740 running rough, any idea's

[gaspump]

After a 250 mile drive friday, where the car drove great, it now is sick. Bringing it to the shop this week but wanted a few idea's from the brain trust first. The service engine light is on all the time, it starts slowly, runs rough. Sitting at a stop light I have to give it some juice and keep the idle up. This car has never given me one problem. So, any idea's of what to look for before it goes under the knife. Thanks for any help.

[Thresher]

Obviously you will need get to get the fault codes read.

That said... Possibly cylinder(s) misfiring? I got my E38 in 2018 and it now has ~ 174,xxx miles. Within the last two weeks my check engine light lit up on two separate occasions. I have a diagnostic scanner and checked the fault codes... I saw messages for misfire on cylinder #6 the first time, and then ~ 10 days later it was on cylinder #3.

Remedy for misfire on cylinder #6 was to replace the ignition coil and for good measure I replaced all 8 spark plugs at the same time. Remedy for misfire on cylinder #3 was to replace the ignition coil. Haven't driven car much since then. My mechanic mentioned that he has seen multiple ignition coils fail in close proximity to each other time wise. I thought about that long and hard. Since this is my first E38 and past history of the car is sketchy at best, I opted to purchase a brand new set of 8 ignition coils and boots. I will install the full set in the car this week. The two previously replaced ignition coils will be moved to my spare parts box as they also fit my '96 E36 which is still running on its original ignition coils.

Back in March I also had to replace the throttle body assembly; upcoming work that my improve my engine includes fuel injector cleaning. Ignition coils, spark plugs and throttle body assembly are easy to replace if you like to DIY. If you have a trusted shop, take it there. Otherwise, find a local store like AutoZone which can run a diagnostic scan for you to learn more about what is causing the check engine light. Don't have a scanner? Consider getting one. I have both a Blue Driver and a Foxwell scanner. Good enough for me and the stuff I am willing to do myself. My mechanic has better tools.

Assuming your battery and alternator are fine? I have a voltage indicator I can plug into the cigarette lighter... voltage when engine is off is normally about 12.6 - 13.8; with engine on and alternator working its magic the voltage reads 14.0 - 14.1.

Hope that helps... good luck!

[Timm]

This should help you as long as you have INPA:

[acspecialists]

I'll put my money on vacuum leak or MAF. The MAF is easy just unplug it and see what happens? I was about to pull my transmission but bought an OEM MAF for $400 and all of my problems went away. Smoke test for vacuum leaks or pull the dipstick and listen for gurgling? Timm has an awesome page on proper vacuum for an E38 as well.

[gaspump]

Today it was warm enough to work on the car. I sprayed out the MAF unit, reinstalled it and the car ran good. Gone is the rough idle, etc. The service engine light is still on. When i got back from my drive, the idle surges from 500 up to 1000 and back down. There's still something wrong but its better then before. Im taking it in tuesday for a better lookee. Thanks for the help.

[KinkedChrome]

Today it was warm enough to work on the car. I sprayed out the MAF unit, reinstalled it and the car ran good. Gone is the rough idle, etc. The service engine light is still on. When i got back from my drive, the idle surges from 500 up to 1000 and back down. There's still something wrong but its better then before. Im taking it in tuesday for a better lookee. Thanks for the help.

I was fighting idle and misfire under load issues with my M62tu engine. I replaced anything that could cause a vacuum leak, had the injectors professionally cleaned, and I put all new spark plugs in it. All my issues went away. I feel confident though that it was the injectors and the brake booster line causing most of my issues.

[gaspump]

Second run today. Still a little rough idle, rpm goes from 500 to 1200 and then slowly down. A side note is that the tranny wants to stay longer in a gear then what I'm used too. It doesn't want to shift down as normal as you accelerate. Any ideas on this?

[acspecialists]

Before I replaced my MAF I almost swapped out the transmission. I still have the spare transmission, you messed with the MAF and the problem changed. It's a lot easier to change the MAF than the transmission. Get OEM, I tried two different aftermarket MAF's before dropping $400 on an OEM MAF. That was two years and 40k miles ago. The TCM and DME work together to decided when to shift the car. If the DME sees something wrong it will effect the shifting. I think it will take four starts to clear the code. I would just get a BT OBD2 adapter then you can drive and watch the signals as well as clearing codes. And don't forget the smoke test, vacuum leaks are bad. If your codes are lambada limits reached then you don't have a transmission problem.

[mikey3396]

What brand and model of spark plugs do you recommend for my 2001 e38 4.4l m62tu