The BMW that came in for repair would run fine for a while and then stop completely for a few minutes before restarting and carrying on as if nothing had ever happened. The trouble with this sort of fault is that all the tuning and diagnostics in the world can't find it unless the fault occurs while the test gear is hooked up to it. I think it was Volkswagen who started the widely held belief that linking the car to a computer will fix everything, it won't,and it can't find a fault that has occurred unless it happens to be a very recent model that records some types of past faults in it's own computer memory so that they can be recalled later. Unfortunately the BMW was not recent enough to have this facility, but fortunately the fault did happen to us while we had the test gear attached. The cause was a greatly reduced ignition spark energy level. With a contact breaker system when faced with this fault it usually had to be either contact breaker, condenser, or occasionally the HT Coil. Since the Contact Breaker and Condenser cost only a few pounds and the price of the labour to renew them was many times more than the cost of the parts then both would be renewed together and the fault would normally be cured. With modern systems the situation is reversed so pinpoint diagnostic accuracy is essential.

The fault was caused by the Ignition Control unit which unfortunately is part of the Injection Control Unit and not available seperately. The price of this was bad news and strongly reinforced the owner's decision to go Diesel on his next purchase. Luckily though the unit had not damaged any unavailable parts within it though so a reconditioned unit at less than half the price cured the problem. For more info on ECU testing click on ECU.

   BMW'S are normally renowned for their smoothness and power but not   this 323I. Cruising on a light throttle was OK but acceleration at   any rate other than slight brought about a series of jerks until   steady state cruising was achieved again. It checked over OK on   normal tuning checks until the engine speed was held at about 3000   RPM when the fuel mixture was found to be running very rich. We then  checked the fuel pressure since the pressure regulator can fail to   give high pressure and consequent rich running - not guilty in this   case though. Next on the suspect list were the Injectors, a sticky   injector or two could cause this sort of fault. A check of the   Ignition pattern on the osciloscope at speed didn't show any  differences here that would be reflected by injector faults.   To make certain we double checked by measuring the comparitive Petrol  quantity injected into each cylinder by using the exhaust gas analyser.   No one cylinder was responsible for the richness,it was across the   board. Checks on sensor inputs to the Control Unit revealed all Ok   except for a higher than normal signal from the Air Flow Meter.   Closer inspection of this unit showed signs of it having been  tampered with, so we took the cover off and reset it back to where it   should be. On roadtest now even worse, it would hardly accelerate at   all. We were certain now that the Air Flow Meter was at fault and   that someone had tweaked it to try to compensate for this. Our   suspiscions were confirmed when a new unit cured the problem -back   to BMW type performance again.

Hessitation, power loss. This type of problem can be one of the most time consuming faults to find because the problem often is only slight and the fault only shows up when the car is being driven under load.

One of the most commen causes of this is fouled or sticking injectors that can be reconditioned at less than tenth the price of a new set. For more information on injector servicing click on injector.

Lacking loss of power. This is a very common problem but can be almost impossible to diagnose if you are not aware of this problem, caused by valve carbonization this stops the fuel being injected from atomizing of the back of the inlet valve. The only real way to cure this fault is to remove the head and clean or replace the valves but this can be very time consuming and expensive, the cheaper and more efective answer is get the ECU (engine control unit) remapped and uprated, we can do this click on ECU for more info.