Misfires dont know where to look next

[Smokphattys]

So it appears this is a pretty common problem but I cant seem to find any solutions and I hope someone has found some new insight. A few days ago the car started running rough, jerking and hesitating as though it wasnt getting a steady supply of fuel. (for a few weeks the cel has been on for the cat) no codes other than the cat code were coming up so I figured it was due for a fuel filter anyway and changed it. took her for a boot and shes running just as bad and the cel comes on. this time its throwing misfires on 1 and 4. so i swapped coil packs 1 and 2 to see if it was the coil pack. then it started to throw codes on all 4 cylinders,WTF? i immediately switched the coil packs back and it is still running rough but wont throw any codes (at least not in a couple blocks)so i dont know where to look now.. help please!

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[contact148]

well if you swapped the fuel filter then got those codes and rough running id double check the filter connections....

you my have just got some air in the system too

I had misfire when i removed my fuel rail due to air

[Jp's_C230k_W203]

Hey man it sucks that this is happening to you but it has happened to me too. I could not figure it out. I took it to advanced auto parts and they read the code and it was a couple of misfires. I just wanted to change the plugs anyways. So after I changed all the plugs the problem went away and it's been running perfect ever since. It might not be your plugs but it's not a bad place to look and it shouldn't cost you much and take a rather short time to do.

[pcy]

Originally Posted by Smokphattys

...for a few weeks the cel has been on for the cat...
no codes other than the cat code were coming up...

Partially clogged Catalytic converter/oxygen sensor may be the culprit.

[Smokphattys]

i changed the plugs to iridium ngk when i got the car about 20k ago.. shouldnt need new plugs yet. how do i get air out of the fuel system? wouldnt it work itself out within a few minutes of running? and would the cat cause a misfire? its been throwing the cat code for a while but the only replacement cat i can find for this model is over a grand. there is no magnaflow application..

[Smokphattys]

oh ya, when i put it in neutral it wont rev past 4000 rpms also

[Jp's_C230k_W203]

Yeah I bought my car in february and they told me they changes the plugs so I didn't think it was the plugs. Anyways best of luck with finding the solution to your issue.

[Smokphattys]

I know the plugs were done because i did them myself, also both o2 sensors were done at the same time. Swapped the coils out a few times and the codes just jump all over (not relating to coil position) so it cant be the coils or wires.. I did a compression test and the first 2 cylinders were at about 125 and the second 2 at 150. A little oil brought the first 2 up in compression and no coolant in the oil so im guessing the rings are starting to go but i cant see that causing random misfire codes.. the car seems to idle almost normal but if its in neutral and i give it lots of gas it will rev even and fast up to 4000 rpm. with medium gas it also appears to rev evenly to the top of the range. but if i just give it a tiny bit of gas it actually goes down in rpms.. could this be a fuel pump problem? egr valve?

[Jp's_C230k_W203]

Yeah, I'm not saying that it's the problem I was just suggesting that because it happened to me. I'm not really sure if it would be fuel related or not. What do you think about it being catalytic converter issue as pcy suggested.

[Smokphattys]

Ya im just not sure if that would cause a misfire.. also i would rather fix the cheap stuff before i dump $1800 on a new cat. wtf mercedes?

[mtnman82]

Did you ever have the 'infamous' $12 hose replaced?

[Glyn M Ruck]

The misfire is not being caused by the CAT as long as a piece of honeycomb has not broken off and blocking the exhaust in which case the car would wheeze.

What fuel filter did you fit? We have had a number of non genuine fuel filters lately with faulty pressure regulators that are built into them.

[Smokphattys]

Yup $12 hose fixed by stealer, mahle fuel filter..

[jedcred]

Originally Posted by Smokphattys

oh ya, when i put it in neutral it wont rev past 4000 rpms also

That's limited by the ECU by design.

[Smokphattys]

ok so it looks like i only replaced the downstream o2 sensor. is it possible for the upstream sensor to stop working properly and not throw codes?

[Glyn M Ruck]

Originally Posted by Smokphattys

ok so it looks like i only replaced the downstream o2 sensor. is it possible for the upstream sensor to stop working properly and not throw codes?

The downstream O2 sensor will not cause any running problems. All it does is monitor Cat performance. The upstream sensor can cause engine stumble under acceleration. Should throw & code & if bad a fuel trim code.

You say your plugs are new - I would once again be suspicious of your coils. Did you cancel misfire codes when moving coils around? If not the system would remember stored codes & record new ones.

[Smokphattys]

ya i cleared the codes when i moved the coils.. one time i got a misfire twice on the same cylinder. unhooked the upstream o2 sensor and no change in idle so it cant be the o2 sensor..

[Smokphattys]

ok i think i might be getting somewhere..did some research and found the pcm gets its info from the camshaft position sensor (2 on our car- intake and exhaust) as well as the crankshaft position sensor. starting with the easiest to access i unhooked the intake camshaft position sensor, started the car and it seemed to idle normal. took it for a test drive and she runs smooth (not as much power but i imagine thats because the sensor is unhooked) definitely no missing or lagging. took it back and ran the odb (CEL on) and the only code i got was the sensor i had unhooked. no misfires! so im going to get a new sensor asap and we will see how it goes.

[Glyn M Ruck]

Camshaft sensors can do strange things if you end up with too much overlap.

This is not common but good luck!

[Smokphattys]

overlap?

[Glyn M Ruck]

Overlap = Inlet & exhaust valves open at the same time.

[Smokphattys]

and how would i know if thats happening? put the new sensor in today and it still runs like crap... Im not getting any codes but the following monitors..

A/C Refrig Mon - N/A
EGR system - N/A
MIL status - OFF
Catalyst mon - INC
HTD Catalyst - N/A
Evap system - INC
Sec air system - INC
Oxygen sens mon - INC
Oxygen sens htr - INC

[Smokphattys]

Did a compression test and im coming up low across the board. about 145 from every cylinder., the book says it should be 172-217. only went up 5 psi with oil. unhooked the egr and it runs smoother but still no power.. theres no coolant in the oil so im at a loss..

[Glyn M Ruck]

Did you compression test at WOT? Must have throttle wide open to get an accurate reading.

I'm worried you have the early M271 head fouling problem & that valves are not sealing.

Do you have access to a boroscope to check for heavy deposits on valve tulips?

Otherwise you can do a leak down test to see where you are loosing compression.

Does the car use much oil? What mileage?

[Smokphattys]

did test at wot.. but had the air filter and maf off also.. no boroscope and not sure how to do a leak down. It has 185000kms (canada) around 115000 miles. it wasnt burning much oil, tho the exhaust did smell a little and one of the plugs was too short. I have new filters, plugs, oil. the cat is plugged from the poor treatment by the previous owner but that wouldnt affect compression would it? I know i need to fix it i just dont want to dump all that money in if the engines f#&*d.