350i fuel pump

[jamo]

According to 'the bible' some of the wedges use a resistor to quiten and also prolong the life of the fuel pump. Does anyone know which cars this applies to, where the resistor might be, and how it works? The reason for asking is that my '86 350i has twice now refused to start due to lack of fuel. On checking there is only 8-9 volts at the pump whilst cranking the engine. If I run a wire directly from the battery + to the pump and run it for 2-3 seconds (to effectively prime the fuel system) the car will then start on the key and runs perfectly from then on. The first time this happened was from cold when the car had been standing for about a week or so, the second time when hot but the car had then stood for about 15 minutes - noted comments in bible about fuel vaporisation which could possibly have explained second instance(?). Has anyone experienced anything similar?

[shpub]

The pump is energised by the ECU through a relay. 8 to 9 volts is good/normal as the voltage drops cranking the engine and the pump will have primed before hand.

If the pump is not working, chcek the fuse, pump relay, immobiliser and ECU connection for a lack of power.

[jamo]

That's an interesting comment Steve. If the pump has already primed then should I be hearing it run for a few seconds when the ignition is first switched on, and then stop(as with most modern cars)? I was under the impression that this didn't happen on the TVR/Rover setup.

[danny hoffman]

I find that holding the throttle open very slightly while cranking helps if my 350i has been standing a couple of weeks. I suspect that maybe the airflow meter is slightly sticky and the open throttle sucks harder to shift it. I am presuming the airflow meter position affects how much fuel is injected. My cold start injector/thermotime switch are fine.

As it always fires instantly using this technique I haven't bothered to investigate further

Danny

[jamo]

Thanks for the suggestions guys. I guess it's a case of waiting for it to happen again and then delving a bit further.

[jmorgan]

I remember my 350 pump wouldn't start until the engine turned rather than the key to ignition on (early 350).

[wedg1e]

The fuel pump relay isn't energised until you crank the engine. Then it pulls in to start the fuel pump. Once air is drawn through the airflow meter, the flap moves, operating a switch that parallels the cranking feed. So once you release the key, the airflow meter switch keeps the pump running. If you crash (or stall ;-), the airflow flap drops the switch out and turns the pump off until you crank again.
The combination of a non-return valve in the fuel pump and the fuel rail pressure regulator ensures that the fuel rail retains enough fuel to allow the engine to fire until the pump pressurises up again.
I'm surprised, Danny, that lifting the throttle helps, since you'd expect max vacuum at closed throttle, so the air flap would lift sooner. Or is the Guinness fuddling my thought processes on this one...?

Ian

[jmorgan]

I was going to add that the 400 does. Pump going etc on ignition on before turn over.
Its a hot wire. So what happens there?

[wedg1e]

jmorgan said: I was going to add that the 400 does. Pump going etc on ignition on before turn over.
Its a hot wire. So what happens there?

Dunno. Haven't got a circuit diagram to look at!
Could be the fuel pump has a pressure switch in it to turn off the pump once the rail is up to pressure (IIRC the V6 Wedges did this).
Jamo: could be something as daft as the non-return valve isn't holding, or your pressure reg faulty? Just guessing really, not a fault I've had to sort. I would get a pressure gauge on the fuel rail and see what happens to the pressure during cranking, and after switch-off. What makes you so sure it's a fuel problem?

Ian

[danny hoffman]

Ian , not sure I follow you, the more the throttle is open the more air would be sucked past the air flow meter flap?

Your descrption of the fuel pump supply sounds spot on as I can hear the relay clicking if it struggles to fire.

Danny

[jamo]

I'm fairly sure it's down to fuel because when I've checked the plugs they have been bone dry. At that point I ran a lead to the pump for a few seconds (and could hear it was running), removed the lead and it fired up first time. I guess that tends to support the idea that fuel pressure isn't being held up by either the non-return valve of the pressure relief valve? It looks like one of those intermittent problems that needs to happen a few more times if I'm going to solve it. Test with fuel pressure gauge sounds a good idea though. Thanks again guys.

[wedg1e]

danny hoffman said: Ian , not sure I follow you, the more the throttle is open the more air would be sucked past the air flow meter flap?

Your descrption of the fuel pump supply sounds spot on as I can hear the relay clicking if it struggles to fire.

Danny

Yeah, I think it was the Guinness, Danny
I was confusing manifold depression and airflow. I think with the throttle closed, you get max depression (e.g on over-run, when things like the crankcase purge valve are operated, so the depression sucks all the fumes out of the engine), but of course that's downstream of the throttle, i.e. in the plenum. Cracking the throttle would allow the pistons a harder suck on the airflow flap, as you state.

:signsupforcollegecourseinbasicmechanics:

The manual I promised you a copy of has arrived: alas it isn't as informative as I hoped. It was described as covering 'diagnostics', but all that chapter shows is a plug-in diagnosis machine and effectively says "Use this". Doh! Still it was aimed at motor mechanics :runsforcover:
However it does give some good descrptions of the various components and I twigged a couple of things I'd missed previously. Don't think it was worth 25 quid though...
My scanner is putting a wide stripe down the side of everything I scan, will have to sort that out otherwise the copies are crap.

Ian

[shpub]

jmorgan said: I was going to add that the 400 does. Pump going etc on ignition on before turn over.
Its a hot wire. So what happens there?

It all depends on the ECU more than anything and the pump relay arrangment. The pump will be primed and then turn off until you start the engine when the ECU will power the pump. When the engine stalls, the ECU kills the power.

[jmorgan]

Cheers

[danny hoffman]

Thanks Ian.

Jamo is your cold start injector firing ?

Danny

[jamo]

Must admit I haven't checked that on the basis that quite a few people have said that the cold start injector is not really that necessary, but it is still connected up.

[danny hoffman]

The thermotime switch was faulty on my 350 when I first bought it - I only noticed 6 months later when it became difficult to start in the winter.

Danny

[tcpc]

have you got your fuel problem sorted jamo,

sorry i have not been in touch , but have had a few problems off my own to sort out, and i think i have done them


tony

[jamo]

Tony - haven't been able to reproduce fuel problems despite numerous attempts. Don't know whether that's good or bad? I've been busy on cooling side though - got a set of those twin fans recommended by GAZ666 a while back. At present I've only connected up one of them as don't think standard wiring is up to both. I've now got all the relays, switches and connectors so hopefully next weekend will get the job finished (complete with override switch and LED indicator). However, even the single working fan is a massive improvement over my old one. I'll mail you separately to find out where you are up to - seen numerous threads on camshaft, oil pressure etc.

[tcpc]

i think i have bored the hell out of everyone on hear
lol,

i didn't know you had cooling probs or are you just being safe, i have one fan with a over ride and it stays well on top off the cooling,

tony


>> Edited by tcpc on Tuesday 13th May 07:19

>> Edited by tcpc on Tuesday 13th May 07:22