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BMW Intermittent Stalling – 325i & Others

December 17, 2010

This answer is applicable for many BMWs in addition to the one listed below.

Q
car year: 1988
car model: 325i Convertible
Every so often while I’m driving down the road my BMW stalls without warning there’s no pattern to it, it may go for 500 miles and then it happens or it may happen twice in one day, but if I just wait maybe a minute or so it will start and go and sometimes just a few seconds and it will start and go, all I get is the check engine light. Can you give me a hint on this problem?

A
Without any diagnostic info to go on, we would suggest three potential faults as the first considerations, in relation to the symptoms that you described.  These points can be applied to most of the ’80s and much of the ’90s BMW models (those with the Motronic engine management system), such as; E23 (7-series 82-87), E24 (6-series 82-89), E28 (5-series 82-88), E30 (3-series 84-91), E31 (8-series), E32 (7-series 88-94) E34 (5-series 89-96) and E36 (3-series 92-98).

Before proceding, since you have noticed the CHECK ENGINE light coming on, we should check to see what engine management fault codes are being generated.  BMW models with the round under-hood diagnostic port (such as your 88 325ic), record and store a fault code whenever the warning light comes on.  We can use a fault code reader and reset tool to check the codes.  Once we have the code and its corresponding fault description, this may help us in specific diagnostics.  Bavarian Autosport offers a tool that you can use to check and reset the codes, as well as reset the oil change and inspection reminders.  The tool will also reset the oil change and inspection reminders on the earlier models with the “D” shaped diagnostic ports (no fault codes are available on these models).

Fault code and reset tools:
BMW

* Main relay for engine management system – A weak relay can open its contacts when it gets hot. When this happens the engine will shut off , both the engine management and the fuel pump will “turn off” (no spark, no injector pulse signal and no fuel pump). As the relay cools a bit, it will function again and the engine can be started.

*  Fuel pump relay – Same issues as with the main relay (above), but only the fuel pump shuts off.  The engine management would still be operational (although the engine will not run due to the inoperative fuel pump).  In this case, you would still have spark and injector pulse, if you were to test during the no-start/no-run period.

Location of main relay (and fuel pump and O2 sensor heater or purge valve) for E30 models (3-series 84-91):

Main ECU Relay:
BMW
Fuel pump relay:
BMW

* Crankshaft position sensor - This applies to only the later models that have a round diagnostic plug (the system on the earlier models will not cause the engine to stall if the crankshaft position sensor is faulty, but the engine would not restart if it were shut off).  A failing sensor (and/or the harness wire for the sensor) can also be somewhat intermittent. If the signal from the sensor is interrupted, the engine management will shut down.

Crankshaft position sensor (models with round diagnostic plugs):
BMW

If you are just going to “throw a part at it”, we would suggest that you start with the main relay and the fuel pump relay. The relays are inexpensive and easy to install. If the problem persists, you can either replace the crankshaft position sensor or go into further diagnostics.

2 Comments
  1. mmorgan permalink

    My 1986 E24 635CSi had the same problem. It would run fine for four or five days and then wait until I got in heavy traffic and die. When I checked the fuel pressure I was only getting about 12 lbs. I found that the primer pump was working but the main fuel pump was not. I replaced both because I assume the ethenol fuel had at least a small part in the problem and the primer pump would have been the next to go.

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