1998 740iL idle misfire in cylinders 1-4 only

[oxwood]

Hey Bimmerforums,
Last year I bought a 1998 prefacelift 740iL (non vanos M62) with bad timing chain guides, replaced the guides and chain and have been driving it ever since then. But for as long as the car has been running, the engine has been shaking mildly at idle and will throw a CEL, but only in neutral. The CEL will absolutely never come on if the car is in drive even though the engine shakes just the same at idle whether in drive or neutral. No difference in idle whether the engine is hot or cold, although the CEL never comes on if the engine is stone cold. CEL sometimes flashes at idle, but stops flashing as soon as I put the car in drive. I pulled the codes multiple times and the scanner records misfires in cylinders 1,2,3,4 (passenger side bank) and a code for "map cooling activation" (I am assuming this has nothing to do with misfires, cooling system works fine. Engine gets up to temp quickly and stays hot). Freeze frame data shows the misfires only occur at or near idle, never above 1200 RPM. Engine runs beautifully above idle RPM and gets great fuel mileage. 181k miles.

The troubleshooting I've done so far is:
-Replaced intake manifold gaskets, thinking that they were damaged on the passenger side and causing an air leak. Gaskets were fine, no change.
-Cleaned and swapped fuel injectors from passenger to driver side. No change.
-Cleaned and lubricated idle control valve. Valve was already clean and functioned fine, no change.
-Replaced upstream O2 sensors. Old sensors looked new, no change even with new ones.
-Sprayed carb cleaner and water on every plastic and rubber bit on top of the engine, in at least 5 separate sessions. No signs of any air leaks, which makes sense as previous owner replaced most of the intake and vacuum system, including PCV and hoses.
-Car passes California emissions testing, as long as I get lucky and the CEL stays off for just long enough. Which rules out a bad passenger side catalytic converter.

Notes:
-I replaced valve covers when I did the timing chain, so no oil in the spark plug wells.
-Exhaust smells clean, no gasoline odors coming from exhaust or engine area.
-Idle RPM increases a bit when I unplug the MAF but the idle stays rough, indicating the MAF probably works fine
-the lumpy idle is always present, but occasionally goes from mild to violent with no warning. During some of my diagnosis sessions I left the car idling for up to 20 minutes, and the engine might shake mildly for a minute, before getting very rough for another minute, before going back to mild.

Thoughts:
-Could a clogged cat cause misfires but still let the engine run clean enough to pass emissions testing?
-Do the M62s ever develop a vacuum leak that only affects one cylinder bank, but would elude carb cleaner and water spray? I was very thorough in my spraying.
-Can the DME ever send low voltages to the fuel injectors, on just one bank? Wiring harness looks fine. I have heard a voltage problem would cause the biggest issues at idle and would not mess with higher throttle settings, which matches my symptoms. Any way to check?

So for as long as I've had the car I have been baffled by these misfiring issues. Looking back through the paperwork from the previous owner, it appears the left side cylinder bank has been misfiring at least since the previous owner bought the car from a VW dealer in 2013. I normally would not bother continuing to chase ghosts like this when my car otherwise drives great but I plan to sell the car soon and I do not want to sell a car with mechanical issues. So after a year of browsing the internet looking at other similar misfire cases and finding no relief, I figured it was time to put my case up. Absolutely any help with troubleshooting is appreciated. Feel free to insult my mechanical skills too.

[Timm]

As you have diagnostics to check the adaptation values - they will tell you if you have a bank-specific vacuum leak - more info here:



Yes, catalytic problems can cause misfires at idle - this can be checked by getting to check the back-pressure at the Lambda fittings.

But, I would actually put money on the timing being too far retarded on bank 1.....

[oxwood]

Thank you for the response! Unfortunately my scan tool doesn't show the adaptation values, but I was able to find that the long term fuel trim values for Bank 1 cylinders are in the -4 to -7 range at the different times I checked, suggesting the computer is compensating for a rich condition by decreasing the amount of fuel injected, which to the best of my knowledge rules out air leaks. I will check out the catalytic converters next time I can have the car on the lift and will report back.

[oxwood]

Update: I had the car up on the lift today and I unplugged the pre-cat O2 sensors on both banks with the engine running and nothing change, the engine shook just the same. The puffs of air coming out of the O2 sensor holes on both exhaust banks felt the same to my hands and sounded the same, with the same volume and intensity. I then took a mallet and struck both cats and neither rattled. Neither cat is discolored from possible heat damage. Exhaust still smells as clean as it could possibly be from both muffler tips. Verdict: my catalytic converters are not causing the issue.

Next step: I will test voltage going from the DME to the ignition coils and fuel injectors to see if one bank has a voltage issue.