Air conditioning stalls car

[mattie4859]

In restoring my CSi, we retrofitted an aircon system, but the problem is it overwhelms the efi ...when the aircon is turned on, the car's idle drops down and often stalls. Is there a fix for this with the original efi (I understand that because it is so basic and doesn't have the sensors that modern systems do, there is no way for the system to compensate when the aircon is switched on?) What did the original BMW aircon system do to overcome this?

 

[MMercury]

In restoring my CSi, we retrofitted an aircon system, but the problem is it overwhelms the efi ...when the aircon is turned on, the car's idle drops down and often stalls. Is there a fix for this with the original efi (I understand that because it is so basic and doesn't have the sensors that modern systems do, there is no way for the system to compensate when the aircon is switched on?) What did the original BMW aircon system do to overcome this?

What you are describing is more typical of smaller displacement 4-cyl engines. The typical treatment is to merely increase the idle speed.

Prior to the advent of sophisticated engine management systems, some cars had several recommended idle speeds depending upon whether the vehicle was equipped with manual transmission, auto transmission and/or air conditioning. With carburetor equipped vehicles, especially on lower powered engines, the addition of an electronic solenoid might also be used. It would be triggered by engaging the air conditioning blower and/or compressor, and mechanically engage the throttle to increase the idle speed - much the same as a fast idle cam working with the choke on a cold engine. Fuel injected engines typically use stepper motors that permit more air flow, much the same as your auxiliary air valve that works in tandem with the cold start valve. I am sure one of these has been adapted for use with a d-jet, but in my limited experience with the djet equipped M30 engine running air conditioning, it wasn't necessary.

Your engine, if properly adjusted, is not particularly wanting for low end torque and should not have that much trouble accommodating the drag of a compressor. Sure, the drag is noticeable; however, if your idle speed is too low, you will likely experience stalling. Same is more apt to occur if your engine is suffering from retarded timing perhaps because of reduced octane petrol.

From memory, the recommended idle speed for your engine is 850 to 1000 rpm. I would imagine that with air conditioning, your idle should be favoring the higher side of those numbers (1000) and possibly slightly higher, depending upon other parasitic accessories such as high amp alternator charging a low or almost-dead battery with a full complement of lights and tunes.

There is always the possibility that the compressor is placing too much of a drag on the system, but before considering this further,

1. What is your current idle speed? and

2. How many revs does the idle drop with the compressor engaged?

 

[Bwana]

Do you have an old York or a newer style Sanden compressor?

I'm doing the same thing on my car ('72 CSi with the original D-Jetronic). My mechanic (for wifey's 540) mentioned off hand one day about using an solenoid air valve off a late '70's, early '80's engine that is plumbed into the manifold, much like what MMercury mentions above for the cold start valve. When the compressor kicks in, the air valve opens and the FI system compensates for the additional load.

I'm not that far into the conversion yet, it will be a few weeks before I cross that bridge. It will be a "custom" plumbing job but my BMW parts guy seemed to have a flicker of recognition of what I might need when I described to him what I was looking for.

 

[61porsche]

a/c and d/l jet

Try this for inspiration:

http://www.firstfives.org/faq/aux_air_valve/aux_air_valve.html

In one of the pictures is the a/c valve for a slightly later car.

Besides dialing up the idle, it might be a tad rich and the load just makes it worse.

There are some threads on this board regarding adding FI with stand alone that have the parts diagram. My guess is that it's a 528 circa part for the a/c idle valve.

 

[Sven]

Here is a solution I used with an earlier version of Megasquirt. There is a small one-way valve from the 1991 735i engine I am using for my 3.5l conversion. I connected one end to the lower vacuum port on the throttle body and the other end to a spare port on the manifold log. This valve is open by default (but in one direction only - the larger hose goes to the manifold). When powered on the valve will close. So, using a special relay (two pole - basically a reverse relay) you set it up as follows: When the engine is running the relay provides power to the valve (keeping it closed). When the AC comes on the relay cuts power to the valve, thereby allowing it to open and letting additional air into the manifold and, with Megasquirt, more fuel is added, hence slightly higher RPMS. It worked nicely. The valve size was about right for the amount of air it let through. I am assuming that when the D-Jet system senses more air in the manifold then more fuel is added?

The D-Jet may not have a spare port on the throttle body, in which case you could tap somewhere between the air cleaner and the TB.

Two-pole relay wiring: 12v to pin 30, Ground to pin 86, AC ON wire to pin 85, and one valve wire to pin 87a. The other valve wire goes to ground. You will need to tap into the wire going to the AC compressor for pin 85.

Valve PN# Bosch 0 280 142 150
Rubber holder PN# BMW 1 717 292 (this is the rubber part, it may not include the metal bar)

I will not using this valve, as the newer megasquirt 3/3X has AC control built-in to control the main idle air valve. But I plan to hang on to the parts until I can test this new setup.