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I have recently been having some issues with my W204, and it has progressively become worse. When I start the car in the morning, it will start a bit rough and when I look at the misfire count on my OBD2 there is roughly between 2-7 misfires on each cylinder. Cylinder 1 is the outlier and will have more. If you let it sit and run for a while, it is not as severe. Once you get over 2000rpm, the issue is not noticeable and it seems like under load there is nothing wrong with the car.
Then the next issue is that if you are cruising at a constant speed and low RPM, you can feel a slight stumble/misfire every 3-5 seconds. It is more noticeable at 60 km/h than on the highway. So when you are driving along for a 20min trip, there will be roughly 70-100 misfires on only Cylinder 1 which is weird. I can also note that there is a slight surging feeling when going up an incline. At idle, you can sit in the car and feel the stumbling as well as the misfires. I have revved the car up at idle to 2000-3000rpm and the misfires become very noticeable. I have also taken it to an Indy and they thought there was nothing wrong with it - so may need to go to another guy for a second opinion.
Last edited by JHEYD; 05-14-2023 at 09:06 PM.
Reason: Edit
Anyone? I’ve got negative fuel trims of -14% ant idle and the stumbling/misfire is becoming worse. I am thinking fuel injectors but don’t want to throw money at parts that are fine.
A few misfires at start is not unusual as these engines age (>150,000 km). But it should not be severe nor should it last more than 30 seconds ->2 minutes (i.e., after engine goes to closed loop).
2 things come to my mind:
1-Did you use MERCEDES spark plugs? The standard NGK plugs are good but not indexed. The 150 kW version of this engine can be a bit sensitive to ground electrode position. I needed 6 standard NGK plugs to get 4 with acceptable indexing. The MB plugs are indexed. If the problem started before changing the spark plugs, this is not the problem.
2-You have a scanner, so I assume you have looked for codes and do not have any aside from the misfires. Go to real time data and look at the fuel rail pressure. It will vary slightly but not greatly. Large changes (>10%) can be leaking injectors or failing high pressure fuel pump.
If all this checks good, you may want to pull spark plugs and compare number 1 to the others since it has the most misfires.
A few misfires at start is not unusual as these engines age (>150,000 km). But it should not be severe nor should it last more than 30 seconds ->2 minutes (i.e., after engine goes to closed loop).
2 things come to my mind:
1-Did you use MERCEDES spark plugs? The standard NGK plugs are good but not indexed. The 150 kW version of this engine can be a bit sensitive to ground electrode position. I needed 6 standard NGK plugs to get 4 with acceptable indexing. The MB plugs are indexed. If the problem started before changing the spark plugs, this is not the problem.
2-You have a scanner, so I assume you have looked for codes and do not have any aside from the misfires. Go to real time data and look at the fuel rail pressure. It will vary slightly but not greatly. Large changes (>10%) can be leaking injectors or failing high pressure fuel pump.
If all this checks good, you may want to pull spark plugs and compare number 1 to the others since it has the most misfires.
Thank you for the response Odd Piggy! I thought it was quite unusual as well for a car with 100k on the clock. I actually had this issue a couple months after I bought the car, I replaced them with NGK PLKR7B8E laser platinum spark plugs and it fixed the problem for about 2000kms before the issue returned (I believe these plugs are the stock ones). I replaced 1 spark plug for the second time on cylinder 1 as I thought I may have got a dud and when I pulled out the newish one it was coated in black. All I can say is that after replacing the plugs, it’s actually worse than when I first got the car. What plugs do you recommend that are as close to indexed ones? Are Bosch the way to go?
I have attached a screenshot of my fuel pressure rail, it sort of jumps around from 800-890. I took this after a short drive and this was at idle.
I actually prefer NGK spark plugs to Bosch. One of my acquaintances tells me that indexing is not really necessary for the M271 variants even though I have always done it. The NGK PLKR7B8E that you used are correct. Your fuel rail pressures vary a bit more than mine, but they’re probsbly okay.
I think the most diagnostic thing is the black coated cylinder 1 spark plug. That cylinder is either running very rich or is misfiring, which it is by your measurements. Cylinders running that rich should show codes for fuel/air ratios or O2 sensor readings out of range with leaking injectors. Are you sure that your coil packs are working correctly and spark plug boots are competent? I would want to be sure my ignition was working correctly before moving to change injectors.
It could be an ignition issue. When I crank the car after leaving it overnight it will misfire ever so slightly across all cylinders and then after 3-4 seconds it idles a bit better.
How do you check the spark plug boots? These are just the connecting plugs to the coil packs I assume.
I have gone through 2 sets of coil packs. I’ve tested a second hand set and then bought brand new ones but the problem still remains. I’ve attached some images of my O2 sensor readings, let me know if they are out of range.
Last edited by JHEYD; 05-14-2023 at 09:07 PM.
Reason: Edit
If your O2 sensors were out of range, they should trigger a CEL. You’re not getting any trouble codes, yet the engine isn’t running well. That’s puzzling. The misfires themselves should create a trouble code.
The check of spark plug boots is visual. If they are cracked, hardened, or oily, they should be replaced. Also be very certain that no thread anti-seize compound has gotten on them [that is NOT recommended on this engine, but some people use it anyway].
I’m down to guessing — have you used your scanner/code reader to check the readiness monitors? If the ECM is not recognizing a completed drive cycle, the OBDII will never go to READY to be able to read codes.
It is very odd, I think the misfires are so minimal that it doesn’t trigger a code. I’ve also had an Indy check it out on one of their expensive OBD2’s and they had no codes come up.
The boots last time I inspected them look like they are in good condition. I’ve sprayed some electrical cleaner on the contacts too just for reassurance. Do you know if the wiring harnesses on the M271 engines tend to go?
I have checked the readiness monitors on my OBD2 scanner, however, there are some that are not ready/incomplete. I always thought this was just the OBD2 not being compatible with my car, might be worthwhile getting the iCarsoft one that is compatible with Mercedes.
It is also worth noting that I have had a P0171 (System too lean) code come up a couple months ago, but I considered it really odd as my fuel trims were very negative and running rich. When I do go for a drive, my short terms are all over the shop. They get up to +20% positive which is why that code was probably triggered in the first place.
When I put my ear up to the throttle body I can also hear a pulsating noise of short bursts of air every now and then. Checked all the hoses connecting to it and they seem intact but thought it was a bit odd.
Last edited by JHEYD; 05-14-2023 at 09:08 PM.
Reason: Comments didn’t attach
It is very odd, I think the misfires are so minimal that it doesn’t trigger a code. I’ve also had an Indy check it out on one of their expensive OBD2’s and they had no codes come up.
The boots last time I inspected them look like they are in good condition. I’ve sprayed some electrical cleaner on the contacts too just for reassurance. Do you know if the wiring harnesses on the M271 engines tend to go?
I have checked the readiness monitors on my OBD2 scanner, however, there are some that are not ready/incomplete. I always thought this was just the OBD2 not being compatible with my car, might be worthwhile getting the iCarsoft one that is compatible with Mercedes.
It is also worth noting that I have had a P0171 (System too lean) code come up a couple months ago, but I considered it really odd as my fuel trims were very negative and running rich. When I do go for a drive, my short terms are all over the shop. They get up to +20% positive which is why that code was probably triggered in the first place.
When I put my ear up to the throttle body I can also hear a pulsating noise of short bursts of air every now and then. Checked all the hoses connecting to it and they seem intact but thought it was a bit odd.
I wonder if you have a vacuum leak? If so, that would cause the short-term fuel trims to act erratically. The symptoms may also manifest themselves as a misfire which would then explain why there are no misfire OBD codes.
Just wild guesses from me now:
-Along the vacuum leak line - maybe something that can cause fluctuation on vacuum like the crankcase vent valve?
-Turbo waste gate?
-MAP/MAF sensors?
-The ECM itself?
Most of those things should show other codes, however.
The wiring and connectors to the ignition coils on the W204 seem to be much better than the ones on the older Benzes from the 2000s so unless there is a noticeable problem, I wouldn’t suspect them.
How is the fuel quality where you live? Lower quality fuel can damage pumps, injectors especially if it contains methanol, more than 10% ethanol, or is stored in tanks with a water bottom seal.
Would you happen to know the parts that tend to fail that cause surging? I am getting this weird feeling that the car surges at at around 60km/h and then the car kicks into life after I get over 70km/h under load. Is that specific to injectors? It also bucks back and forth really bad at low speed.
I’ll have a look at the crankcase vent valve tomorrow, good idea. Fuel is good in my area, I have switched to 95 at Shell and occasionally fill up 98.
I’m at a bit of a crossroads here. Thinking it might just be worthwhile taking it back to my mechanic as he has done the work on the timing chain. It has actually come back worse than when I left it with him.
Would you happen to know the parts that tend to fail that cause surging? I am getting this weird feeling that the car surges at at around 60km/h and then the car kicks into life after I get over 70km/h under load. Is that specific to injectors? It also bucks back and forth really bad at low speed.
I’ll have a look at the crankcase vent valve tomorrow, good idea. Fuel is good in my area, I have switched to 95 at Shell and occasionally fill up 98.
I’m at a bit of a crossroads here. Thinking it might just be worthwhile taking it back to my mechanic as he has done the work on the timing chain. It has actually come back worse than when I left it with him.
I have attached my misfires on each cylinder, anything out of the ordinary?
You have misfires on all four cylinders, so the source of the issue has to be common all four. Aren't misfires the result of the spark not occurring at the correct time rather than a fuel or air problem? Do you happen to know if the mechanic replaced the intake or exhaust camshaft position sensors when the timing chain was replaced? There would typically be a code associated with these sensors, but it may be intermittent and not frequent enough to trigger an OBD code. Swapping out one or both of these is quick and inexpensive.
You have misfires on all four cylinders, so the source of the issue has to be common all four. Aren't misfires the result of the spark not occurring at the correct time rather than a fuel or air problem? Do you happen to know if the mechanic replaced the intake or exhaust camshaft position sensors when the timing chain was replaced? There would typically be a code associated with these sensors, but it may be intermittent and not frequent enough to trigger an OBD code. Swapping out one or both of these is quick and inexpensive.
When I had the timing chain replaced, the mechanic also put in a brand new intake which cost an ungodly amount. He said all the plastics were brittle and needed replacing.
I am thinking camshaft position sensor would be a good start, what about camshaft magnets? I’ve heard these go on this engine as well. Either way these parts have not been replaced
I think I may have found the source of my issue…. Camshaft magnets! Just inspected them today for oil in the harness as this is a common issue I hear. Intake side is fine, no oil or anything. However, the exhaust side was covered in oil. Have a look at the photo! I have ordered parts for the camshaft sensor and magnet. Going to replace all 4 parts as a round of maintenance. Will report back on the results in a couple days. The mechanic did a good job of sealing the old ones back up (see orange sealant) but not so good at inspecting for oil seeping through the connector.
Reporting back on the results of changing over my camshaft magnets. It was not the problem as it turns out. It has made the cold start a bit better but overall the problem remains, if not worse. I’m not really fussed about replacing them as they were drenched in oil and leaking through the sensor so was probably due. I haven’t changed the camshaft sensors - could this need replacing too?
So back to the drawing board again. I’m going to dump some fuel cleaner in it in the hope it might reduce the problem. I’m also getting this weird rattling noise when the air con turns on, sounds like it’s coming from the top side of the engine (not where the AC compressor is located. Could that be part of the issue?
Last edited by JHEYD; 05-14-2023 at 09:09 PM.
Reason: Fix number
Just wild guesses from me now:
-Along the vacuum leak line - maybe something that can cause fluctuation on vacuum like the crankcase vent valve?
-Turbo waste gate?
-MAP/MAF sensors?
-The ECM itself?
Most of those things should show other codes, however.
The wiring and connectors to the ignition coils on the W204 seem to be much better than the ones on the older Benzes from the 2000s so unless there is a noticeable problem, I wouldn’t suspect them.
How is the fuel quality where you live? Lower quality fuel can damage pumps, injectors especially if it contains methanol, more than 10% ethanol, or is stored in tanks with a water bottom seal.
Hi Piggy,
I have replaced all the fuel injectors with refurbished ones. There are significantly less misfires but I am now getting a very rich condition. Talking -30% to -40% fuel trims and it has triggered a P0172 code. Do you think this may have something to do with the fuel pressure sensor or the HPFP? I am getting as low as 640psi at idle. Here is the best screenshot I have of the psi. Let me know your thoughts.
The car has been out for at least a 45 minute to one hour drive since the work has been done, right?
Your your rich condition and trims are so far out that the ECU can't correct. I don't think the fuel rail pressures are out of line, so I wouldn't suspect the HPFP or the fuel rail pressure sensor. FRP sets codes P0190-4 also.
Does the short term trim change toward zero when you increase the RPM to about 2500 and hold there? If it ddi, that would have made me to suspect leaking injectors, except you have just replaced them.
Look at your O2 sensor readings. Make sure the engine is up to operating temperature. They should be fluctuating and averaging about 0.45 - 0.5V.
Also, check to make sure that the air induction system is tight and not leaking after the MAP/MAF sensors. Not likely, but maybe.
The car has been out for at least a 45 minute to one hour drive since the work has been done, right?
Your your rich condition and trims are so far out that the ECU can't correct. I don't think the fuel rail pressures are out of line, so I wouldn't suspect the HPFP or the fuel rail pressure sensor. FRP sets codes P0190-4 also.
Does the short term trim change toward zero when you increase the RPM to about 2500 and hold there? If it ddi, that would have made me to suspect leaking injectors, except you have just replaced them.
Look at your O2 sensor readings. Make sure the engine is up to operating temperature. They should be fluctuating and averaging about 0.45 - 0.5V.
Also, check to make sure that the air induction system is tight and not leaking after the MAP/MAF sensors. Not likely, but maybe.
Thanks Piggy. I have taken it for a 45min drive and it drives fine on the highway etc but once it coasts at a constant speed, I still get the slight hesitation. I might add that I have put in refurbished injectors that have been rebuilt with better parts etc. So my thinking is that the injectors I originally had in were so blocked up that the ECU adjusted for them throughout the life of the car. Once I put the new injectors in, they are so free flowing that the ECU has to cut back on the fuel trims. I have also noticed gas accumulation in the oil, so did an oil change yesterday as I think this may be enough to throw off the fuel trims too. Replaced the PCV valve as there has been oil entering the intake manifold. Car sits around -30% to -35% fuel trims which is just enough for a P0172 code not to be triggered. I can live with that. It might take a week or so before the car gets used to the injectors. I have also had the mechanic smoke test the engine and he has found no leaks.
The fuel trims do change towards zero when you apply throttle, but I am sure that there is no leakage as I applied petroleum jelly to the injector seals on the fuel rail and made sure they were seated correctly. Do you think I might have a faulty intake manifold? I think all the linkages are fine. That’s my last resort at the moment. Replaced the 02 sensor recently too, so I think that is operating fine.
04-19-2023, 07:26 PM
