Hi all, I'm hoping you might be able to help diagnose a problem with my 2015 Octavia VRS 2.0 TDI Estate.

Symptoms:

• Struggles to start from cold (e.g. if left for a few hours)

• Sometimes the engine judders and has cut out on me when going up hill before. Usually drives lovely though.

I think it's a fuel starvation issue caused by the fuel transfer pump not running when it should.

What I know:

• All glow plugs replaced recently (last owner swore that was the cause of hard starting)

• Fuel transfer pump works (I can hear it running when I run test in VCDS)

• Fuel transfer pump doesn't run when I turn the ignition on.

• Pretty sure the fuel transfer pump doesn't run when the engine is running either.

Any advice on likely causes or sensible diagnostic steps would be appreciated.

My guess was maybe the fuel pressure sensor but I think that seemed ok in VCDS.

Thank you.

Hello, welcome to the forum.

Are you able to source a diagram for the fuel transfer pump?

I'd check to see what controls there are in the pump circuit.

Failing that, I'd take another good look at the pressure sensor.

@Warrior193 Thanks for you response.

Here's the wiring diagram for the lift pump. It looks to match my system, at least the connectors and wire colours seem to match.

It just looks to be a direct signal from the ECU which turns the pump on via the fuel pump control unit. That part of the system seems ok since I can activate the pump via VCDS.

So I'm more inclined to say that it could be a sensor playing up.

I've tried to measure the low pressure fuel sensor but I'm not sure it I'm reading the right thing. I've been looking at IDE00589 (Fuel Pressure) in VCDS but I think that may actually be the high pressure system (?)

With the ignition on and engine off the "Fuel Pressure" appears to fluctuate between 0 and 170kPa:

(The lift pump isn't energising)

But with the engine idling the "Fuel Pressure" fluctuates at around 22kPa - 28kPa, then rises if I rev the engine.

So I feel that I maybe need to identify the low/supply pressure sensor. I'm not sure which reading it is in VCDS.

I also monitored these values but didn't see anything other than 0 at any point.

Any advice on better testing the low pressure fuel sensor please?

Is there even a supply pressure sensor? ChatGPT suggested there is but the part number is provided (04L 906 051) came up as either an exhaust pressure sensor or a fuel temperature sensor.

Perhaps it could be a fuel temperature sensor issue? Apparently "Fuel Pressure" is a calculated value and I wouldn't be surprised if it uses fuel temperature in the equation. A dodgy temperature reading might explain flickering between 0 and 170kPa with the engine off but reading normalish values with the engine running since fuel temperature would have a more negligible effect if the actual pressure is high.

Thank you

Edited 21 September, 202521 Sep by TheSwagga

Well I have made some incorrect assumptions...

With the back seat out I can hear that when I crank the engine the lift pump is running. I expected that it would run when I turned the ignition on (like many other cars).

So there is some other cause of this problem.

I did check the fuel temperature sensor, resistance was 2770Ω and rising (engine had been running and was cooling).

A while back I put a piece of clear fuel line on the input to the fuel filter. The next day there was some air in the line:

My brother suggested I buy a pressure test kit so I will order one now.

Also perhaps the check valve could be faulty? A cheap part to replace I assume but wouldn't explain the juddering when running.

Edited 21 September, 202521 Sep by TheSwagga

The problem turned out to be a bad seal of the fuel pressure regulator onto the fuel rail, I eventually noticed diesel dripping from that spot.

Frustratingly three garages I took the car to couldn't see this obvious sign, I thought I might have been going mad.

I ended up taking it to a well reputed Skoda guy far away (Tadhg O'Riordan in Kanturk) and he saw the problem straight away and fixed it in an afternoon for €80!