Pulling plug wires to diagnose an issue...

[Tomcat BMW]

My 1991 850i seems to have a very slight, idle/off-idle "rumble". I wonder if one of the cylinders/spark plugs is not firing?
Whenever one of my six cylinder cars has this, I have been able to pull a plug wire (one at a time) until one does NOT affect the idle...that is the bad one and then I can start figuring out what is wrong.
There are no codes/warning lights/etc...

I pulled the first plug wire on the 12 cylinder and nothing seemed to change...so I though, how lucky that the first plug is the problem one. However, I replaced that wire and tried the next plug wire...same thing, it seemed to have no affect on the idle. I tried every plug wire (one at a time) and could not perceive any changes at idle. It seems that the 12 is too balanced/smooth to let one plug cause an issue.

Has anyone else tried this technique with the 12 and were their results different? Any trouble shooting suggestions? Car/engine is smooth as silk once off idle.
Thanks,
Tom

[shogun]

Could be many things such as DK's dirty or the coding material in the area where the brushes are in idle is defective, http://www.mwrench.com/Whitepapers/DKMotorcleanup.pdf
vacuum leaks, weak fuel pumps, spark plugs, distributor, distributor rotor, ignition coil, fuel injectors, ground cable from engine to chassis, defective hoses in fuel tank breather system http://www.realoem.com/bmw/enUS/show...diagId=13_0354 etc.

Ignition cables you have to test
M30/M70 ignition system data accdg to Bentley
Coil primary, coil code # 2051118335 terminals 1 (-) and 15 (+) resistance 0.50 ohm
Coil primary, coil code # 20510171101 terminals 1 (-) and 15 (+) resistance 0.37 ohm
Coil secondary, coil code # 2051118335 terminals 15 (+) and 4 (ctr. resistance 6.0 kohm
Coil secondary, coil code # 20510171101 terminals 15 (+) and 4 (ctr. resistance 9.0 kohm
spark plug ends 5.0+/- 10% kohm
shielded plugs 1.0 +/- 20% kohm
spark plug wires 0 ohm (approx.)
rotor 1.1 +/- 10% kohm

M70 firing order: 1-7-5-11-3-9-6-12-2-8-4-10
crankshaft position/rpm sensor:
540+/- 10% ohm

from workshop manual
distributor rotor 1+/-20% kohm
angled/shielded connectors 1+/- 20% kohm
spark plug connectors 5+/- 20% kohm
cylinder identification sender coil resistance at 20 degree C (68F) <1 ohm
pulse sender/crankshaft position sensor coil resistance 540 +/- 10% ohm
temperature switch for e-box cooling E32 750: switch on at 44 +/- 3 degree C, switch off at 36 +/- 3 degree C.

Fuel injectors you can test, I let the engine run in idle and then pull the electr. plug from the fuel injector one by one and when idle changes, you found it.

[Tomcat BMW]

Thanks!!!!
Ps-My plugs are going on 50k miles...what is the recommended gap for these ngk (2288) bkr6ek?

[dragon850]

Thanks!!!!
Ps-My plugs are going on 50k miles...what is the recommended gap for these ngk (2288) bkr6ek?

Those are the correct plugs, as far as wires are concerned you need to pull the entire wire set and test each one on the bench they should be around 6k ohm resistance, you also need to flex them while testing to see if there are brakes in the wire.
Possible your DK motors are out of spec, when you ready I can rebuild them and bring them back to spec.

[Tomcat BMW]

Those are the correct plugs, as far as wires are concerned you need to pull the entire wire set and test each one on the bench they should be around 6k ohm resistance, you also need to flex them while testing to see if there are brakes in the wire.
Possible your DK motors are out of spec, when you ready I can rebuild them and bring them back to spec.

We (well YOU) did my DKs last July...so I know they are perfect. I did all the plugs and it seems to be better (acceptable).
The automatic transmission MAY be "catching"...so I might try some Lubegard's 'Dr Tranny Instant Judder Fixx'

[dragon850]

We (well YOU) did my DKs last July...so I know they are perfect. I did all the plugs and it seems to be better (acceptable).
The automatic transmission MAY be "catching"...so I might try some Lubegard's 'Dr Tranny Instant Judder Fixx'

Yes, in that case the DKs are Perfect!
I would test the plug wires first.

[Tomcat BMW]

Yes, in that case the DKs are Perfect!

100% true!
Just in case, do you have part # and brand/vendor of OEM wires?

[Mikekugel]

Thanks!!!!
Ps-My plugs are going on 50k miles...what is the recommended gap for these ngk (2288) bkr6ek?

NGK BKR6EK plugs are dual electrode plugs one heat range colder than the no longer available stock single ground electrode plugs. They are the correct 1000 ohm resistance, and work great on my M70.

[shogun]

M70 and M73 ignition wires I have F/S in black and red color, see in E31 parts for sale. wires only = no donuts, brackets, marten protection, how to add the donuts onto my wires is shown in detail in the offer.
That is a cheap alternative when your original brackets are still intact/can be re-used and you want to re-use the original donuts or new BMW donuts. The boots have 'woodscrew" type connections.
Quite a number of forum member use my wires since years.
In case the old plastic brackets are brittle and broken, better buy cpl. sets incl. the brackets and donuts.