FSI Fuel Pump Relay

P

Philip

Member
Does the FSI have a separate relay for the in-tank fuel pump, and, if so, is it under the nearside floor?
 
Philip said:
Does the FSI have a separate relay for the in-tank fuel pump, and, if so, is it under the nearside floor?
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@PlasticMac and @Andrew are probably the chaps for this.

There's some great images and descriptions of the FSi fuel system in various threads on this forum if you use the search function.

Here’s one;
1726688847485.png
 
Good morning all.
Yes, it does, it's relay 5. Under the floor. (Fuse 18).
It's not a simple relay though, has some electronics in there too.
Mac.


Screenshot 2024-09-19 08.16.23.png
 
Thanks, I’ll have a look. Current thought is that the (third) in-tank pump has died (can’t hear it prime when the ignition is turned on). No VCDS or tool kit here, so I’ll try to get it out and give it 12V from a booster pack.
 
Philip said:
Thanks, I’ll have a look. Current thought is that the (third) in-tank pump has died (can’t hear it prime when the ignition is turned on). No VCDS or tool kit here, so I’ll try to get it out and give it 12V from a booster pack.
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That would be unusual. There seems to be a few original FSI electric pumps failing recently, but that's not unreasonable after twenty years. Were the pumps you fitted new, or pre-owned?
They should last well, liquid cooled and lubricated.
Mac.
 
I preemptively swapped the original for a new Pierburg when I first went through the car, because it wasn’t looking great - that failed about 10k later (possibly because the tank top seal split and it was sucking air); I then replaced that (at short notice via a garage) with a new pattern assembly from ECP (which had the wrong potentiometer on it and had to come out again). That pattern part has done about 40k and there is definitely no priming noise with it exposed.
 
Thinking further - I have a generic VW/Audi code reader with me which showed no relevant codes after the car stopped; I would have expected to see cylinder misfires recorded if the low pressure pump had failed. Are there any Schrader-style valves on an FSI fuel rail?
 
Philip said:
Thinking further - I have a generic VW/Audi code reader with me which showed no relevant codes after the car stopped; I would have expected to see cylinder misfires recorded if the pump had stopped.
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Hi Philip, generic code readers are akin to a candle in the dark with FSi's.

You really need someone with VCDS who understands live values and measuring data etc. From my experience with FSi's is that the number of components in the fuel system you could really be playing parts darts replacing different things.

I had a similar issue on a Caddy some years ago which blew the fuse on the lift pump. Obviously in your case the lift pump has been working (if replaced a few times!) Might be worth checking the relay contacts for corrosion/poor connection as it can get wet/damp under the passenger floor.
 
If the electric pump failed, then the engine would stop, OK, obvious, but, I think, there would be no DTC, because the engine isn't running.
Same when you try to start, the ECU will know there's no fuel pressure, but I don’t think DTCs are generated until the engine is running.
All based on "I think"
The pump shouldn't ever suck air. The pump pickup is at the bottom of the tank, in a small reservoir, called a "swill pot" that holds enough fuel to keep the fuel supply going, if the tank is low, and what fuel is remaining, swills to the other side of the tank when cornering.
You probably have, but check fuse 18, 20 Amps.
Mac.
Edit: The pick up is an impeller, like a single stage turbine.
 
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No EML, no warning lights now or then. Was in a 40 limit on light throttle and then heard a very brief noise, almost like a rustle as if you’d run over a piece of plastic sheet or similar - engine dead and wouldn’t restart. Swapped fuel pump fuse, no change. Cranks fine with no odd noises, but not even a hint of it catching; no fuel smell in exhaust despite lots of cranking, no leaks or anything obvious untoward. Reseated fuel pump relay under the floor, no change.

Will have to try to get it to a garage for some diagnosis (RAC useless and expected me to leave it outside an “approved” garage over an hour away from where it stopped at 11:30pm, no further recovery possible).

(Regarding fuel pump tank seal - I think the system works on expecting everything to be airtight)
 
It's probably me, but I'm confused.
Looking at the schematic, the pump supply is bl/ro, (blue/red), but there is no bl/ro wire at the relay ...

Edit: Mystery solved. The Fuel Pump fuse, number 18, is where the pump supply changes colour. The relay is before the fuse, where the wire is gn/ge, (green/yellow), and after the fuse, it changes to bl/ro, (blue/red), and goes to the pump.
Mac.
Screenshot 2024-09-19 15.01.04.png
Screenshot 2024-09-19 15.01.46.png
 
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Philip said:
No EML, no warning lights now or then. Was in a 40 limit on light throttle and then heard a very brief noise, almost like a rustle as if you’d run over a piece of plastic sheet or similar - engine dead and wouldn’t restart. Swapped fuel pump fuse, no change. Cranks fine with no odd noises, but not even a hint of it catching; no fuel smell in exhaust despite lots of cranking, no leaks or anything obvious untoward. Reseated fuel pump relay under the floor, no change.

Will have to try to get it to a garage for some diagnosis (RAC useless and expected me to leave it outside an “approved” garage over an hour away from where it stopped at 11:30pm, no further recovery possible).

(Regarding fuel pump tank seal - I think the system works on expecting everything to be airtight)
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It does expect the tank to be at a positive pressure, and it's vented via carbon canister, and an ECU controlled solenoid, into the inlet manifold. If there's a air leak, you'll get an EML, but it won't stop it running.
There may be a fault conditions that will disable the fuel pump, not sure, but I'd expect there to be. Crash protection etc.
When you have VCDS access, check out Group 140, Fuel System, Block 4, Readiness.
Mac.
 
PlasticMac said:
It does expect the tank to be at a positive pressure, and it's vented via carbon canister, and an ECU controlled solenoid, into the inlet manifold. If there's a air leak, you'll get an EML, but it won't stop it running.
There may be a fault conditions that will disable the fuel pump, not sure, but I'd expect there to be. Crash protection etc.
When you have VCDS access, check out Group 140, Fuel System, Block 4, Readiness.
Mac.
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The electronic circuit, within the Fuel Pump Relay can start and stop the fuel pump. Opening the driver's door will, (should), start the pump, (improves starting).
The same circuit can stop the fuel pump, when the ECU receives a "Crash" signal from the Air Bag controller.
So, if the fuel pump does not start when the drivers door is opened, but the door open symbol comes up on the DIS, check the 12 volts to the fuel pump. If there's no 12 volts at the pump, then the fuel pump relay is suspect.
So, a dodgy door micro switch, or CCCU, could cause poor starting, because the fuel pump will only start when the ignition switch is turned, rather than be running, and up to pressure before turning the key!
Mac.
 
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