BMW 3-Series E90 (2005 - ) - 08 3.0 - Diesel at cold

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Thu 15 May 2008 11:07
08 3.0 - Diesel at cold - Ubi
Does a diesel engine run initially on a rich mixture at cold in the same way as a petrol one does?

Edited by Dynamic Dave on 15/05/2008 at 11:40

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Thu 15 May 2008 11:57
08 3.0 - Diesel at cold - DP
My understanding is that cold starting/running assistance on a diesel is done through advancing the injection timing and increasing the idle speed. The injection timing change is why diesels are noisier / rougher on cold starts than they are once warmed up.

My old Mondeo had a Heath Robinson type setup of an electronic solenoid valve on the inner wing, connected to a long cable stretching across to the injection pump, and controlled by the engine temperature sensor via the ECU. On cold starts, this solenoid would pull a linkage, which caused the cable to mechanically increase the idle speed on the pump. It also operated as an anti-stall device to "catch" the revs as they dropped towards idle.

On our old Golf mk2 1.6D, the cold start rev pick-up was a manual process and worked via a "choke lever" on the dash. The engine would start and drive perfectly without it, but would idle like a bag of spanners for the first mile or so.

Of course on a modern common rail, the whole system is under direct control of the ECU, and so such clumsy linkages and cables have been eliminated.

This only affected idle speed as far as I know.

Cheers
DP

Edited by DP on 15/05/2008 at 12:02

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Thu 15 May 2008 12:16
08 3.0 - Diesel at cold - Ubi
Does that mean that idling at cold harms a diesel engine less than a petrol one? Less risk of lubricant being displaced by fuel etc.
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Thu 15 May 2008 12:22
08 3.0 - Diesel at cold - jc2
Pre 1990 diesels used a richened mixture to start but most modern ones use timing advance-some use an external control to advance but the external control is usually there to up-speed the idle-it was also used to up the idle speed when reverse was selected and was called the"driving school fix".Other pumps used the increase in fuel pressure when the engine started to restore the timing to normal.Most early car diesels required you to put the accelerator to the floor when cranking-now they tell you to keep your foot off.
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