hello! got a bit of a mad one. this week after not driving all weekend i started the car and the engine was extremely rough and sort of surging over and over.

unplugging the MAF makes it smooth out, i managed to drive it around the estate easily and it drove great until i slowed down to park and it was very lumpy. Plenty of white smoke and my friend who was there said it stinks of petrol!

Im going to check the breather hose under the airbox at the weekend and look for any leaks or split pipes. last time i saw it was fine, i even have a replacement on hand anyway! i will try and post a video of what it is doing.

air filter is ok, fuel filter is in the fuel filter is in the tank so i cant access it. hasnt lost any coolant. brand new spark plugs a few months ago.

thanks in advance guys!! hopefully something simple!!

 

not the best video but you can hear it surging. it was actually way worse than this!

 

Hi dbclc123, I hope this will help.It sounds like you're experiencing a rough idle and performance issues with your M271 engine. The symptoms you've described, such as rough idling, surging, white smoke, and a strong petrol smell, can be indicative of several potential issues.

Given that unplugging the Mass Airflow Sensor (MAF) seems to affect the engine's behavior, it might be worth considering checking the MAF sensor itself. Sometimes, a faulty or dirty MAF sensor can cause similar symptoms. You might consider cleaning the MAF sensor with specific MAF sensor cleaner to see if it improves the engine's behavior.

Additionally, since you've mentioned checking for leaks or split pipes in the breather hose under the airbox, that's a good starting point. Vacuum leaks, especially in the intake system, can cause rough idling and performance issues. Inspecting hoses, connections, and intake components for leaks or damage is a smart move.

Considering you've already checked the air filter, replaced spark plugs, and have a replacement breather hose, another possibility could be issues related to fuel delivery or fuel system problems. Although the fuel filter is inside the tank, issues with fuel pressure or injector problems might also result in the symptoms you're experiencing.

Using a diagnostic scanner like the Youcanic scanner is indeed a good idea. Scanning for fault codes could provide crucial information about potential issues with sensors, fuel trims, or other components that might not be immediately obvious.

Since there's white smoke and a strong petrol smell, this could suggest an issue with the fuel mixture, combustion, or even potential internal engine problems. If the issue persists after checking and potentially fixing these points, it might be advisable to consult a professional mechanic or a Mercedes-Benz specialist to perform a more in-depth diagnosis and address the problem accordingly.

 

Thanks for the reply.

I do have a spare but used MAF i can try. I have a cheapo eBay obd scanner that I will try tomorrow, can't get to my usual MB guy as he is too busy/unavailable. .

Is it safe to run the engine with the airbox removed for a few seconds as I'm diagnosing? How many things can I remove and safely start the car without destroying it?

Also. I had it in with MB for the sunroof recall on Tuesday last week then it seemed a bit lumpy after that, by Monday morning it was gone. Is there anything they would have done that could have altered something or knocked something loose/damaged a hose or pipe? Seems weird it was fine until the car was in at MB!


Thanks.

 

Running the engine for a brief period without the airbox is generally safe, especially for diagnosing issues. However, this should be done cautiously and for short durations to prevent any debris or foreign objects from entering the engine.

As for the service at the Mercedes-Benz dealership, it's possible that during the sunroof recall, various parts of the vehicle might have been moved, adjusted, or checked. It's not uncommon for minor issues to arise after servicing or repairs, especially if something was inadvertently disturbed or if a connection or component was touched.

Regarding the lumpy performance after the dealership visit, it's plausible that something might have been inadvertently affected during the service. Check for any disconnected or loose hoses, wires, or connections around the area where the sunroof was serviced. Inspect for any obvious issues such as hoses that might have been accidentally disconnected or components that might have been disturbed.

If the problem persists or if you're unsure about inspecting it yourself, it might be worth taking the vehicle back to the dealership to have them re-examine the areas affected during the sunroof recall to ensure everything is properly connected and functioning as it should. If necessary, inform them of the performance issues that arose after the service for a comprehensive recheck.

 

thanks again for a detailed reply. after taking the airbox off today and disconnecting various sensors it is hit and miss. sometimes runs smooth and sometimes is extremely rough.

the last thing i just checked before it got too dark was the fuel pressure which is sitting around 50-55 (psi?). started the engine and it held the pressure steady.

turned the engine off and left it 15-20 minutes and it still had around 50psi fuel pressure.

when i started it the second time it was extremely rough again and the fuel pressure gauge was bouncing up and down like mad!!

any ideas with the new evidence? seems likely its fuel related.

thanks again.

 

Have you checked the fuel filter? A clogged fuel filter can restrict fuel flow, causing pressure irregularities and rough engine performance.

 

Is it accessible? From what I can tell it's part of the pump enclosure under the rear seats, is that right?

How do I check it?

Thanks for the reply again! Car is still on the drive

Edited. Is the filter in the pump or is it in the other side, the sender side? Can't find anything about it and most sources say the filter is under the car which is wrong. Thanks.

 

For the M271 engine, the fuel filter is typically located within the fuel tank itself, as part of the fuel pump assembly. Replacing the fuel filter in an M271-equipped vehicle, it might involve replacing the entire fuel pump assembly.

 

Check engine light on?

Sounds like a vacuum leak the PCV is located below the air box under the supercharger that is known to fail and crack causing rough idle. In additional to PCV along with it's hoses the vacuum booster in the rear of the head also fails and needs to be replaced. While your at it you can remove the throttle body and give it a good cleaning.

The fuel filter is located under the vehicle you can remove the driver side by rear.

 

Check engine is off. It came on because I unplugged the MAF for testing. Cleared the code and it stayed off.

I cleaned the throttle body out not too long ago.

Top pcv hose looks brand new with latest part number. The 2 under the supercharger and the valve look fine as far as I can see with my camera scope thing. Checked all hoses I can see and they all look fine with no splits or hissing.

Fuel filter is part of the pump assembly


The only other thing is a ticking/tapping that sounds like it's coming from the rear of the supercharger. You can hear it in the video, not sure if that's normal or not.


Cheers.

 

unplugged the coolant temperature sensor on friday and it ran smooth! so i ordered a new one and a thermostat as i think it was stuck open.

changed them both today and its no different. still seems to run ok with the temp sensor unplugged.

opened the oil cap again and its all milky so guessing theres water/coolant getting in the oil.

head gasket? would that cause the roughness? or could that water have gotten in when i changed the thermostat?!

thanks.

 

Here's a couple more videos I've just taken. In the video of me revving the engine, it makes a loud almost grinding noise. Doesn't do that every time though, only heard it that one time.

Starting it without the MAF still allows it to run smoothly for a while.

No overheating, no oil or coolant loss.

There is a loud ticking, tapping coming from down where the injectors are or maybe even inside the engine?!

 

did you fixed this problem ?

 

Timing chain had slipped and timing was way out!! Had it all replaced. Wasn't cheap. Probably should have scrapped the car