Fuel pipes - which is feed, which is return?

[Melchior]

I thought the feed was the bigger diameter pipe and the return was the smaller one coming from the pressure regulator but I'm not 100% sure and need to know so I can attempt to diagnose my non-starting problem.

Here's a photo - the disconnected one is feed or return?

Thanks!
M

[citronut]

on a GTI the feed is the one fuel shoots out of if you switch on the ignition

regards malcolm

[Melchior]

No need to crank it over for the pump to run? Hmm.

Neither the fat nor thin pipe shoots fuel out when I crank so I think I found the cause of my non start problem. Now to figure out WHY the pump is not pumping *sigh*

Thanks for the quick answer Malcolm

[Way2go]

NO Fuel will shoot out without cranking as the pump will only run with the ignition in the "start" position until the ECU takes over after a successful start.

The feed is the thicker pipe usually that fits to your right of the rail. The return comes off the other end of the rail through the pressure release.

[citronut]

the pump dose run till it gets up to system pressure/a few seconds, then stops till you crank the engine

regards malcolm

[Way2go]

the pump dose run till it gets up to system pressure/a few seconds, then stops till you crank the engine
regards malcolm

I can assure you Malcolm, that it does not do that on my BX GTi with the Motronic 3.1 injection system. With the feed disconnected from the rail, no petrol emanates from the pipe until you crank and then it comes out in spurts.
Cranking pressurises the rail and after a start, the ECU/relays take over. (Other injection systems may behave differently)

[Melchior]

Whilst the car was working (hah!) I have to say I never heard the pump running when turning they ignition on. I did think it was a little odd (all other fuel injected cars I've owned have primed the pump at that point) but thought it's either very very quiet or only runs on the starter and once the engine is running.

Either way, with the pipe shown in the photo disconnected, no fuel comes out either with only the ignition on OR whilst cranking so I'm pretty sure the pump is not.. err.. pumping.

Later, for my next trick, I shall attempt to feed 12v directly to the pump and see if it does anything.

For future reference, there's a bit more info on the Dutch Citroen forum here:
http://www.citroen-forum.nl/forums/view ... 20834/P15/" onclick="window.open(this.href);return false;

[citronut]

the pump dose run till it gets up to system pressure/a few seconds, then stops till you crank the engine
regards malcolm

I can assure you Malcolm, that it does not do that on my BX GTi with the Motronic 3.1 injection system. With the feed disconnected from the rail, no petrol emanates from the pipe until you crank and then it comes out in spurts.
Cranking pressurises the rail and after a start, the ECU/relays take over. (Other injection systems may behave differently)

i dont know for sure if Motronic reacts any different to other injection systems,
but maybe yours holds its pressure between engine off and re/starting

regards malcolm

[mat_fenwick]

I think we established you have the Jetronic system, which is the same as I had on an Alfa 75 around 10 years ago. That most certainly only ran the fuel pump with the ignition on AND engine turning - the fuel pump relay was operated by a microswitch in the AFM so that whenever the flap opened, the pump would run. I remember looking into the wiring in some detail, as I developed a poor connection somewhere in the loom about 30 seconds after overtaking a long queue of cars on a Scottish A road. It was most embarrassing to coast to a halt and have them all try and squeeze past me!

As we were on holiday I temporarily 'solved' that by running an ignition controlled feed straight to the fuel pump, and you could then clearly hear it running whenever the ignition was turned on. Not ideal in a crash, but it got us mobile again.

[Way2go]

the pump dose run till it gets up to system pressure/a few seconds, then stops till you crank the engine
regards malcolm

I can assure you Malcolm, that it does not do that on my BX GTi with the Motronic 3.1 injection system. With the feed disconnected from the rail, no petrol emanates from the pipe until you crank and then it comes out in spurts.
Cranking pressurises the rail and after a start, the ECU/relays take over. (Other injection systems may behave differently)

i dont know for sure if Motronic reacts any different to other injection systems,
but maybe yours holds its pressure between engine off and re/starting

regards malcolm

They all (multi-point rail) hold pressure between engine off and restarting which is why even in Haynes they warn you to depressurise the system before disassembly. The pump does not run under ignition live alone without a running engine.
As Mat pointed out above, this is probably for safety reasons in a crash because otherwise fuel would continue to circulate and be a potential hazard. In your scenario with a stalled engine you would have to think to turn the ignition off which would not be uppermost in the minds of most drivers in a shocked and injured situation.

[Melchior]

The systems which run/prime the pump up to pressure before you start the engine will have an auto shut off switch which trips out when it is physically shocked. My old Rover 420 had pretty hard suspension and once I went over a very sharp bump - the engine cut out! I wondered wtf I had broken - the fuel cutoff had been triggered. Pushed it back in and off I went

[mat_fenwick]

Just found a bit more information - my Alfa had the L-Jetronic, not the LE3-Jetronic fitted to the BX. So ignore my recollections about the AFM flap.

I found this giving a good description of the operation:

The LE Jetronic electrical system is controlled by a single tachometric system relay with one pair of contacts. The tachometric type relay is energised from a speed signal provided by the ignition system. Without the speed signal the relay will not function.
A permanent voltage supply is made to relay terminal 30 from the battery positive terminal. Once the ignition is switched on, voltage is applied to the fuel injection relay at terminal 15. When the engine is cranked upon the starter, cranking voltage is applied to relay terminal 50. The relay windings are energised, the relay contacts close and voltage is output at relay terminal 87 and 87b (sometimes labelled 87a). Voltage is thus applied to the ECU at terminal 9, the fuel pump, AFS, TS and AAV.
When the engine speed rises above 400 rpm, a speed signal to terminal 1 of the relay from the ignition coil terminal number 1 holds the relay contacts energised. If the engine speed falls below 400 rpm, the relay will be de-energised and the relay voltage output will cease. The engine will stop.
When the ignition is switched on, the relay winding is momentarily energised which closes the relay contacts and connects terminal 30 to terminal 87b or 87 (varies according to model), thereby providing voltage to the fuel pump circuit. After a moment the circuit opens and the pump stops. This brief running of the fuel pump allows pressure to build within the fuel pressure lines, and provides for an easier start.
The relay contacts will then remain open until the engine is cranked or run. Once the relay receives a cranking signal from the starter circuit, the winding will again be energised by the ECU, and the fuel pump will run until the engine is stopped.

[Defender110]

The feed is the thicker pipe usually that fits to your right of the rail. The return comes off the other end of the rail through the pressure release.

I don't know about BX's but low pressure returns are generally bigger bore than higher pressure feeds?

[Way2go]

After a moment the circuit opens and the pump stops. This brief running of the fuel pump allows pressure to build within the fuel pressure lines, and provides for an easier start.
The relay contacts will then remain open until the engine is cranked or run.

Ah, so this is one of the differences then between the Motronic and Jetronic.

[mat_fenwick]

And also a difference between L-Jetronic and LE-Jetronic