Case Study - E38 - Stalling

Date of manufacture	May 1999
Mileage	90,000
Model	740i
Engine	Vanos M62

Symptoms

When the engine was started the revs would rapidly rise to 1500 RPM and then almost immediately stop again. The car could be started by holding the throttle and keeping the revs around 2000 RPM for a few seconds and then slowly reducing the revs. The car would then keep running but the revs would be around 400-500RPM which is a little low for the M62.

These symptoms slowly got worse to a point where even if started and held at 2000RPM the revs would keep dropping. Once the revs settled at 400RPM, engaging Drive would make the car move with a distinct lope as the revs hunted around.

Also noticeable was a drop off of power over 3000RPM, the Vanos M62's should keep on heaping on power all the way to the red-line and the drop in power was quite pronounced.

Lifting the dipstick with the engine running changed the revs slightly, lifting the oil-filler cap made the engine nearly stop completely. The E-OBD lamp was not illuminated (known as the 'Service Engine Soon' indicator in the USA).

The fix

Very simple, this one was caused by a faulty MAF. The facelift M62's which have Vanos have a MAF with an integrated temperature sensor and is not compatible with the earlier versions. When an M62 is started the revs should rise to around 1000RPM and then slowly drop to 600RPM and then remain steady.

Any hunting to find 600RPM means that the fuel/air mixture is wrong. This could be a faulty PCV but when this happens, lifting the dipstick makes no difference and there is a distinct slurping and gurgling. Once the Lambda sensors have reached temperature (which takes 30 seconds or so) the closed-loop control of fuel/air mixture can compensate for the poor mixture.

It might be thought that the power over 3000RPM should not be affected as the Lambda's should compensate. However the fuel/air mixture is set from a look-up table during harsh acceleration rather than operating in closed-loop mode. There are other areas of operation that are not fully under closed-loop control, this is because at very light throttle settings the time period between the air entering the MAF and reaching the Lambda's is too long for proper control. Trying to operate in closed-loop mode in these situations would cause hunting between over-rich and lean mixtures.