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post #1 of 3 (permalink) Old 01-11-2009, 05:40 PM Thread Starter
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Vehicle: 1997 E420 MB
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MB1997 E420

Hello everyone: i have a 1997 E420 and it's been giving me some trouble with some codes. one reads PO 300 which is random miss firing. does anyone have a good clue how to fix this...
and the other code was PO307 which is cilynder 7 mis firing and i replaced the coil and spark plug boot, and now it's reading PO300 which is random misfiring........

any help would be very welcome..

thanks, Sergio
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post #2 of 3 (permalink) Old 01-11-2009, 06:58 PM
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Go to W210 section and you will find TONS of topics about MAF failure what yours probably is.
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post #3 of 3 (permalink) Old 01-12-2009, 02:14 PM
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May wanna check the spark plugs as well.

If you do replace the plugs make sure you blow some compressed air into the plug holes before you remove the plugs to get rid of all the dust and debris so it does not fall into the cyllinder when you remove the plug.(BAD).

You would be suprized at how much crap can get into those areas.

Your random misfire codes could be the mass airflow sensor.

A good way to determine if that sensor is bad is to start and run the car, then knock on the MAF sensor like you are knocking on a door.
If the idle suddenly drops and comes back to normal, chances are the air mass is bad and very sensistive thus causing your issues with misfires, but this test does not always work but is a good indicator of a bad sensor.

The dealer CAN get the cheaper bosch inserts per your request, but only for certain models so check first.
Also look at the rubber seal area around the mass airflow sensor for a snug fit and all the plastic tube and pieces going into the intake area.
Alot of times these tubes and the housing that connects them are cracked (junk) and should be replaced.

BUT I would check all of the spark plugs and coil supressor ends (spark plug boots) for discoloration(white milky stains) or cracks, water evidence, mud, whatever.
I have seen some packed with muddy residue.
If your plugs have a dark, rusty colored circle or black spots at the base of the insulator chances are they are worn out.

It can't hurt to start small with plugs and supressors ends, then take it from there, but try knocking on the Air mass sensor first while engine running.

Probably a good idea to change the fuel filter and add a can of BG fuel system treatment.
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