Loss of power

[Bryan Baker]

The car is a 1990 BMW (E34 3.5 litre petrol)
On a trip London to Manchester suddenly the engine would lose power but keep turning since I didn’t disengage the clutch. Then, just as suddenly, it would catch and run as normal (unlike my heartbeat!).
On the 200 mile trip it happened maybe 5 times but not on the last 100 miles. Plenty of fuel in the tank. Someone has suggested the cause may have been the fuel pump relay. Just thinking about that (as I write) that makes some sense because if the fuel pump was supplying fuel then when the engine caught and fired up the exhaust muffler would have been full of fumes – but there was no noise each time the engine caught and ran again.
Does the fuel pump relay switch on and off frequently whilst the car is travelling? For example if the car goes on over-run and the engine needs no fuel does the relay open and cut the fuel, closing again when the pedal is depressed once again?
Not too easy when the fault is intermittent, is it? I could just put in a new relay but I’d rather be sure. So far as I know all the relays are original.
Any thoughts?
Bryan.

Topic

[Ty]

If there’s another relay in the box that’s the same as the fuel pump then swap the two and see if the problem goes away.

[Mike]

Maybe you just got some bad petrol.

In any case, you need to be more specific as to what exactly was happening.

Was the engine rpm increasing when you pressed the accelerator but the car would not go faster OR was the engine speed slowing down and both the car’s speed and the engine’s speed did not increase when the accelerator was pushed?

[Bryan Baker]

Thanks for your response.
1. Fuel quality did cross my mind but the car did not hesitate or splutter.
2. It was not that the car slowed and the engine speed increased as if, for example the clutch was slipping. It was as if the ignition switch had been turned off. Power was lost, speed fell away then, suddenly, power came back the engine revved freely and off we’d go – until the next time. Which might have been a few minutes or an hour. But it was not the ignition switch going off since all the gauges were still operating.
But what do you think about my point that the fuel supply must have been cut since if unburnt fuel was passing into the exhaust then I would have heard something when the engine did cut in again?

I hope that you can point me in the right direction on this – I realise it isn’t an easy one.

Bryan

[Mike]

You may very well be “on the right track” in thinking that the fuel supply was dropping out.
It could very well be that the fuel pump is failing or perhaps the fuel filter has become clogged.
If it has been a long time since the filter has been replaced, I would try that first before blaming the fuel pump itself.
Testing the fuel pressure will confirm or refute a fuel delivery issue.
You will have to connect a fuel pressure test gauge to the fuel rail and measure the fuel pressure.
The pressure should be a steady 3.0 BAR (45 psi of fuel pressure) when the car is idling,

[Author]

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