M266 poor idle and low speed hesitation

PunkyB

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Aug 31, 2021
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15
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W169 Classic SE A150 Berggris Metallic, Parktronic, electric folding mirrors and comfort seat pack.
Hi All,

I desperately need help diagnosing a problem with my 2009 Mercedes A150 with the M266 petrol engine just hit 90k. It's driving me mad because I have no fault codes and there seems to be no obvious reason for it I can see. It is also a bit random with some days better than others. When I initially bought the car I scanned it and found only two faults relating to the charge air cooler sensor. I cleared them and they have never returned. About a week after owning the car (it was raining if that has any bearing) the engine started running very badly and I had a misfire on cylinder 3. As the car had been sitting around a while at a dealer I put loads of good fuel and fuel system cleaners through and the misfire never happened again. The main problem is poor low rpm operation, the inability to smoothly hold say 2k rpm without fluctuation. When driving there is hesitancy and a little jerkiness but they usually disappear when warmed up and higher rpms are better. 3 weeks ago the car was fully serviced and since then it has run much much better, I cleaned the air temp and map sensors too when we serviced it. I've included a picture of the plugs that were removed to show how bad they were.

The issue is still there though even if it's not as bad, this hesitancy and poor throttle response at low Rpms and a slightly rough idle with the odd little shudder felt every now and then are still there . Fuel economy is good, performance in terms of acceleration and top speed match the claimed figures so no apparent loss of power. There is only one noise I can hear from the drivers seat which I have shared, like a tapping or knocking that sounds like it's coming from the left hand side of the car (RHD CAR) but under the bonnet I can't really pin it down it's more in the centre. It comes and goes and don't know if it's related to my problems.

Fuel trim figures short/long term never going into double digits. Short term tends to be minus all the time in normal driving and only a small amount positive at full throttle. I sprayed carb cleaner to try and find a vacuum leak with no luck and even tried using my vape as a smoke machine but did not see it gravitate anywhere. Only one error with Torque pro constant scanning which is rich to lean sensor switch time fail.

I'm thinking either vacuum leak, injector failure, coil pack or leads as likely sources of my issues. If it was any other car it would be so easy to just test all these things but given how difficult access is to the engine I'm trying to be sure about what might be wrong before deciding what path to take. So if anyone who has an opinion that has knowledge of engines all the A class can figure this kind of problem out please let me know I would be so grateful!

Sound mentioned :
 

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sambam2006

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Dec 18, 2008
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132
Car
S63L AMG, GLC COUPE
I had very similar symptoms back when I had my CLS 63 AMG, after replacing every spark plug, coil packs, camshaft sensor it turned out to be the o2 sensor that caused my misfire. The issue was the least expensive but the diagnosis by Mercedes was close to £700ish
 
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PunkyB

PunkyB

New Member
Joined
Aug 31, 2021
Messages
15
Location
UK
Car
W169 Classic SE A150 Berggris Metallic, Parktronic, electric folding mirrors and comfort seat pack.
Thank you for your reply. Does anyone know if I were to do some graphing with Torque Pro what sort of figures I should be looking at to identify a faulty sensor? I'm aware that there are voltages that I can monitor.
 
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PunkyB

PunkyB

New Member
Joined
Aug 31, 2021
Messages
15
Location
UK
Car
W169 Classic SE A150 Berggris Metallic, Parktronic, electric folding mirrors and comfort seat pack.
Sorry just to add my research tells me that I need I need to do my OBD graphing based on whether I have a narrow or wideband o2 sensor. I'm not sure which I have there are many I could see, but I THINK it may be the Bosch 4 wire sensor which is wideband? One thing I should say is that there has never once been smoke from the exhaust of this car, ever. No loss of coolant or oil either and no combustion gases present in the coolant in a test I did. The knocking sound can be eliminated by revving the engine up but once it settles to idle it kicks in after about 10 seconds? Could this perhaps be a bearing that is not being lubricated sufficiently with the oil pressure generated at idle?
 

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