drewk88

Hello all,

I need some help with a misfire problem on my 2004 S600.

My Star Diagnostic shows random misfires on cylinders 7, 8 & 9.
I can feel them slightly whilst sitting at stop lights.
While driving, they can be almost unnoticeable, unless I give it more than about 3/4 throttle.
Then the computer shuts down the injectors for the misfiring cylinders to protect the catalytic converters, and of course that's noticeable.

I've replaced all 24 spark plugs, 24 red insulator boots, the left side coil pack and the fuel pump & filter.
But I still get random misfires on the same three cylinders.
I even replaced the spark plugs and boots for the three bad cylinders a second time. Nothing changed.

I've not replaced any injectors yet.

This problem appeared within a day of washing the engine and has been consistent ever since.

I've also tried injector cleaner in the fuel, with no positive results.

Has anyone heard of anything like this?

Any ideas or suggestions would be greatly appreciated.

Thanks,

Drew

wallyp

Try cranking the engine in the dark, lifting the hood, and looking closely for visible sparks on the plug wires or coils...

drewk88

Thanks for the reply, Wally.
I just finished doing what you suggested.
No sign of sparks whatsoever, neither visual nor audible.

vmystikilv

After replacing everything are you still getting a check engine light? This sounds similar to an issue I had with my first ml. I got jump started by an old car called a dart and my car wouldn't act right . Mercedes replaced plugs , coils , so much. For a month the car was in the shop. Finally replaced the entire engine harness and all was well. Turned out a resistor went bad in the harness

drewk88

I can clear the check engine light, but it will come back on if I give the car enough throttle to trigger the injector shut down.

tusabes

Don't wash your engine

drewk88

I checked the component N91, ECI Ignition Systems Mains Unit.

The N91 provides four outputs, two to each coil pack:
Primary voltage ~ 180 volts DC, and Auxiliary voltage, ~ 25 volts DC

In addition to following the testing instructions in Star Diagnostics, I also tested the four voltages with the engine running,
so that the mains unit was supplying power to the coil packs during the test.
The Star Diagnostics test procedure only checks the voltages while the mains unit is not supplying power to the load.

All four DC voltages were well within specifications.
Additionally, I checked the four outputs for any ripple or noise.
They were all very clean, with less than 50 millivolts of AC.

drewk88

Update: In addition to the items listed in post # 1, I've now replaced:
Inter-cooler water pump
Fuel injectors
O2 sensors

I've also checked compression, 170 - 175 PSI in all cylinders.
I still have the misfires. Any ideas would be much appreciated

ibeforreal

Apparently when you washed the engine water probably created havoc with some of the electronics on the car. These cars are not like they were years ago when you could take it to a car wash, lift the hood and spray the you know what out of it.

tusabes

The proper way to "wash " an engine these days is to wet a rag with cleaner, then wipe the rag around the non electronic parts to clean off the dust

Follow up with a different rag that's had protectant sprayed on it if you want the plastic and rubber pieces to be shiny

Always spray into the rag, Don't ever spray water directly in the engine compartment

imusikpro

miy S500 happen the same. The Catalyc converter make some noise. Fix it last week . Misfire gone .

drewk88

imusicpro,

Did you have any diagnostic codes related to the downstream O2 sensors which indicated that the cats were defective?

Here's another interesting piece of information:
After clearing the misfire code P0300, the check engine light will not come on if I drive the car "easy" no matter how long I drive. But after the car is stopped for a short while and then re-started, the check engine light will come on after only a few minutes of "easy" driving.

imusikpro

last week , when i drive to work, the check engine on, but the car still driving fine . But when i drive home i notice loss power, check engine flash and the engine start shaking . I tow the car to the shop, they take out the spark plug, clear the code . But check engine came back only 1 day . They suggest to fix the Catalyc converter , problem solve .

Billh13

So that fixed it?

imusikpro

yes , problem came from catalyc converter