Mis-fire codes & catalytic converter

[fema]

Following up on recent posts in another thread; my local shop read the codes for my check engine light as "mis-fire codes" and suggested I need plugs & wires replaced - something which I have NOT done in 128K miles. The tech said if this were left untreated it could affect the cat converter - which is expensive from what I hear! Does this sound like the right diagnosis?

They also suggested the fuel filter may need to be replaced, though he didn't mention a specific code - I have had the flashing low fuel light indicator recently which according to the owners manual means a fuelleak or loose cap. I have a call into the dealer to find out if the fuel filter (and supposed) line update has ever been done. Does this diagnosis make sense?

And finally, the thermostat may be failing because the running temp seems to be about 15* low; 175 vs. 190, according to the tech.

[azinn]

Originally Posted by fema

Following up on recent posts in another thread; my local shop read the codes for my check engine light as "mis-fire codes" and suggested I need plugs & wires replaced - something which I have NOT done in 128K miles. The tech said if this were left untreated it could affect the cat converter - which is expensive from what I hear! Does this sound like the right diagnosis?

They also suggested the fuel filter may need to be replaced, though he didn't mention a specific code - I have had the flashing low fuel light indicator recently which according to the owners manual means a fuelleak or loose cap. I have a call into the dealer to find out if the fuel filter (and supposed) line update has ever been done. Does this diagnosis make sense?

And finally, the thermostat may be failing because the running temp seems to be about 15* low; 175 vs. 190, according to the tech.

Plugs should have been done 28,000 miles ago, wires should be okay. If you have the old fuel filter it should be upgraded. As for the thermostat thats your call. My 2 cents.

[mbdoc]

on a 1998 ML the plugs should have been replaced in 2002.

1998 models list 4 years OR 100K max plug life...keep waiting & you will need 6 new coils as well!

[fema]

I understand I am behind on the plugs - can you tell me if this diagnosis makes sense and replacing the plugs will solve my problem? Will (could) it affect the cat converter?

Thanks.

[mbdoc]

An out-of-tune engine can cause "early" catalyst failure.

The W163 chassis (ML) already have a BAD reputation on catalyst failure.

There is even a RECALL on many models & MBUSA just was fined by the EPA (1.4 MILLION) for many cars from 1998 not passing emissions

[fema]

Thanks Doc!

[fema]

These are the codes that were pulled by the tech - can someone confirm what these mean exactly, thanks.

[mbdoc]

P0 304 means cylinder #4 in misfiring. #4 is the on the DRIVERS side, front.
Possibly a bad plug, plug wire, or coil.

P0 300 means that a "random" misfire was detected, but the misfire wasn't severe enough to detect which cylinder

[fema]

I appreciate this info - now the CEL has gone out, but don't worry I am doing the plugs! I've done some searches but its still not clear to me - (assuming the light comes back on) will the CEL go out if the problem is repaired, or must it be cleared by a tech (special "tool" required)? Does the light having gone out mean anything further?

[MSpielberg]

Can you double check this? I have a question about cylinder numbering on the ML320. How do they number to get driver side front to be #4? I have a CEL for misfire on number 1, and need to know where to start.

[azinn]

Originally Posted by MSpielberg

Can you double check this? I have a question about cylinder numbering on the ML320. How do they number to get driver side front to be #4? I have a CEL for misfire on number 1, and need to know where to start.

Cylinder numbering is shown in attachment -

[MSpielberg]

Originally Posted by azinn

Cylinder numbering is shown in attachment -

Thanks, never would have guessed it, based on the numbering of the Mercedes V8's