940 B234F -91 running rough/3 cyl?

Ayy!

I'm trying to figure out what is wrong and feel quite stuck on this one.
Car is a 945 GLE -91 B234F (auto cambelt tensioner) with an M46.

I replaced the head gasket (0.1mm of head removed) because of tiny amounts of oil ending up in the expansion tank.

All 4 cylinders have compression, just around 12.8-13 bar.
I've cycled the spark plugs between all 4 cylinders, yet cyl 3 wont run smooth. Safe to say plugs are OK in my opinion.
Rotor, distributor and plug wires are all new. All plugs have a spark.
Cams+crank are all zero'd as you can see in the short film I've attached. The balance belt was removed years ago.
No fault-readings according to the fault-code box
Fuel injectors are all clicking.

Film:
https://www.youtube.com/watch?v=vLx0Kz-ziOU

As you can see she starts up and runs great for about 1 second. Then she dips down to the rough idle where I can't decide if one cylinder is out or just idling really bad. If I give her some gas she smoothes out. Pulling cyl 3's spark plug wire won't change anything, however when pulling the injector cable, you can hear that RPMs dip and engine starts shaking even more (that's where my hesitation whether no 3 is running or not comes from). Plugging it back will rise the idle to normal for 1 second, then back down to the rough.

Oh, and I realised that there's one cable not plugged in under the intake manifold. Is is supposed to attach somewhere? The 3 sensors all are plugged in, and the idle air controller has got its cable. Maybe it's just an extra one that has always been sitting there without me noticing??

Pic:
https://i.imgur.com/yDoyGqr.jpg

Thank you in advance guys!

Sounds like a bad injector. Try swapping those around.

Do a cylinder drop test and see if it is just one cylinder misfiring. = unplug one injector at a time. (sounds like you did this..)


spray around with brake cleaner/carb cleaner/propane/hair spray etc and see if you can find an air leak.

Harlard said:

Sounds like a bad injector. Try swapping those around.

Click to expand...

Swapped them around, no difference. Ohm-measured all 4 as well, all good there. However today there's a hissing noise around intake to cyl 3. Probably a simple leak...

ZVOLV said:

Do a cylinder drop test and see if it is just one cylinder misfiring. = unplug one injector at a time. (sounds like you did this..)


spray around with brake cleaner/carb cleaner/propane/hair spray etc and see if you can find an air leak.

Click to expand...

Only one cylinder misfiring. Today I noticed a loud hissing noise coming from the intake about where 3rd cylinder is located. Will definitely spray some engine starter around there and investigate further

Maybe you didn't torque a lower intake nut.

Problem solved! Tiny crack in the intake manifold gasket turned out to be the issue, not larger than a couple of mm. Never thought it would be this easy to solve.
She now runs perfect again

Thank you, you two, for taking time with the issue!

Thanks for posting back. Glad you got it!

I always perform a cylinder drop test on these engines by unplugging a fuel injector. It is safer for you, and also for the cat. Unburned fuel going into the cat can damage it.

A local guy with a 240 had a misfire after a "tune up". I narrowed down the offending cylinder in 20 seconds doing a cylinder drop test by unplugging the injectors one at a time. EASY!

Touching secondary ignition components with the engine running is just asking to get a strong ZAP. It hurts!

Yeah, the injector unplug is the the better choice. Being the swede I am tho, I've felt that zap countless times working on these ol' bricks while growing up. It is a part of the "midnight roadside redblock engine diagnosis", but it doesn't have to be