L jet info - running rich

Post your E12 technical questions and comments here. Please, no off-topic posts.

Moderators: Mike W., Pierre

Post Reply
Posts: 390
Joined: Thu Jan 29, 2009 9:05 pm
Location: perth, australia

L jet info - running rich

Post by garyinwestoz »

found this today


" Document number: T0067
For further technical information regarding testing, repairs or to search for New or Remanufactured Automotive electronic products, please visit www.injectronics.com.au, call our office on (+613) 8792 6999, or email sales@injectronics.com.au

Make: BMW

Model: 5 & 7 series, early to mid 80’s Subject: Running rich

Some early to mid 1980 BMW 6 cylinder engines used the Bosch L Jetronic system. This was the earliest of the air flap type engine management systems and used a 35 pin ECM.
The ECM pin 4 monitors crank signal from the starter motor and uses this signal to give start up enrichment. There should be zero volts at this pin when the engine is running or the key is just in the on position. When cranking there should be a crank voltage of approx 10 volts depending on the voltage drop.

Injectronics has had many customers call to say they have a running rich BMW and upon questioning we have learnt that starter motor work has been performed. In some cases a relay has been wired into the starter circuit, which causes a voltage to be present on pin 4 when the engine is running. This of course will make the engine run rich. To confirm this is your fault check the voltage at pin 4 with the engine running and it should be zero volts. If there is a voltage present, disconnect the relay or temporarily cut the wire going to pin 4.


This publication is distributed with the understanding that the authors, editors and publishers are not responsible for the results of any actions or works of whatsoever kind undertaken on the basis of information contained in this publication, nor for any errors or omissions contained herein. The publishers, authors and editors expressly disclaim all and any liability to any person whomsoever whether a purchaser of this publication or not in respect of anything and of the consequences of anything done or omitted to be done by any such persons in reliance, whether whole or partial upon the whole or any part of the contents of this publication. Injectronics Australia Pty Ltd. © Copyright 2001.
74 525 auto
76 525 - 4 speed, LPG and holley carb- restoring
77 528i- 4 speed manual
77 528i -4 speed manual getting rusty
79 528i auto - a SA car?
80 528i auto - ready to relicense
81 528i +3.5ltr - auto - restoring
86 525e 5 speed manual- dd
User avatar
Mike W.
Posts: 2798
Joined: Wed Aug 30, 2006 12:12 am
Location: Sonoma County

Re: L jet info - running rich

Post by Mike W. »

I did that by accident when I put L jet off an E12 onto a Bavaria. It wasn't rich, it was pig rich, black smoke rich. It ran, but not well. Didn't take too long to figure it out going over the schematic.
Mike W.

1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter
User avatar
Posts: 137
Joined: Mon Nov 14, 2011 12:58 am
Location: Colorado

Re: L jet info - running rich

Post by tschultz »

Good detail here.

Today I plugged in my spare exu into rich running m90. This car had black smoke out the tailpipe at idle. Spark is controlled by the distributor and ignition control unit where fuel is vis the pump and ecu. pressure is good and 2.5bar and the u jectors are new aftermarket units.

With the new ecu the black smoke and rough running disappeared. Previously, the cat had flooded and fouled out spark plugs. Spark plugs were 3x heat too cold which I thought was the problem. now things make sense with the flooding and fouling out due to the ecu and incorrect Spark plugs!

I just thought this may be helpful to some other ljet owners...
1980 635CSi
1985 535i
Post Reply