Fuel Pump - Odd Behaviour

[ROOODBOY]

Hi folks.

My 1989 350I has for a long while now suffered from an intermittent rough running and hunting problem.
I have tried new cap, rotor arm, leads, ignition module and temperature sensor but to no avail.

I have tested the fuel pressure and it's spot on, even when running rough.

My question is, with the ignition on, sometimes the fuel pump runs and sometimes it doesn't, should it do this? Why would it do it sometimes and not at other times?

Thanks,

Martin.

[hallsie]

Impretty sure it should run all the time and return any unwanted fuel to the tank like a recirculating system!

Are you sure its not running loud when in need of fuel and quieter when its recircing?

Stu

[mrzigazaga]

Hi Mate..
Not sure about 350i but mine has a plug on the fuel head that runs the pump when cranking when its plugged in or on ignition if unplugged..Also my fuel pump was well loud when the fuel lines were messed up as it was sucking more air than fuel..It was running like a donkey too..Mind you at least it was running more than it is now..

[carob]

With the ignition in the position turned on but no cranked. take the air filter off the AFM and push the plate inside the AFM and listen for any differences without the wedge running. Maybe of help or i may be talking crap.

Rob

[ROOODBOY]

Thanks guys!

carob said:

With the ignition in the position turned on but no cranked. take the air filter off the AFM and push the plate inside the AFM and listen for any differences without the wedge running. Maybe of help or i may be talking crap.

Rob

Ok, I can try tonight, but what will that tell me?

[Mr Tank]

ROOODBOY said:

My question is, with the ignition on, sometimes the fuel pump runs and sometimes it doesn't, should it do this? Why would it do it sometimes and not at other times?

Thanks,

Martin.

Martin

A 350i fuel pump will run when the engine req fuel!
If you just turn on the ign the pump will turn on and pressurise the system and then it should switch of! If the system is already pressurised (Due to very recent running) it may not switch on.
This I think is what you have happening above!
The pump will switch back on once the engine is running and any unused fuel will return to the tank!

Sorry it doesn't solve your running problem but explains your fuel pump!

Andy

Edited by Mr Tank on Wednesday 8th June 10:29

[carob]

You will be able to listen to the fuel pump better for erratic running.

Rob

[TOPTON]

Flapper or Hotwire????

HOTWIRE--
ignition on but not cranking, the fuel pump should be heard pressurising the system for 2-3 seconds then the pump relay switches it off. Once the engine has started the pump should be running again ALL the time the engine is running, keeping the required pressure, returning any unwanted fuel back to the tank via the pressure regulator.

If you pressurise the system (ignition on, no cranking) then switch the ignition off for a few seconds, when switched back on again it may not need to pressurise again untill the engine has cranked, effectivly de pressurising the system.

Flapper--
The pump only runs when engine is cranked, pressuring the system on start up. Again running ALL the time the engine is running to maintain req'd pressure.
As carob says, take air filter off the AFM and push the flap inside while the ignition is on (not cranking over). The fuel pump should then be heard as long as the flap is pushed, releasing the flap switches off the pump.

If on either system the pump switches off while the engine is running, even for a couple of seconds then the problem could be the pump relay, fuel pump itself or AFM

[ROOODBOY]

Hmmm, interesting.

It's a flapper system by the way.

I definately have something going on that shouldn't be then.

For example, just yesterday, I took the car for a run of about six miles, pulled up at home, switched engine off, switched ignition back on (to wind up windows and open boot etc.) and fuel pump was running continuously. It does this sometimes but not at other times.

So could the flap in the afm be stuck slightly open, and thus also cause part throttle running and idling issues..?

Thanks for the info guys!

[TOPTON]

ROOODBOY said:

Hmmm, interesting.

It's a flapper system by the way.

I definately have something going on that shouldn't be then.

For example, just yesterday, I took the car for a run of about six miles, pulled up at home, switched engine off, switched ignition back on (to wind up windows and open boot etc.) and fuel pump was running continuously. It does this sometimes but not at other times.

So could the flap in the afm be stuck slightly open, and thus also cause part throttle running and idling issues..?

Thanks for the info guys!

Its a lazy day so just had a fiddle with mine

The AFM flap, being spring loaded makes a deffinate 'thup' (nice descriptive word eh) when pushed and let go to rest/closed. To make the pump work I had to push it about 3-4mm from rest, it then remained on as said earlier untill I let go of it.
It certainly sounds like an AFM problem on yours, maybe it's full of gunk inside, time for some carb cleaner. Also I think it can be adjusted once cleaned.

ETA if the flap is not closing/opening the correct amount, then it is also sending the wrong signals to the ECU to fire the injectors at the wrong times, it works as a variable resistor, the wider open for air--more fuel please

Edited by TOPTON on Wednesday 8th June 12:12

Edited by TOPTON on Wednesday 8th June 13:22

[ROOODBOY]

TOPTON said:

Its a lazy day so just had a fiddle with mine

The AFM flap, being spring loaded makes a deffinate 'thup' (nice descriptive word eh) when pushed and let go to rest/closed. To make the pump work I had to push it about 3-4mm from rest, it then remained on as said earlier untill I let go of it.
It certainly sounds like an AFM problem on yours, maybe it's full of gunk inside, time for some carb cleaner. Also I think it can be adjusted once cleaned.

Thanks for having a look, much appreciated!

The afm was actually the first thing I replaced a while ago when chasing this intermittent running problem, and I did clean it out with carb cleaner before fitting it, although it wasn't bad anyway.
I suppose it's quite possible that the afm I bought was just as bad as my old one!

Another odd symptom it has when it's running rough is that the pops and bangs on the overrun disappear.

I'll take it off again tonight, inspect it and look into adjusting it.

Thanks!

[ROOODBOY]

Well, I've just had a play with the afm, and I see what you mean about moving the flap and the fuel pump coming on, so I'm guessing it was sticking open a bit..?

So I changed the afm back to my original one (after giving it some more carb cleaner, but its spotless in there anyway), and it's exactly the same!

I'm running out of parts to waste my money on (and patience), although I do half suspect that both afms are dodgy, I've heard so much about them!


Anyone know how hard it is (if possible at all) to change to a hotwire set up?

At this rate me and Ziggy are going to end up having a Wedge burning party!

[mrzigazaga]

Sorry mate..I think i might of dropped some of my bad luck on your garage ramp..I told you my car is evil..And i still think its haunted..Remember when we lifted the bonnet and the coil lead was off..And that was after i just drove to you..Or when you was about to check my timing and you asked me to crank the engine one time and it started instantly..3 times and then when the garage had my car last week he said my vacuum pipe was laying across the engine and it was disconected at both ends?..Very strange..

[ROOODBOY]

Don't worry mate, I'm sure we'll have them exorcised in no time!

[The Hatter]

My 350i flapper was behaving similar to yours with the pump running sometimes and not others; it would also have erratic idle. Eventually it went into heavy overfueling and lots of spluttering/hesitation. I suspected the temp sensors and the Air meter but both checked out OK.

I seemed to fix it by simply taking out the ECU, disconnecting and reconnecting it a couple of times. I guess it must have been a dodgy connection in the main ECU plug. It's been OK for the past month or so and has much better idle and low speed running characteristics.

I also found that someone had hotwired the air meter so that the pump runs all the time with the ignition on so it must have been a longstanding problem. Good for cold starts... not so good for fuel cut out in an emergency!

[ROOODBOY]

The Hatter said:

I seemed to fix it by simply taking out the ECU, disconnecting and reconnecting it a couple of times. I guess it must have been a dodgy connection in the main ECU plug. It's been OK for the past month or so and has much better idle and low speed running characteristics.

I've already done that, several times.

I even fitted another ecu, still the same!

Thanks anyway, keep the ideas coming!

[adam quantrill]

Could be the multiplug itself (female connectors isn't it?) or an intermittent fault in the loom?

[ROOODBOY]

Yeah, that would be my worst nightmare!

I hate electrical problems!

[Wedg1e]

Mr Tank said:

A 350i fuel pump will run when the engine req fuel!
If you just turn on the ign the pump will turn on and pressurise the system and then it should switch of! If the system is already pressurised (Due to very recent running) it may not switch on.

Sorry Andy, it's not that clever! The hotwire system may control the pump electronically but the flapper system activates the pump while cranking to get system pressure up, then once the airflow meter flap opens, it takes over and keeps the pump running. That way if you crash and the engine stalls, the fuel pump shuts off to prevent fuel spraying from ruptured lines. If it was pressure-sensitive then a split fuel line would see gallons of BP's finest everywhere...

[Wedg1e]

ROOODBOY said:

I do half suspect that both afms are dodgy, I've heard so much about them!

Probably not, although that's not to say 'experts' haven't been at work.

Does the flap swing through its full range without sticking, scraping, rough noises and with about the same amount of resistamce across the range? If not, the pivot bearings could be shot (can be replaced) or there's crap in the main passage.

Does the black top cover have a semi-clear, hard sealant retaining it or is it bathroom silicon, roofing sealer, sellotape, BluTak or chewing gum? If the first, it's probably factory-spec and untouched. Usually if it's 'touched' there will be knife, chisel, screwdriver, Dremel, jemmy and/or teethmarks in it.

Apart from this, unless you have a high-mileage unit and the carbon track of the resistor pack is worn through, it's almost certainly saveable. The usual bodgfication is that people (often well-intentioned, technically-competent people) change the tension of the flap bias spring. This results in a completely different response of the flap to a given airflow (and thus incorrect fuelling), but if it's wrong there are a couple of ways to adjust it back to normal. None of them involve witchcraft, paying out money or trawling through endless websites. If the car even starts it suggests the basic adjustment isn't far off.