Fuel pump issue

[Tuffy]

Thanks mate
Yes it’s the only thing we can think to try.... I’ll order one from TPS on Monday... connect it without sinking into the tank ... if it doesn’t purge I guess I could return it??

Are they around the £120 mark from TPS?

[pudding]

Probably not wise running a fuel pump motor dry, they don't like it!   Do it for a few seconds only.   The thing with returning VW parts though is they insist on seeing the fault themselves, so you'd have to take the car to the dealer with the new pump not working so they can diagnose it before refunding you.  Annoying.

Yeah around that.  I bought one 2 years ago and I'm sure it was £130ish with the VAT.  I was getting fuel surging issues on fast corners when the tank was 1/4 full or lower.  Turns out VW made some modifications to the pump to stop that, and also an overheating issue on long journeys, so you will get the latest and greatest pump.

[Tuffy]

Thanks again mate.
To be honest there’s not much else it could be now. Just making us doubt ourself a bit when it purged with a direct 12v feed

[ROH ECHT]

The direct 12V feed from the power probe would run it flat out, whereas the controller varies the current according to ECU demand. 

This is possible for a worn out pump. The max feed from the controller to the lpfp is 16 amps. I doubt 16 amps is supplied when the prime command is sent from the ECU to reach target line pressure in the low side. 16 amps is likely only supplied when the lpfp duty cycle is at 92%(its max) and has a draw from the high pressure side. Anyway, hope the new lpfp works finally.

[Tuffy]

Sorry...so are you saying the pump could well be goosed although it powers fine with a direct feed 🤔🤔??

[pudding]

Yep.  I suspect it is worn out / lazy and needs a full current kick to bring it to life.   As above, the PWM controller does not give the pump full current unless commanded so by the ECU.  I think a new pump will sort you out 

[ROH ECHT]

Right, the controller is a PWModulator...so, feed power from it varies in order to maintain fuel's low pressure within its line. The car's battery will supply 300 to 1000 amps(range for most car batteries)...so way more amps than what the pump normally receives when it was directly connected to the battery. I have a Torqbyte PM3 between my controller and lpfp and it takes the signal from the controller and increases the amps up to 36 amps for the upgraded lpfp.

So, Pudding is correct in that even a badly worn pump, if not completely shot, will run given more amps.

[Tuffy]

Ok guys. Another update. What with the bad weather and my mercanic being flat out busy we still haven’t solved the starting issue

Have now established that fuel is getting through to the hpfp, the reason the pump wasn’t purging was pressure had built up enough in the hpfp, when pressure was released it purged fine and the fuel came through.... but still turns over and doesn’t fire🤔🤔🤔

. Have removed the hpfp, spring pushes freely and no damage to follower.... will screw a gauge to it and see what psi it’s pushing on crank over. Would it be fair to say that it should be pushing around 1700 psi ?

The only code that’s flagging up now....which wasn’t there before is for the throttle body.... could that cause a non start

[pudding]

You're looking for around 50 bar at idle mate.  I'm not sure many cheap fuel pressure gauges can even cope with that, let alone the 110bar in boost!  Will have to be logged with VCDS.

The mechanical pumps are known to fail, so don't be too surprised if it's busted.  Around £250 from the dealer for a new one.   Aftermarket ones like Loba are better quality than OEM and set you up nicely for tuning 

I would also confirm the ECU is commanding the HPFP.   During cranking, the engine relies on the LPFP to start the engine (should see 7 bar after turning the ignition on) and as soon as it fires, the ECU tells the HPFP to kick in.  I would confirm that signal is present, which is the sensor on top of the HPFP.

[Tuffy]

Cheers pudding.
Are you saying if it’s not giving the command it could be an ECU fault?? Because I assume if it was a goosed sensor on top of the hpfp it would log as a hard fault

Also it’s not actually firing, so would the ECU be talking to the sensor on top of the hpfp until it actually fired ??

Could the throttle body fault cause a non start??

[pudding]

Yeah it would log it as a fault.  I doubt the ECU is faulty but it's just a process of a elimination thing.  Sometimes old school diagnosis is required because ECUs don't fault code everything.

If the HPFP was dead I would expect coughing and spluttering at least, but if it's just not firing at all, it could be other things.  Crank sensor, ignition faults etc.  Have you confirmed you're getting a spark?

Throttle body would defo be fault coded, but with VCDS you can run an adaptation on the throttle in 'Basic settings'.  Channel 06 if I remember.  You will hear the throttle moving and VCDS will say "ADPT. OK" if all is well there.