SJMercLover

Here are the symptoms and early diagnosis by my regular shop:

1. Got check engine light. code : P0305 ( Cylinder 5 misfire). Note: the light goes away and then comes back ( usually when gas is lower half of tank)
2. Symptoms: Rough Idle. hesitation, slight shaking when speeding up.
3. Notice this more when gas is below the halfway line
4. My mechanic checked ignition coils, spark plugs to be good
5.. Did dynamic compression test to find carbon deposits on some of the valves. Recommended walnut blast, none of the shops perform that in the surrounding areas and research shows there are other ways to clean the valves too.
( like Sea foam spray through the intake manifold)
6. Questions:
- Does Sea foam spray work and worth trying ?
- I see some posts reporting bad O2 sensor causing it - how to verify that ?
- what else can be causing this ?
Love to hear some expert ideas on this forum

S-Prihadi

Check fuel pressure of Low Pressure sensor while idle and while doing redline 1st and 2nd gear.
If you read below 4.5 to 4.7 BAR, that is a clogged fuel filter. Must be redline/WOT yah.

If gas below 1/2 line is one of the repeating factor, you could also have leak on the plastic hose venturi/jet-pump output of fuel filter.... the one going back to fuel pump.
Fuel pump is at RIGHT saddle, Fuel filter is at LEFT saddle.
MB uses that jet-pump or venturi effect to bring back fuel from LEFT to RIGHT saddle.
If fuel tank is 3/4 or full, both saddles are in parallel, but not when fuel level is low.

Here is how MB twin saddle fuel tank looks like, so you can understand what I have explained
https://www.megaparts.eu/uploads/pro...1591704204.jpg

CaliBenzDriver

You are on the right track!
Read your Long Term Fuel Trim at idle fully warmed up.

What your current mileage?

Fuel delivery pumps can help cause these hesitation - Scan live fuel pressures during drive conditions.

Chassis has goofy voltage control that can dish out chaos in modules. Not primary here.

Any other Stored/Current fault collection??

SJMercLover

Thank you Prihadi - I will show this thread to my mechanic. What is puzzling is why does the code repeatedly show "Cylinder 5 misfire" ?

SJMercLover

Thank you CaliBenzDriver. No other codes - always the same cylinder and only one code. I am seeing some other posts suggesting changing the ignition coil(s). But my mechanic checked it out to be fine, same for the spark plugs. I would ask him to move the IC to #4 and see if the code moves.

CaliBenzDriver

Let your specialist be his own man. He knows about swapping ignition parts - I still don't know current mileage .

He already says it's not a spark issue. If you push him hard enough he will replace a piezo injector that will fix the misfire for a while with relearn procedure.

Compressions tested ok... good for you, this engine gets to stay in.

Next I would compare Fuel Trims - What are the numbers?

Camshaft wheels are not recognized to have shifted.

Low pressure or voltage would involve more cylinders.


What really helps is steady Cyl5-only misfires....

SJMercLover

Dear forum readers,

Thanks for all the great advise.

I had a hunch that my original shop (but new mechanic, looks like the shop changed hands in the last year) had not been very diligent. While there may have been carbon deposit on the valves, that may not have been causing the CEL.

So I took my car to another mechanic who came strongly referred and described the history and the tips from you all experts on this forum. He was happy to see I had done some research and helping him. The plan was to go through all the checks advised by the kind members of this forum on this thread.

One of the first things he did was to swap ignition coil to #4 and the error code changed to #4 and that made it very clear it was the ignition coil. He changed the ignition coil and also changed the Spark plug. I have been driving the car for the last two weeks, have gone through two gas tanks in that time, so far it's driving pretty smooth, no issues even when gas indicator is near the last line. Check engine light is gone. No pending error code either.

I am just wondering was that the real fix or just one of the issues. Also, if there are carbon deposits on the intake valve, will that also show up in similar symptoms ( rough idle and shaking) ?

Thanks again for all the great pointers.

S-Prihadi

Sweet......