paulzale

I've done the search and cannot find anything exactly the same. I have the following issue:

Under hard acceleration, right at the shift point ~5,000-5,500 rpm, the check engine light will come one. It runs rough until I turn it off and restart. Then it is fine. I have OBDII reader and get the following codes P0300, P0304 and sometimes P0306. I know these are all misfire codes for cylinders 4 and 6. I changed the plugs this weekend, but the problem persists.

My thoughts are fuel pressure or coil packs. I changed the fuel pump/sending unit about 9 months ago, so my next step is to change the fuel filter. My thought is that maybe it is Fuel Pressure related due to it being only on cylinders 4 and 6, the furthest away? Next I will move the coil packs to see if the problem follows them.

Any other possibilities? It runs silky smooth normally and even under acceleration as long as I let off and let it shift below 5k rpm.

FYI, 187,000 miles. Thanks

PaulX608

I would certainly start with the filter. Would have been a LOT easier to get to all the lines when you did the pump assembly IMHO, but to each his own. The coil packs are pretty tough, they work or they don't. Other than that, maybe try not pushing your 12 year old, almost 200,000 mile V6 SUV quite so hard?

paulzale

Ha Ha Ha, I don't drive it hard at all and never push it, problem is that I have a 17 year old and its funny that it happens to him. I am even torn on fixing it as it keeps him in check! Also, yes, I'm kicking myself about the lines now!

paulzale

OK, I've replace the fuel filter and lines, but the problem persists. It is on cylinder 2 most often, but also get it on 4 and 6, all on far side of engine. It seems like it doesn't want to shift right at peak rpm, then the CEL, if I feather the throttle at that point it will shift and be fine. Thoughts? It shifts silky smooth at all other points without issue, never through a CEL only when hard acceleration or to pass on highway when it down shifts and rpms increase toward the high end.

paulzale

Any thoughts on this? Last time it happened I got P0173 as well as P0300, P0304 and P0306. Again, fuel pump, and filter have been changed. It runs fine all other times, just happens at wide open throttle right at shift point...

DrX

Did you swap the coil packs around? You've already changed the plugs, so next you need to replace the spark plugs wires on those 2 cylinders.

There's more info in this diagnostic manual. Read page 11/39, which explains that when too many misfires occur, fuel to that cylinder is shut off (causing the rough running your son experiences). useful-w163-documents-engine-112.pdf

paulzale

I'll switch the coil packs and wires with the other side to see if the problem follows them.

DrX

Before you start swapping things, peruse this diagram.

paulzale

I'm still having this issue under hard WOT acceleration. I am thinking the driver's side CATs are blocked up? P0300 P0304 P0305 P0306, any way to test this before spending money on new ones?

Maj. Dundee

Yes there are two ways and you choose which one.

The proper way to do it is, remove the front O2 sensor and attach an Exhaust Back Pressure Gauge (about $30) in its place. Bring the RPM's up to 2,500 RPM's. Any reading over 1 1/2 psi you have a cat. problem.

The second way is to remove the front O2 sensor and take it for a ride. If the misfires do not occur or are more diminished, then it is a good indication you have a clogged cat. This of course is not a foolproof test.

paulzale

Thanks, small investment to buy the gauge and do it correctly. Do you think this is my proper course of action given the steps I have taken so far and the OBDII codes I'm getting?

Maj. Dundee

Absolutely. The investment is much cheaper than the cost of what you already invested.

tonycurtis20

try replacing your PCV valve

plasticbaldy

OK Paulzale, Please don't keep me in suspense. I've got the same vehicle and the same problem - exactly - codes and all.
What was the solution please ?
I even have a son who drives it and it happens. lol
regards, Andy.

03MBML350

Same problem here. 2003 ML350. 200k miles. New OEM MAF, plugs and wires.

Misfire Cyl 2 above 4k rpm. Sometimes mis 2, 5 and 6 with and without random multiple code.

What was the solution please ?

plasticbaldy

Not sure if your's is the same symptom. Just Cyl 2 sometimes ? And I'm not sure that Cyls 2, 5 & 6 are on the same bank.

I just love these selfish *****s like Paulzale who come onto forums getting free assistance but won't offer something in return. He would have received 2 emails now as a result of my posting asking him what the solution was. I wish I could help you but I haven't solved mine yet. I have a feeling that it's the O2 sensors. Maybe try switching the left side to the right to see if the other bank goes into limp mode. That's what I'll do when I have time. I had to do a 1200km trip and had the
O2 sensors disconnected so the vehicle would run properly but this doesn't prove that they're faulty. Apparently it's the O2 sensors that tell the computer of misfire and other problems (by sensing unburnt fuel) so the misfire might actually be the cause or a faulty O2 might be telling the computer BS resulting in it going into limp. Please let us know if you diagnose something. I'll do the same.

BTW my cats blocked at around 190,000km and needed replacing. Have yours been done ?

DrX

This is completely normal behaviour. These posters not only ignore others who contact them to find out what the solution was, they usually don't post once more after they've resolved their issue, even if just to say thanks for the help.

No point feeling bad about it. It's a problem with the selfish society we live in.

plasticbaldy

I totally agree and I'm not feeling 'bad' about it. I'm bringing attention to his selfishness so that hopefully the next time he comes here for help somebody remembers this and tells him to go shove his head up a dead bear's asre.

Professor21

At the risk of being told to shove my head up an animals ***, I'm one of those "newbies" looking for help. I have the same scenario going on with my 2001 ML55. Keep getting misfires on 7-8, mostly 8. I've gotten to the sensors but haven't removed them yet. Am I on the right path? I've already replaced plugs/wires. Same problem though. At 4K rpm's, CEL comes on and the car runs rough. Turn it off and it will start fine and actually drive okay but at 4k, it does the same thing... HELP...

03MBML350

Have you recently done any maintenance work or opened anything prior to having the misfire issues?

Mine is a 2003 and misfiring above 4k on cyl both banks. I was going to do the fuel filter/regulator first before changing the front O2 sensors but the clips require a special tool and I don't want to cause any problems. Fuel pressure at rail was 58 and dropped to 30 after about 30 minutes.

I plan on changing the intake boot next, then either the O2 sensors or fully removing the tpi

My number is 804 nine nine one eight five five four. If anyone has feedback or just discuss their issue or situation.

plasticbaldy

Have you missed the point buddy ? Being a 'newbie looking for help' is not the problem mate. Plenty of generous people will help you. But if, when you solve the problem, you're not willing to reply to others with the information you've learned when THEY ask, then frankly, **** off now and don't expect others to help you.
On the other hand if you're willing to give as well as take then don't take offense with ridiculous comments like your "At the risk of being told . . . ."
There is NO risk to normal people who not only 'take' but also 'give'.

Now to provide the little bit of information that might help you - but I have to say that I'm not a mechanic and don't know anything about ML55s.
A W163 is a V6. When the computer senses a misfire it sends the faulty bank - either cylinders 1,3,5 or 2,4,6 (I think) into limp mode. My mechanic tells me that the misfire is sensed by the O2 sensor smelling unburnt fuel. He says that this is one of 2 feedback systems to the computer. I think he said the other doesn't sense misfires. Shutting the computer down and restarting lets the engine run normally until the misfire is sensed again. So we either ;
1 - have a misfire or
2 - the O2 sensor is sending faulty messages or
3 - the computer has a fault.
Reading the thread indicates that people who replaced most of the ignition and fuel supply components to cure a genuine 'misfire' didn't solve the problem.
I had an urgent trip to make so my mechanic disconnected the O2 sensors and the vehicle has been running well. However this is because the computer has sensed that there is no O2 feedback and set a default mixture setting for the engine. There is a chance that this default setting may be too lean and result in burned valves or pistons or worse.
I guess that now I should switch the O2 on the left exhaust with the right, reconnect them and see if the misfire fault codes changes to the other bank. This may indicate a faulty O2 sensor.
Somebody with more technical knowledge please feel welcome to comment.
Somebody who solved this fault please let us know what their solution was.
It's beginning to appear that the O2 sensors or their wiring are at fault. Luckily aftermarket O2 sensors are much cheaper than coils, leads, spark plugs, fuel pumps and filters or a newer vehicle. lol

Professor21

Thank you so much... I'm having the exact same issues only 7-8 show the misfire but 8 sometimes shows a current fault as well as a stored fault. I've changed all the wires/plugs and moved the coil in an attempt to see if the problem would follow the coil. It didn't. I tried to find a gauge to read the volume through the CAT but none of the parts stores knew what I was talking about. I'm going to try to remove the O2 sensor this evening and see if the misfire happens. It's usually around 4k rpm's that it fails. I'll be sure to follow up with what I've learned or not but I will follow up.

Thanks again,

plasticbaldy

I think that removing the O2 sensor will stop the misfire code appearing because the O2 signal is the only way the computer knows there's a misfire. But this doesn't answer the question. Is there really a misfire ? OR Is the signal from the O2 sensor faulty ?
I'm assuming here that your engine only has one exhaust pipe ? If this assumption is correct then try purchasing an appropriate sensor on eBay & having it connected. 'Appropriate' - because (from memory) there are 2 wire and 4 wire O2 sensors.
Also - are you certain that your catalytic converter isn't blocked ? I had to change mine (all 4) at ~100,000km and that seems to be the usual life of a cat in the V6 engines running on Australian fuel.
The symptom of my blocked cat was permanent limp mode but I don't know if the codes showed a misfire or not. I didn't ask the mechanic what codes were showing at the time.

Professor21

Thanks... I actually have dual exhaust. It's the 5.5 V8 AMG. After removing the O2 sensor, the misfire did not occur. I'm going to try to have the cats tested for back pressure before replacing them just to be sure. I'll follow up with the outcome.

Thanks...

art_arev

Your CAT is blocked. By removing the O2 sensor you created a way for the exhaust gases to come out, hence no misfire. Get under the car & tap the Cats in the bank were you removed the sensor & see if they rattle. If you want to buy a tool to check exhaust pressure check this out:
If the pressure at any time goes above 1psi then your CATs are blocked.