87 420sel idle fluctuates after she is warm. The lights also seem to pulse at all times when the car is running? Could the be a product of a poor voltage regulator, bad sprakplug gap? Any help would be great. JD

What light fluctuates? Headlights?

With a poor idle there should still be no change in light output unless the poor idle is a near stall. You may want to check the voltage regular. You do not have to remove the alt to do so, making it a quick thing to check. But a simple voltage check at battery can also hint at any issues.

If voltage is fluctuating greatly, it can affect idle. If you are concerned about spark plugs then maybe you want to change them, but they should not affect idle unless they are way past due for changing and have some heavy deposits. But when this happens there is usually more of a miss than an irregular idle.

Really, there are a bunch of things to check. Lambda adjust and vacuum leaks to fuel filter and air cleaner. Can you provide an idea of what has been done to the car?

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

All the lights pulse as soon as you start the car, interior, heads, tails.. I am waiting on new distrib cap, rotor, spark wires. I changed the plugs not too long ago. I ask about the spark gap cause i dropped one of them a few times, oops.. I've yet to do a voltage test on the battery, lacking a meter at the moment but i'm sure it will be irregular. I am looking at the voltage regulator as a suspect. JD

I replaced the cap, rotor, plugs, wires and tossed in and oil change. Started her up and runs the same, lights pulse, guages bounce. I put a meter on the battery, while off 13.0 volts, while started the voltage is up to 14.4 (normal I think) but then the voltage jumps around from 14.0 up to 15.0. I am waiting on a new voltage regulator. Anybody have other suggestions? JD...

Wow, 15 is really high. Your OVP may be kicking in or something else causing a huge voltage demand/spike.

I would replace regulator and check voltage. Make sure everything is turned off (HVAC, etc).

Check the OVP relay fuses. Its a little box between the two firewalls. When standing on the side of the car by the fuse box, look past the brake booster. Mine has a clear cap that pops up. Inside there is a single 10 amp fuse. Some apparently have two fuses.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

hey, thanks for the feedback... I am still waiting on a new voltage regulator, in the meantime.. I blew that fuse (red top between fire walls) last night. I was lucky, while driving the radio kinda fizzled off and the lights got very dim. able to pull over to a lot where she died. i touched the red fuse that fell apart in my hand, with a back up i made it home. i did notice that the dash, heads, etc. lights were all much brighter with a new fuse but still i've the voltage bouncing and waiting on the new voltage regulator...

I would not drive it. The OVP (over voltage protection) does not protect everything. You may end up burning something up!

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

she will be sleeping in the garage untill i get that voltage regulator. should be any day now.. i'll let you know when i am back up and running.. thanks again.. jd

Got the new voltage regulator installed and that seemed to be a good fix. Voltage is about 14.4 steady at idle. The guages and lights do not fluctuate or pulse anymore. Took a quick shake down ride and all seems well.. thanks again JD...

Cool. Sounds like it may have been it. Keep an eye on voltage readings for a bit just to be safe.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Voltage is still cool at 14.3 or so steady, lights do not dim, pulse.. cool.. But now i think i've a fuel issue. I can touch the OVP and the engine revs up. I've read that these ovp's and fuel pump relays can loose soder contact as a result of heat. I am on the wait for a new fuel filter, ovp and fuel pump relay. she starts and much like the voltage pulse the motor runs a bit uneven but still very drivable.. also looking for the gray color paint on the cladding and bumpers for a 87 420sel artic white... thanks.. jd