BMW D-jetronic cold start

Hallo, I have 1974 BMW CSi 3.0...It will NOT start in the morning(or any other time if the car has been standing for more than say, 12 hours) It generally takes 3 turns of the key for approximately 5-10 seconds before it fires into life ...It then runs perfectly... If I then go for a drive and stop , when I return to the car it starts first time without trouble ...please help me diagnose the cause if possible ?

regards Mark

Hi Mark,

welcome with your beautful BMW 3.0 CSi on jetronic.org !

There are 2 things that control cold start:

1. Cold start valve. It is controlled by a thermo time switch and a relay. Thermo time switch switch opens cold start valve only on FIRST start for a few seconds. Time depends on engine temperature. Above 35°C no action. Also no action on 2nd start as it has an internal heating. Check for 12V on first start on cold start valve.

2. Non-return valve in fuel pump. You should hear your fuel pump run for 2 seconds after you switch on engine. That should increase fuel pressure in ring line to 2.0 Bar. To hold that fuel pressure fuel pump has a non-return valve that tends to break. Replace wih external valve from Pierburg. So check fuel pressure after siwtching ON ignition. Does it build up to 2.0 Bar and how quickly does it drop?

3. I guess you have leaking fuel hoses or injectors. Leaking hoses also cause fuel pressure decrease. Leaking injectors also let fuel into manifold and that can create trouble when starting your engine. It will also let fuel pressure drop. I had it myself with an injector leaking above 1.8 Bar. It will also show in fuel pressure drop. Since I replaced that injector my car start in 1-2 seconds at all times.

4. In your mail your guessed that it could be linked to AAV. That is not the case as AAV only controls additional air to make the engine run faster after it has started up and as long as it is still cold.

BMW has a specialty in controlling cold start valve. The relay will also be started by ECU after engine startup to do an after start enrichment. That is why that elecrical circuit is a bit more complicaetd than normally. Unfortunately the Bosch workshop manual for your car was never translated to English. I have asked Bosch for allowance to do so and reuse the pictures of the German workshop manual. But I have not yet received an answer.

greatly appreciate you sharing your in depth experience..thank you