Off idle accelleration stumble

[CBradio]

My car has been running pretty well lately,

But Ive been having an intermittent "stumble" on accelleration.

The car idles perfect and for the most part drives okay, and it seems to happen a little less when its warmed up but Im not positive- it could be in my head.

I changed the combo relay like 3-4 years ago and I opened it for inspection..
All looks okay inside! Clean, no cracks!

I have a 530i but it has the 528i ignition system.

If its not the combo relay, what else would be causing an off idle stumble?

Also, it does NOT always happen at the same RPM range.

[Mike W.]

Is the vacuum retard on the distributor hooked up and working? Idle speed should go up substantially when you disconnect it. Next I'd check fuel pressure.

[CBradio]

Fuel pressure guage inline w cold start injector tells me I have pressure,
But it does feel like fuel delivery cuts out on accelleration...

I also do have retard disconnected on my distributor because timed to spec (or slightly advanced) w retard connected I idle at TDC which makes my car run too warm if stuck in traffic. Disconnected I idle 8-10 degrees BTDC and I havent had issues with it... runs fine.

The thing that drives me crazy is the problem is intermittent.... sometimes it does not happen at all on a drive.

I have a spare AFM and will try that, You also mentioned in another post the plug for the FI temp sensor, I will need to look at that. Maybe Ioved it or something when changing oil filter.

Im just wondering if it could the ignition module or coil on my 528i ignition system?

[Mike W.]

It sounds like fuel to me not ignition, although I bet it would go away with the retard hooked up, but that's timing.

Here's something I wrote up on another forum to explain how it all works. Still might not be an option, but at least for background.

OK, you have a dual diaphragm distributor, one side is advance, the other retard. Sounds nuts, but actually works fairly well. It is critical to have the vacuum hoses connected to the right places, the advance side is just before the butterfly and the retard side is just after. The way it works is at idle, the retard is in effect. Which is fine. Once you open the throttle a bit, you lose vacuum at the retard port so it advances. The advance works conventionally, as you open the throttle you get vacuum on the other side of the butterfly. When you hook up the retard side you should get a big dip in idle speed. The advance side actually makes less of a difference, and none at idle. You can test them with a vacuum pump, with the cap off, pump one side then the other, you should see movement in both directions from the respective diaphragms. I had a bad diaphragm on one some time ago and found it didn't quite run right, but nothing dramatic like you have, more of a low speed slight bucking. As far as checking the mechanical advance, while not precision, just check to see if it has any spring action to it when you turn the dist rotor. It should have light resistance and then spring back when you release it. It is supposed to get occasional light lubrication from a couple of drops of oil on the felt underneath the rotor.

[CBradio]

Thanks for that!

Im gonna check all my "plugs" (not spark plugs)
And if I cant fix it I will start messing with the distributor.

Also, The distributor I got had a rubber cap over the retard port...
I think there were a couple different distributors for the 528i ...

Im going to take a look now; will write of any progress

Thanks !

[CBradio]

So I checked all my plugs, connectors, etc, all the wires going into the plugs-

All is well.

I also tested my advance and retard using the retard hose-
Both work but I didnt connect the retard.

Maybe unplugging/plugging things worked, or maybe my car is just in a good mood, but no bucking or stumbling this time around the block...

Sometimes itll happen just off idle, other times when I give it a bit more...

I didnt check the TPS! I usually use a voltmeter to check it but I need to replace the battery. Maybe it somehow undid itself

[RonDwyer]

I have a 69 Mercedes 280SE and it was not running well. I pulled the distributor and removed the advance plate. Be careful doing this, dont break the copper connector, clean it well with carb cleaner. Separate the 2 parts and put dry Moly on them. Now that they slide freely on each other the car runs properly.

Sometimes keeping these cars running involves checking and cleaning things that don't seem obvious. It could surprise you.

[CBradio]

That makes sense, if its "sticky" that might cause intermittent issues.

So far this weekend its driving fine.. i have a spare distributor too, so I can swap if it acts up again.

Thanks!

It is my only car!

[CBradio]

I figured it out!! It was the AFM- i adjusted the metal clip by bending it a drop and no more issue for last two weeks!

My car seems to be running rich now, but no more random stumbles!!

^_^

Love when things get figured out!!

[RonDwyer]

Try the distributor trick. You will be surprised at how gummed up the 2 plates likely are. I don't lubricate with anything but a dry lube, oil holds dust.

Try moving the plate toward the vacuum dashpot with a screwdriver to see how difficult it is to move.