E30 320i fuel pump replacement ...
Anything I should be aware of before I have a go at fitting a new pump? I'm
only going to fit a second-hand one as I'm still not convinced that the
pump is faulty. Do these pumps run noisily and overpressurise the fuel rail
regularly? I've checked out everything that I can think of and it seems
that the pump is putting too much into the fuel rail and the FPR can't dump
enough of the excess back to the tank.
This sounds like a failure of the fuel pressure regulator. It works as a
pressure relief valve to prevent the fuel pump from putting too much
pressure into the system. The normal failure mode for the relief valve
leaves it closed so then you have a positive displacement fuel pump trying
to pump about a gallon a minute through the injectors so the fuel pressure
goes up over 100 psi and I'm sure the fuel pump would be noisy trying to
pump at this pressure. I had this happen on my '84 318i a few years ago.
Leaks started showing up at every hose clamp and the engine would barely run
at low throttle settings -- wouldn't run at all at idle.
Hope this helps
Exactly what I thought, but disconnecting the return hose shows a healthy
flow of returning fuel. Maybe it's not enough - Difficult to tell.
A bit of history - A had the same fault (same car) a few years ago. Local
garage (non BMW) declared the fuel rail pressure to be around 32-34psi,
i.e. normal. The fault just went away after a while and the car ran
perfectly for a couple of years!
This time the leak was at the bulkhead end of the fuel rail - A very nice
spray that went everywhere :-( Remade the connection, moved the hose clip
up a little and promptly blew apart the coupling from the fuel filter to
the fuel rail :-)
Seems to be the same problem. I work around it by putting a resistor in
parallel with the temperature sensor, which fools the ECU into giving
shorter injector pulses and the engine runs fine (apart from being
difficult to start of course).
Maybe I'll just take a chance and fit a new FPR - It will cost more than
the car is worth though ... ;-)
This is really sounding very much like my experience except that mine only
took several months to evolve into something that I could diagnose instead
of several years like yours. It will all make perfect sense once you see
the inside of the pressure regulator.
I will try to describe the innards of the regulator.
Imagine a vertical piece of 3/16" ID metal pipe -- this is the overflow back
to the gas tank out the bottom and the top is covered by a round flat disc
that blocks the exit. This disc is attached to a rubber diaphragm that will
lift the disc off the end of the overflow pipe when the correct pressure is
reached. The outside diameter of the rubber diaphragm is sealed between the
two chambers of the housing. In the upper chamber of the housing is a
compression coil spring that preloads the disc onto the end of the overflow
pipe. The lower chamber of the housing is connected to the fuel rail and is
full of fuel at the regulated pressure. The upper chamber has a port that
is plumbed to manifold vacuum so as to subtract the manifold vacuum from the
spring pressure. As fuel is pumped into the system the pressure rises until
it is enough to overcome the spring preload and raise the disc up off the
overflow pipe thereby sending excess fuel back to the tank.
The peculiar behavior of this malfunction occurs because the way the
regulator disc is attached to the rubber diaphragm allows it to become
disconnected when the fuel pressure is high and then to reattach itself when
the engine is turned off and the spring pushes the diaphragm back down onto
the disc. The disc has a small sphere the size of a bb welded onto it's
upper surface. This ball is held to the diaphragm by a thin sheet metal
inverted cone that is part of the diaphragm. The cone has slits on all four
sides as a way to form it around the ball after the ball is welded to the
disc so the 'leaves' of the cone are springy. After many years wear of the
ball and the leaves of the cone make it possible for the fuel pressure to
pull the ball out of the cone so that the regulator no longer works and the
pressure goes sky high. After the engine is turned off, the pressure goes
away, the spring pushes the diaphragm back down, and the ball snaps back
into the cone -- perhaps at a little different clocking so it may stay
attached for some time.
In the beginning my car would only stall at idle once every week or so and
would always start right back up and be fine for another week. At the end
it would run OK for 15 minutes in the morning and stall at every stop after
that. I finally put a fuel pressure gage on it and ran it for 15 minutes so
I got to see the pressure shoot up to over 100 and my fuel hoses all try to
Excellent description - Many thanks!
I'll find a new FPR (Hopefully at a sensible price; This is an old wreck of
a car after all!) and give it a go.
My only other thought now is that maybe the return hose to the tank is
partially/totally blocked, so the FPR can't actually dump the excess fuel.
Unlikely, and given your explanation I'm inclined to believe that a 22 year
old FPR just *might* have worn out ;-)
The pressure is set by the regulator. This is controlled by engine vacuum
- the idea being high vacuum reduces the pressure to counteract 'suck'
from piston vacuum on the injectors. Which all contrives to keep the flow
*rate* through the injectors constant regardless of throttle position. The
actual amount of fuel injected is controlled by the time the injectors are
open - they pulse.
If the regulator has failed closed, the pressure will be too high and the
pump noisy as it's running into a closed end - the regulator controls
pressure by returning excess to the tank.
First thing I'd check is the vacuum pipe from manifold to regulator - if
this has failed the pressure will be too high and constant.
*People want trepanners like they want a hole in the head*
Dave Plowman email@example.com London SW
The regulator is certainly letting at least *some* fuel back to the tank,
but I suspect that it's not opening fully, or is sticking shut after a
short time. Or maybe the return pipe to the tank is partially blocked -
Seems unlikely though.
I took the vac hose (OK, narrow tube!) off and made sure it was clear.
Pulling the regulator end off with the engine running and putting a finger
over the end feels like a vacuum. But, it seems to make very little
difference to the running of the engine (eventually the engine does die,
but it takes a few minutes). So, I really do suspect that the regulator has
had it. 'Jack' explained in another post to this thread, and now I'm
convinced. I suspect that the garage that tested the fuel pressure were,
erm, 'mistaken' when they said it was normal ;-)
So, cheapest FPR seems to be from GSF at an eye-watering 53.40! With a full
tank the car is only just about worth that :-( But, it's got decent tyres,
long MOT and tax, and I've owned it for 8 years, so maybe it's worth it.
Anyone got any cheaper prices for an FPR? I've only tried Euro Car Parts
and GSF so far ...
It can quite easy to test the pressure yourself depending on the pipe
layout. I'm not familiar with the E30 - has it got a separate cold start
injector? That can be a convenient place to remove the pipe and replace
with a gauge. But best to de-pressurize the rail first.
You could always get an adjustable one off Ebay - some have a pressure
gauge fitted. Then keep it for your next car. Or try a breakers yard -
they're not something that normally totally fail. They're often not that
Check Ebay for a secondhand one? I've got an old but working 36 psi one
from a Rover EFI somewhere.
*Women who seek to be equal to men lack ambition.
Dave Plowman firstname.lastname@example.org London SW
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