316 -84, waving idle.

[Tomi Tuominen]

Does anyone know how to get rid of Pierburg 2BE - carburettors waving idle? Second problem is too low idle. There is no idle adjusting screw at the carburettor so how do I raise about 100 rpm?

[SimonP]

Many mechanics are afraid of these carbs because they don't understand them. Mixture richening for acceleration is performed by electronically closing the choke flap, rather than squirting an extra dose of fuel in!
From personal experience, I would recommend repairing the Pierburg carb.
I had an '87 model 316 and made the big mistake of getting the Weber 34/34DMTL conversion.
I hated it. The throttle opening on the Weber is linear, whereas the Pierburg is exponential. What this means is that you can redline the engine with about 1/8in of pedal travel. Not good for city traffic jams.
Oh, and economy went down from average 33mpg to 28mpg.
The Haynes Solex/Pierburg Carburettor manual (UK) gives detailed setup instructions.
If the screw heads on the adjusters are snapped off, these are available at BMW dealers (special order).
Maybe there is a small vacuum leak. Thoroughly check ALL pipes and hoses, refer to the manual to see if they are in the right place.
Check the DECELERATION AIR VALVE. This is the thing that connects between the manifold and a stub on the air filter pan. It has a third hose going to the base of the carb. There shoud be NO vacuum on the air filter stub at idle. It is needed because unlike a conventional carb, the throttle butterflies are COMPLETELY shut on deceleration, the valve then admits fresh air into the intake to avoid over-vacuum.
Also check the filters in the throttle actuator and the outgoing connection to dash temp sensor.
Check for vacuum leakage at the throttle butterfly pivots. If applying some oil to these restores smooth idle, you need to replace the throttle plate. However that part will cost you the same as the complete Weber conversion kit.

A small cheat tip:
If you just need to raise the idle speed slightly, there are two single wire connectors taped back to the harness adjacent to the ECU under the dash (to the right of the steering wheel, UK RHD model). These can plug together and raise the idle speed as the ECU now thinks you have switched the Aircon on!

Hope this helps,
Si

[Tomi Tuominen]

I have already checked and connected those two wires at ECU (plugs 13 & 38). By listening the idle raised about 20 - 30 rpm. The Pierburg manual promised that the raise would be about 100 rpm. Is there any other way to raise the idle?

Ya, I have checked and replaced all vacuum lines and replaced electromagnetical valve. Still idle waves, especially when warm and after long drive. It waves for some time, for 20 to 60 sec and then it calms down.

Does the system have some kind of PI-control, and adjustable amplification? And what is Aircon?



(Sorry about the language, written with the help of dictionary)

[SimonP]

Aircon = Air Conditioning or A/C.

Most 316 models were not fitted, but the facility was built into the ECU for all models.
If possible, check for correct air:fuel ratio at idle, with an exhaust gas analyser or colourtune plug.
If the CO is too low (weak mixture) and cannot be adjusted, there is likely to be a vacuum leak. Unlike the Weber and other carbs, the 2BE will not normally adjust to an excessively rich mixture.

Check that the lever on the carburettor, which is operated by the throttle positioner, is not bent out of shape. (mine was when I got the car!)
Check the action of the throttle positioner. As the ignition is switched off, the plunger should retract to allow the throttle to completely close.
A few seconds later, the plunger should push out and open the throttle by an amount greater than at normal idle. (roughly the amount for about 2000-2500rpm)
As the engine is re-started, the plunger should retract slightly in order to maintain the correct idle speed.

If all else fails, here's a last-resort cheat. However, it will result in higher emission levels on deceleration (like a standard carb). DO NOT do this if your car has a CATALYTIC CONVERTER.

There is a throttle stop screw on the primary throttle, which is factory set so that the throttle is completely closed but does not jam.
If you adjust this (try some shims first, to keep factory setting) to obtain the correct idle speed when warm.
If the throttle positioner plunger retracts so it is no longer in contact with the lever on the carb, the ignition timing will advance.
(Ignition advance vacuum is controlled by the electric valve on the bulkhead. The switch for this is on the tip of the throttle positioner plunger.)

It is also possible (with some adaptation) to fit the "standard" version of the carb, the 2B3 which was fitted to the Audi 80 and a few other VW-Audi models. However I think the throttle linkage is on the opposite side.

If you have to get a weber conversion, it will take some getting used to.
However, the idle is smooth and steady. It's just a lot less economical to run.