D-jetronic system repair for BMW 3.0 CSi

Recently, I upgraded the D-jet system in my 1972 BMW 3.0 CSi :

1) A stock BMW ECU - red dot - was installed (replacing the Volvo 164 ECU that was in use).
2) A new throttle position sensor was installed (original was not working with the old ECU I don't think and also wiring repair was needed)
3) A repaired MAP sensor - green dot - was installed (the copper diaphragm had ruptured).

After this, the tune up was done, with CO adjusted to 1.5% and I think the RPM reference value was 1950.

Without question, the car is running much better now - and very smoothly. There isn't the level of raw fuel odor due to the overly rich mixture caused by the faulty MAP sensor (although there is still something - always has been with this car). On full throttle acceleration, I also see much, much less smoke from the exhaust, and less pinging (I'm not sure I have seen any pinging, although I haven't pushed the car often enough to be sure yet). Power delivery seems good as far as I can tell. Before the recent work, at low speeds (in second gear typically), I would occasionally see the car "surge" rhythmically l when I took my foot off the gas and coasted - This effect seems to have gone away as well as far as I have seen so far (this sort of thing was expected, I'm told from this "Dr-D-jet" webpage, when the MAP sensor diaphragm is ruptured).

There is one relatively minor issue. The day I got the car back from the shop - at the end of my ~45 min commute (31 miles in some traffic) when I was back on surface streets in San Francisco - I did see the "low idle speed" phenomena that I have seen in the past : The idle was rather low (below 500 RPM according to my tach), and was "hunting", by which I mean it was swinging from a low value to a lower value and back again, with a period of about 2 seconds or so. For many years this has been an issue with the car, although typically it would show up many months past a tune up and in hot weather. This particular day was a fairly warm day although certainly not a hot summer day it was much warmer the following days when I did not observe the effect. I am tempted to conclude that perhaps the issue would go away if the fuel mixture were enriched a bit more - the effects was completely absent during the time the MAP sensor was broken and the mixture was extremely rich - the car would idle at >900 RPM and would never waver.

Are there any comments or suggestions regarding this issue ?

Hi Peter,

welcome with your BMW 3.0 CSi in our forum! A nice car indeed.

Having the right ECU and MAP sensor you seem to have gone a big step forward. Now regarding idle, I first thought of the typical sawing of too much false air and over-run shutoff. But NO, your ECU does not support that feature. So in that case please check the following:

1. Check idle-run switch 17 versus pin 14 voltage on your ECU wiring harness. When you release gas pedal, it should be 0V. If idle-run switch does not close, ECU does not stabilize the idle by itself.
2. Close the air pipe to your auxiliary air valve (AAV) and watch whether that has an effect on your rpm. If it has, your AAV does not close properly and could cause variaations in idle-run speed.
3. If both above are okay, you might have other sources of excessive air. Use brake cleaner to spray everywhere on manifold and on vacuum. During that rpm must not change.

What a pity that I cannot ask you to our workshop in June. I am still missing a BMW and a Volvo between all Citroen, Mercedes-Benz, Opel and Porsche.

Hi Peter,

it is good to know that the right ECU and an overhauled MAP sensor helped to make the car run smoothly. How about the ilde-run, did my hints help?

Now I understand why you did not have the original Bosch workshop manual for BMW. It simply does not exist in English, only in German. And that would be the best manual to work with.

One issue that confuses me ... In your hints, you suggest that excess air may be an issue. But wouldn't that tend to increase the idle speed - that is to say - the "auxiliary air slide" opens when the engine is cold and increases the idle speed, no ?

In any case, the air hose "clamping" experiment had been done, with no visible effect. Haven't tried the voltage test yet.

There are two very simple things I can try (in either order I suppose) in addition :

1) I can simply turn the enrichment potentiometer on the ECU clockwise a couple of "ticks" (ie, from "12 o'clock to 2 o'clock", for example, to enrich the mixture a bit to see if that helps. If that doesn't help, I can reset it back, and instead ...

2) I can adjust the idle screw one turn, say - counterclockwise - to increase the idle speed to see if that helps too.

It would be nice to avoid a significant increase in enrichment, though - my fuel consumption is much improved now. In fact, I would say that my gas mileage has gone from worse to bad !

In 1972 there really wasn't much attention paid to fuel economy ....

Hi Peter,

that is correct. Excessive air would increase engine rpm. As you were talking of a sweep in idle-run rpm, that was my hint. Sometimes you have a leakage that just happens a t certain temperatures. It can even be the shaft bearings on throttle valve or the rubber sealings under injectors.

I would go to an idle-run CO of 2%. It is the upper limit for your car and it will only effect idle-run fuel consumption.

And yes, an old D-Jetronic system consumes a bit. My MB 8-cylinder 450SL runs ar 14-15 l / 100 km (16-17 mls/gallon). Your car could be a bit better, but only silghtly. But knowing the fun it makes to ride such a car I would say it is damn worh it.