Hi would appreciate some help here. We have in our shop 1999 ML320, first presented 12/7/07 with severe misfire, codes P0300,1,2,3,4&6 stored. Checked spark plugs obvious point of call, platinum tips worn right down. Spoke to last servicing dealer (whom has maintained vehicle from new) Advised they were not renewed in last 2 services. Renewed plugs and cleared codes. Roadtested vehicle, heard slight tinny rattle under harsh acceleration from RH cat, and misfire returned. Carried out live data check on cats which showed to be working OK. Found 4 suspect leads, renewed whole set as cheaper than buying 4 from dealer (Bremi parts) Misfire only now evident on 1 & 2. Tried substitue coil packs, still evident. P0301,P0302
Advised client suspect RH cat failure causing problem.

Spoke to local dealer who said our diagnosis sounds wrong and possible head gasket failure Vehicle has done 143,000klms Dealer is also now stating they replaced plugs 23,000klms ago, yet it would be the first time I have seen that ammount of wear in that short a time. I am very reluctant to spend anymore of customers money, NO engine warning lights, vehicle just fails to react when driven hard and misfire occurs. Thank you

Forgot to also mention misfire evident on cylinder 3 when carrying out live data checks, but only registers in the data and doesn't log a code. Cylinders 4,5,6 all read 0 misfires.

Cataylst failure is VERY common on the ML's. Performing an exhaust backpressure test should show if that is the problem.

The ML's have been involved in a catalyst recall...I would check to see if that truck has had that recall performed..or if it falls into that recall range.

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No recall exstits on Cats in Australia, only seems to be US due to the emmision laws!

LH cat split around seams approx 12 months ago and she had that repaired, not new cat just seam welded.

Will test for back pressure on cats and keep you informed. Thanks

A defective MAF device will set those codes and cause random misfires. And WILL NOT set a proper code. Try to borrow a known good one for a test. I have heard, but not documented, that even new MAF devices have a pretty high failure rate.

Jim in Phoenix

Hi,
Had already tried know good MAF from donor vehicle. Fitted new RH cat last week and vehicle is running like a dream!!

Thanks for the help

thanks for the update.
Sure is nice to hear success stories on the ML320s.

Ive been working on stopping a slight misfire for 2 years.

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Steve Danville
what i did myself.
CPS
rear shocks
MAS x 3.
SparkPlugs (twice)
Fuel pump & Filter
Window switch
replaced both failed horns with Honda horns.
Cleaned ERG Tube
Oil changes
Oil consumption Fix
Brake Pads
A/C Recharge
Anti-Freeze change X 2.
Front Shocks. Monroe. $90
Mercedes Warrenty Work:
Harmonic balancer,power steering clamp. Cat converters.Drive Shaft Bearing.s
Replaced Outer Lower Tie Rods and front lower ball joints at 156,000 miles