Will every misfire generate a code?
 

My 2000 ML320 has stumbled at idle on 2 or 3 occasions. I'm curious if that is enough to generate a code. I've indicated the problem to the techs and they always come back with "unable to replicate". Just wondering if they are even reading the codes.

The misfire event has to be of sufficient duration to set a code. It all depends on what the parameters are for setting a code.
You can instruct the techs that you want a prinout of the short test which will show that there were no codes in the system.

Gilly

Thanks again Gilly!

I've had similar problems, intermittent rough idle. Twice the dealer has determined, from codes, that my air mass sensor is bad.

I usually buy only Chevron, but once in a while I fill up at my carwash to get a discount on gas with a wash. I don't tell the dealer this, but I think I may have gotten a tank of less than 91 octane there. The rough idle sometimes comes after certain fillups. It certainly did the last time.

They reset the fuel adaptations and off I go. What I suspect is that the ML doesn't adapt correctly to the lower octane, on the other hand, it could be your air mass sensor. Have them look at that before you lapse on warranty.

Mass Air Sensors are notorious for failing and aren't cheap to replace (~$250 US sans labor) yet your issue may reside in fouled sparkplug(s), fuel filter, bad gas, etc.

I've run 89 octane in my ML for over a year now with no adverse effects.

Autozone can scan your vehicles for codes FREE and clear them also with a little persuasion. :)

FR

Hi Fast, do you have an 8cyl or the 6? I hear of owners of the 8cyl swearing by 91octane so I've stuck with that. I wouldn't mind saving money on less octane..but my mileage has varied..unless I got 87oct and didn't know it!

ML Dude,

135K mile ML320. Six cylinder.

If I had a 430, it would receive a steady diet of 93 octane, no questions asked.

FR

FR,

93 octane, wish I had a handy source....they only feed us 91 in SoCal.

The question I have is whether the ML AAM or ECU should adapt to a "bad" tank of 89 or less octane. I thought so, but each time I "may" have gotten one, I've had problems. This would not be good going on after the warranty runs out and I would like to identify and correct this malfunction.

Rgds, Ed

The ML engines have knock sensors, so they ought to be able to handle 87 octane. When we first got our 320 my wife would always fill up with 87 and I would always get 91 or 93, whatever the premium was. When Mercedes sent us a letter telling us nothing lower than 91, she quit buying the cheapo gas. I can't sense any long-term affects, though power seems to be better with the premium. Any damage that would happen would be to internal engine components, not MAF sensors and the like.

Contaminated gas, on the other hand, would foul up the fuel system in short order. A dealer in Houston said the MLs are particularly sensitive to this.

dont use regular gas
 

The ML320 requires premium gas.
If regular is used the computer will retard the timing to prevent engine knock. That throws the emmission coltrols off..extra fumes / gas goes into the intake manifold, then contaminates the oil and oil consumption follows in addition to rapid sludge build up.