I really need some help. My 1998 ML320 started misfiring on acceleration recently. I reviewed other posts with similar issues but have not found the final solution. Here are the codes along with what has been done to date:

P0300
P0301
P0302
P0303

When I start my car, all runs well until I hit 3000 RPMs. Then the misfiring starts and does not recover until I turn car off and restart. I am restarting my car at every stop light at this point. It is also sluggish on acceleration from a stop.

I have:

• Replaced all spark plugs
• Replaced all spark plug wires
• Replaced coils on cylinder 1, 2, and 3
• Cleaned MAF
• Disconnected MAF
• Cleaned EGR (fully functional)
• Cleaned EGR Intake (fully functional)

All of the above have rendered no change to the engine. The dealer told me that the engine is in great shape and only needed plugs and wires (for $1500!). Clearly that was NOT the case (glad I didn't spend the money...).

Any help would be GREATLY appreciated.

 

be sure the wires are on the coil packs correctly and securely and also be sure the wires are on the spark plugs all the way. they should click into place. also make sure that the wires are on the correct spark plugs A or B.

also its seems you have misfires on the right bank only.could be a bad or clogging right side CAT
or could be could be a fuel presssure issue...the fuel gets to the far side of the rail last....and might not have enough pressure there on that side...just a possibility.

gotta make sure that the basics are right first though.

 

Would this be caused by a vacuum hose crack or disconnect?

 

no...... a weak fuel pump which is very common on ML or an old dirty fuel filter could cause it

 

How would account for the fact that it runs fine after a restart? That is the piece that makes me think that it is electrical. Thoughts?

 

how many miles on this ML? so at 3000 rpm it starts misfiring again? does the engine light come on right away? if you drive at 2500 rpm will it still misfire?

 

I have 179K miles. it only misfires when I accelerate. However, when the RPMs drop, the misfiring does not quit. It only stops when I turn the engine off and restart. But it is fine again until I have to gun it. If I keep the acceleration reasonable (below 3K rpms), i can drive for quite a long ways before it misfires. It is really perplexing. i will try the fuel filter since it likely needs replacing anyway.

 

the filter does have a fuel pressure regulator built into it....usually when the regulator goes bad youll get an extended crank, hard start after sitting for awhile.. kind of a bitch to replace. the lines on it are hard to remove. if you have a fuel pressure gauge put it on the car and it should hold at about 60 psi always. even after shutting it off.

 

it would be nice to see how many times the 3 individual coils misfire.. if its all at the same time or at differnt times.

try to loosen the right side cat at the manifold so you can by pass a clog if there is one see if it drives better. ive seen clogged cats do that. as you start to demand more gas it creates to much back pressure but going slow its ok.

you can also try to unscrew the o2 sensor from the exhaust before the cat instead of loosening the exhaust. then drive it to see if any differnce

 

could maybe have to do with right side O2 sensors