaken and the fuel filter will have to be ordered. The one I have is for our other far. Correct one is Mahle kl438 and is now on order.

Maj. Dundee

I asked you what you wanted to start with.

03MBML350

Fuel pressure is fine. Let's just keep this going. Your welcome to pick as we go I can keep up.

Maj. Dundee

Please don't tell me that the fuel pressure is fine when you have already stated that you do not have the right equipment. Fuel pressure must be tested at idle, at different RPM's and while going at certain speeds and at kickdown.

I recent purchased a fuel pressure gauge but the hose is not the right diameter nor does it have the correct fitting for the schrader valve. I guess I could remove it. What other equipment/tools?

If you want further help, be honest

03MBML350

Yes. It's fine/ok to start with checking the fuel pressure as the next step.

Fuel pressure is fine meant to start there next.

03MBML350

Fuel pressure at rail is 58lbs. Under various loads, Rpms, and kickdowns remained at 58lbs. Shut vehicle off then it dropped to about 30lbs after 30 minutes.

How would you like me to test the pressure?