Hello to the forum, new member, old tech. I am a professional mechanic with over 40 years experience. This has me a little stumped. Got a '99 ML 320, with 300,000 on the clock. Problem, clear code's with Snap-On Solus, circa 2012, engine will not accelerate above 2500 rpm. Get code's for #4 & 5 cylinder misfire. Alldata say's use a scope to watch coil's. Don't have a scope, however, swapped out coil packs, problem still there, excactly the same. Put in a rebuilt MAF, a little better, but still won't accelerate over 2500. What is this? Anybody? please help, as we put an engine in this vehicle for this problem. The original engie did have lower compression in cylinder's 4,5,. 130 Psi. versus 210 in all the rest. The replacement engine has 210 in all cylinder's. Thank you in advance,,,Gordon

 

Try to replace front O2 sensor on that bank or swap sensors and see if you get the same results on oposit side of engine. If the sensors test good then front cat convertors may be internally broken and building back pressure. Also check fuel pressure (50-60 psi) at idle. My car could not reach more then 35mph when my fuel pump went bad but I made it home from ski trip since the trafic was slow nobody noticed I had trubble taking off but limited fuel supply to engine kept me going for last 25 miles of trip.

 

Thank you for your response. What I found was that the injector's were losing the pulse signal after a few minute's of running. Then the left cat was murdered from the low compression misfire that started this whole thing. So, I ended up replacing the engine, the PCM, and hollowed out the cat. Runs real good now, waiting for the CEL to illuminate from a cat efficiency code. On a side note, I did remove the upstream O2 on that bank, with no change.