[boulderlund]

I have been chasing a misfire problem for several months. I have studied the posts in this forum closely and have performed all of the standard diagnostic tests, yet can not pinpoint the problem. The car is a 2008 335xi, automatic transmission with 121,000 miles on it.

Here are the symptoms:

1) I get misfire error codes for cylinders 1-3 only. I have never seen any errors for bank 2/cylinders 4-6.

2) I get mixture too rich errors for bank 1. Never for bank 2.

3) The car runs rough, especially pulling away from a stop or after the transmission shifts into 3rd gear.

4) I often get a flashing service engine soon light and the care seems to go into limp mode.

5) Noticeable raw gas smell in the exhaust.

Here are the steps that I have taken to diagnose or correct the problem:

1) Replaced all 6 spark plugs --- no effect.

2) Swapped the coils from cylinders 1-3 with those from cylinders 4-6 --- no effect.

3) Did a walnut blast --- no effect.

4) Swapped the injectors from cylinders 1-3 with those from cylinders 4-6 and re-coded --- no effect.

I did the injector swap last night and misfire error codes for cylinders 1-3 appeared on the first 1 mile test drive. While I had the engine cover off, I removed and inspected the spark plugs and performed a compression test. All 6 cylinders measured between 135-150 psi and all spark plugs looked fine. The white insulator on all plugs was just slightly tinted brown and there was little or no carbon build-up on the electrodes. I could not see any consistent variances between the plugs from cylinders 1-3 and those from cylinders 4-6.

Other potentially useful information:

1) The misfire problems started shortly after the dealer replaced the high pressure fuel pump, fuel pressure gauge, and updated the ECU software.

2) I am still using the older style (pre version 11) injectors.

3) My OBD reader shows large and erratic fuel trims for bank 1.

4) I am also experiencing persistent underboost errors. These usually occur around 2500 rpm when pulling in 3rd gear. These errors do not seem to be correlated with the misfire and mixture too rich errors, however.

My analysis:

I don't think the misfires are caused by bad spark plugs, coils, or injectors since the errors to not follow these components as they are moved from bank 1 to 2. I don't think the misfires are related to the underboost problem since this should affect all cylinders, and I seem to have misfires at low throttle settings where the ECU is not calling for boost. I am concerned by the erratic fuel trims and the correlation between the misfires and the fuel pump work performed by the dealer. I wonder if the ECU software update assumes that the car has the newer style injectors installed? I have never seen codes related to the VANOS or O2 sensors, but these seem like components that could cause the problems that I am seeing.

What to do now?

I don't know exactly where to go from here so I am hoping that some of you experts can suggest additional diagnostic steps. I have both INPA and testo running and I have learned how to use these tools. I can log just about any quantity with testo and I think this might be the best way to get more detailed diagnostic information. The only difficulty with testo is that many of the parameters must either be selected via their hex codes or by their German name. Thus if you have a suggestion of quantities to log, I could sure use the German equivalent and/or the hex code.

[byd02]

Hello, I know this is very old now but I am on the same road. Did you figure it out?

[IraHayes]

Originally Posted by byd02

Hello, I know this is very old now but I am on the same road. Did you figure it out?

His problem turned out to be the result of a bad pre-cat.

Read post 16 in this thread, then click on the link he provided in his post:
https://www.e90post.com/forums/showt...3#post22552723