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E24 1983 Staring Issue

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Gareth91971 View Drop Down
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    Posted: 14 Feb 2021 at 3:54pm
My 1983 635csi is a problem starter. The battery is new and on conditioning charge, but if the car is left for more than a couple of days without starting it turns over but will not fire up. If I come back to it a few hours later or early the next day it usually fires up. It runs well and seems to start no problem if it is used daily in the summer. I am finding through winter though it is harder to get it to start. I have been told it could be the computer, or possibly to do with fuel pressure. Any other owner had this problem? Suggestions and things to try welcome as otherwise it will have to be a recovery to a garage.
Thanks, Gareth
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Mike Fishwick View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike Fishwick Quote  Post ReplyReply Direct Link To This Post Posted: 15 Feb 2021 at 10:13am
My bet would be fuel starvation, caused by depressurisation of the fuel system when the car has been at rest for a while.  This can be easily checked by measuring the fuel pressure at the test point on the fuel rail, using a suitable gauge, which I think is avaiable from Halfords.

The cause of this is probably a leaking fuel pressure regulator, which could be due to either an accumulation of gum produced by prolonged use of UK 95 octane fuel, which is filth - try to use 97 octane fuel, particularly if the car spends a lot of time out of use. This will stop the valve from seating fully, so allowing fuel to return to the tank.

A good check for this problem is to use a small G-clamp to compress the fuel return hose frm the regulator, so stopping any leakage from getting back to the tank.  Leavet his for a few hours, remove it, and see how the engine starts.

It would also be worth putting a can of fuel injection cleaner into the tank, a process which should be regularly repeated with 95 octane fuel.

Another not-unknown problem is a porous diaphragm in the regulator - this has fuel one side, and inlet vacuum on the other, which opens and closes the fuel return valve in response to the pressure of a spring and the level of vacuum in the inlet manifold.  A porous diaphragm will allow fuel to leak into the inlet manifold via the regulator vacuum pipe, which strangely enough dos not give any symptoms of rich mixture, although in small engines such as BMW motorcycles it sometimes causes the oil level to increase.

A defective fuel pump can also allow a return flow to the tank if it has a failed roller - the roller-cell pump uses rollers to squeeze sectios of its liner to force fuel against the pressure if the system - but in this case it makes an unusal noise.

Edited by Mike Fishwick - 17 Feb 2021 at 10:42am
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