ViribusUnits

I've finaly figured out how the darn thing works, and what it does.

One of the things it's suppost to prevent is a poping sound in the exaust on lift off. I've knowen for some time that I've got such a sound, but never realy worryed about it much. It's a low priority problem, and I didn't think it was related to anything. The problem that cought my interst is that it can cause a rich idle w/o the O2 sensor, that is a clear problem for me.

Since it's attached to the vacume system controling the fuel pressure regulaters, I'm guessing now that it COULD be a problem, but I'm not sure. I'm looking into it simply because I've got a rich idle, and it's attached to the system that could be creating the problem.

How do I test the vacume limiter?

I've yet to test the fuel rail pressure on my car, mostly because I've yet to aquire the equipment to test it. Hopefuly, I'll get it checked this weekend.

Well, seems there are a number of potential suprises for me this weekend!

Thanks.

PorKen

The vacuum limiter (or decel valve) is used to burp air past the throttle when decelerating with the throttle closed. It's a hold-over from the K-Jet system.

The idea is to help burn gas which has been injected but unburned because there is not enough air after the throttle is suddenly closed. This unburned gas leads to increased hydrocarbon emissions.

This problem is more pronounced on the manual cars, but for some reason the valve was included on the auto and manual 928's pre-83; 83 and 84 have them only on manuals.

I removed mine. After I removed a bunch of stuff and my vacuum was stronger, this unit cycled at idle, up and down, up and down.

For more L-jet info...<a href="http://www.firstfives.org" target="_blank">Link: BMW's with L-Jet</a> Go to the Technical FAQ page and scroll down to the L-Jet stuff = good info.

One of the interesting things I read from the link above is: when accelerating the AFM flap overshoots briefly and acts like an accelerator pump on carbed cars.

ViribusUnits

Well, forget testing the vacume limiter.

Well, mine will hold a vacume, no problem. I don't know if it opens or closes, but that doesn't realy matter.

It's NOT getting vacume. In fact, everything on the front side of the engine that is attached to that vacume systme is not getting vacume!!!!! (The fuel dampaner, and the vapor control valve.) They will hold vacume if I pump it with the mity vac. However, that has no effect on the engine, no matter how high I pump the vacume.

So, the plan is to figure out why. And why it would have no effect. I'm willing to bet that this is causeing my rich idle problem. No vacume at idle, and the fuel rail pressure is too high. Too much pressure, too much fuel. Now to find the problem.

Thanks for the info.

mark kibort

I dont remember exactly, but there is two vaccum lines from the throttle body. one that controls the fuel regs. this should be under vacuum at idle. if you are not getting vacuum , it could be on the wrong vacuum line from the throttle body. that would definietly be a possible reason for over rich condition at idle. the decel valve as someone said, is used to let the idle down slower and burn more of the gas if left in the system. also, the fuel cut off switch (idle switch ) on the TB is for fuel cut off for no backfiring off throttle. I elect to disconnect this on my 84 US Ljet with the 5 liter, and now I found a way to do this on the S4. (put a switch on a white wire leading to the ECU ) it is not as effective on the S4, as fuel cut off is partially automatic on the S4, but near 2000rpm, interupting the switch, definitely helps the rpms come down slower and the engine doesnt hunt for the idle, or add the compression braking I cant stand in a race environment. (at the only cost of a little backfireing)

MK