530i hesitation at 3000 RPM

wkohler · Post by **wkohler** » Mon Oct 13, 2014 12:04 am

7/75 530i, totally stock injection and ignition (points, condenser, etc).

When I bought the car, it would occasionally die, but could be restarted by popping the clutch without an issue. I narrowed that down to the combo-relay being worn out, as shaking the relay could kill the car.

What has continued to happen is that if you are accelerating to 3000 RPM, it will drop to 2800 or so. If you're cruising at just over 3000 RPM, and let off the accelerator, the car feels like it died, but didn't and is a bit jerky when you reapply the throttle.

I don't imagine this would be a normal thing, but I'm not going to lie, coming from the world of Motronic and only having a small bit of experience with electronic ignition L-Jet, I am not sure where to start looking. My first thought is that it might have something to do with the vacuum retard or something in the distributor that happens to act up at that RPM, or maybe the vacuum limiter (part of the EGR system).

I am going to replace the thermal reactors with a set of Stahl headers, and will at that time delete the EGR, but I don't want to assume anything about this and would also like to learn a bit more about the parts I'm hoping to remove if that's at all possible - or even the cause of this issue.

I hope that makes sense. Thanks.

Mike W. · Post by **Mike W.** » Tue Oct 14, 2014 9:40 pm

IIRC and I'm reaching here since I never even had a 530i, but I think there is a little box that runs the vacuum switching valves that run the EGR. And I think it deactivates EGR above 3K. I would guess it's related to that, but may or may not be the way it ran when it was new. Follow the wires going to the switching valves for the box location, it's not part of the ECU.

wkohler · Post by **wkohler** » Tue Oct 14, 2014 10:44 pm

There is a silver box by the combo relay that has wires that go to the valves. Maybe I'm nuts, but I didn't imagine BMW would have this kind of jerkiness, etc. I guess I'm a bit happier thinking you're thinking EGR. Do you think it would hurt anything if I unplugged that box just to see if it had an effect - meaning, rev it up to 3k and see if it stays there or drops? I obviously wouldn't leave it for an extended period.

Mike W. · Post by **Mike W.** » Tue Oct 14, 2014 11:23 pm

By all means, unplug it and see what happens. I'm far from sure it's the EGR, but early ones in particular were associated with drivability issues. 530s were modern at the time, but those were dark days for automakers with 120HP V8s, horrible carbs, miles of vacuum hose and poor driveability in general. Even compared to BMWs carb'd M30s, the ability to stand outside the car, cold, turn the key and have it start and run without any accelerator pedal action at all was an eye opener.

As I said, I haven't experienced what you're talking about, but it makes sense from what I've read and the parts I've seen.

Robert Bondi · Post by **Robert Bondi** » Sun Oct 19, 2014 12:17 pm

FWIW, I think that box you guys are referring to is the "speed switch"
and you can see where it fits in with the rest of the emission stuff in
the FAQ:

http://www.firstfives.org/faq/ljet/lambda530i.html

It basically sits right next to the 530i combo relay on the inside driver fender.
I ran 530i injection w/o reactors or EGR for years and that speed switch was still in there until I converted to 528i FI. I can't say with certainty that the
speed switch was needed for the FI or was just clutter that remained behind for a while.