Hello Benz Lovers,

Thanks for trying to help me out ~

Sooo... I drove this 2004 E320 with 150 thousand KMs. (anymore details I should included?)
It's been almost 6 months now since the problem started occurring.
When ever the misfiring occurs, the CEL would pop on, rough at idle and weak acceleration.
Had a quick scan with an OBDII and my phone, the code read PO301.

* = parts we've replaced
My dad and I started off with the spark plugs* and the problem went away for a while.
(It's actually stupid how fixing or pulling apart some stuff and putting it back made the misfiring go away for a while.)
Since the problem got back, we tried to fiddle with other potential parts that could cause the misfiring.
I think next we replaced the spark plug wire*. Still, no luck.
I asked my older brother for help, he called the engine mounts* are dead.
(Gotta say, I lack a lot of experience in car fixing... But ***** deep, I ordered the engine mounts* and changed it out with my dad.)

Was so happy here that the problem went a way for 2-3 weeks.
Until we had to change out the engine gaskets*. Days later, the misfiring on CYL 1 occurred again.

That's when I gave up and threw the car into Mercedes for a check up, they swapped out the ignition coils*, told me it was fixed, went and collect it. The problem occurred again.
Brought it back into Mercedes, took a loan car, about 1.5 weeks later, they called me back, said some wiring(s) were loose and fixed that (apparently 20 hours of labour, but the charged me 3). Went and collect it again, the problem kind of went away, but you can kind of feel the misfiring is going to come back. (Probably can say wasted $2000 AUD for that, this was in Melbourne.)

(I went and bought a new car after this and returned the car to my mum to drive the problem went a way for the past two months (end-DEC 2019 to FEB 2020))

Since the problem is back we are looking for other potential causes and replace whatever could be broken.
Luckily, I have my own car to drive around, so low-key slowly fixing this car.

We are looking to replace:
- Mass Air Flow Sensor
- Camshaft Position Sensor
- Crankshaft Position Sensor
- SAI, Control Valve and Pump Check Valve

Will be ordering them all at once from FCP Euro (for less $ on shipping).

In the case of after replacing these parts and the misfiring still goes on... What can be my next step?

(Someone is probably gonna roast me for having a ****ty car scanner.)

But nevertheless, thank you,
khei

 

Instead of shotgunning parts at the car (and replacing good parts for no reason)
and knowing that after messing with the car the problem goes away for a while

I'd be looking for a loose cable, connector block, or ground. A loose pin socket will dog you and they're hard to locate but experience has shown me that it could easily cause the issue you describe.

 

Hi rapidoxidation,

I agree shotgunning parts is a stupid idea but, six eye ***** scanning through the engine... We couldn't find anything loose. Ran our fingers through all those connections already, it seems tight and not an issue to worry about.

Any other options I have?

 

Check for vacuum leaks and get a real scanner that can read MB specific codes. Couple other things to check would be fuel injectors and O2 sensors. You could try replacing just the upstream O2 sensors but that's just throwing more parts at the problem although at that mileage they shouldn't be that badly worn out.

 

Hi - thank you to those who attempted to help me resolve the issue I've been having.

Just wanna leave a update here that ended up changing the fuel pump. It has been a few weeks now and the car is running much more smoother. Hope that this post will help someone else out in the future. And of course, no more misfires and check engine light