Hello

I am out of ideas, so I came to ask here. Maybe anyone had the same issue.

I am trying to fix Octavia Mk2 with 1.9 TDI BXE engine and automatic transmission.

When you leave the car, it starts just fine. It keeps running for 5 minutes and then suddenly engine stop, like you kill it with the switch, but all the lights on the dash stay on. It’s not like all the electronic dies. Then you wait for 20 minutes, sometimes more and it starts once again and run for some time and same happen again. Does not matter if you drive or idle.

There is ECU EDC16U34, no faults in the memory. Zero. So far I did:

• Checked all the fuses - all good

• Checked relays and even replace BCM

• Three different batteries, even on support cables, booster

• Replacement of crank sensor

• Replacement of ignition switch

• Checked wires from ECU to crank sensor - all good

• Checked ECU - clean connectors, no water damage

• Checked wires around transmission and under battery tray - visibly no damage

• Checked fuel pump - still running

When the engine suddenly stops I can hear click of the fuel pump relay. Then when you try to start again, this relay is just clicking - I have tried three of them, all do the same. Same with KL.15 relay on BCM.

It looks like, something is shutting off the ECU, but when this happens, I still have connection and communication to ECU, so there is still power.

The only thing I have not tried yet is to change the ECU. But I don’t want to do that because nobody’s going to pay me.

Any ideas? Experience? What else to check?

Sounds like something is getting hot and failing. It might be the camshaft sensor. I have read that they can cause problems without causing fault codes. You could also try without fuel cap in case the tank venting system is blocked and causing a vaccum in the fuel line. Happend to me on my old fiat van as the previous owner had put on non vented fuel cap.

Alasdair

18 hours ago, eMvoc said:

• Checked fuel pump - still running

When the engine suddenly stops I can hear click of the fuel pump relay. Then when you try to start again, this relay is just clicking - I have tried three of the

So this is confusing. You say fuel pump running but relay just clicking....

Which is it? Please clarify.

Or, do you mean the relay runs when the car runs and then clicks when it stops?

Is the power supply and or the signal to the relay and fuel pump being interupted when the engine dies? Check it with a multi meter or test lamp.

Can you bypass this and power directly off battery with a jumper cable to run the engine. That would narrow down the problem.

Does it stop quicker if you are driving or idling, or EXACTLY the same? Again, the devil and the clue is in the detail.

If time is always the same regardless of engine load/driving/idling, and the signal to the fuel pump relay is interpreted then I'd be thinking immobiliser cutting in....

Another fairly cheap sensor is the coolant temp sensor. If its failing then it may cause similar problems although in my case the car wouldnt start when cold.

Alasdair

Edited 3 February3 Feb by Alasdair1

5 minutes ago, BlueWagon said:

So this is confusing. You say fuel pump running but relay just clicking....

Which is it? Please clarify.

Or, do you mean the relay runs when the car runs and then clicks when it stops?

Is the power supply and or the signal to the relay and fuel pump being interupted when the engine dies? Check it with a multi meter or test lamp.

Can you bypass this and power directly off battery with a jumper cable to run the engine. That would narrow down the problem.

Does it stop quicker if you are driving or idling, or EXACTLY the same? Again, the devil and the clue is in the detail.

If time is always the same regardless of engine load/driving/idling, and the signal to the fuel pump relay is interpreted then I'd be thinking immobiliser cutting in....

It’s like this: when the car is left for longer period of time, it behaves as usual. You put the key in, you hear fuel pump running, you start the car and engine is running normally. Then after 1 to 5 minutes, no matter if you drive or idle, the enigne stop. And when it stops the fuel pump relay click. Then when you try to start it again, during the cranking, this relay is just clicking on and off - something is telling not to put fuel in and disconnect this relay. If I hold it manually, the pump is obviously still running but engine not. Time is not always the same. Sometimes it runs one minute, sometimes five. Sometimes you can start is after one minute, sometimes after 30 minutes. Immo usually stops the engine right after start.

To Alasdair:

This engine can also run only on crankshaft sensor. Without camshaft sensor, it just crank for the longer time. Coolant temp sensor won’t stop the engine.

Might be the fuel pump relay thats failing when warm. Have you tried swapping for a known good one.

Alasdair

18 minutes ago, Alasdair1 said:

Might be the fuel pump relay thats failing when warm. Have you tried swapping for a known good one.

Alasdair

Could be a good call, relays can trip when warm.

This may help the OP find them all, specifically ones that supply the ecu

https://www.scribd.com/document/480348716/SSP-54-Octavia-II-electrical-components-pdf

Yes, I tried different relay. I have even changed the whole BCM with all different relays in from different running car.

When you say pump is still running I assume its the intank lift pump. It may be running but not pumping once it warms up or down on pressure/drawing in air

Alasdair

Edited 3 February3 Feb by Alasdair1

Yeah, but it won’t turn off engine immediately. There is some sttutering when the engine is out of fuel.

Maybe as said drawing in air somewhere or blocked filter/fuel line.

Alasdair

what are you using to scan the car ?

VCDS Pro

2 minutes ago, eMvoc said:

VCDS Pro

Does the VCDS cut out at the same time the engine stalls

Also does the rev counter move whilst cranking

I had problems on my sons Fabia with strip fuses near battery. There were two that had hairline invisible cracks that when warm broke contact. Not sure on the Octavia or if they would stop car but worth a look. If VCDS isnt finding any codes its an odd one and may be electrical in some way. It doesnt have an aftermarket alarm/imobilser?

2 minutes ago, Alasdair1 said:

I had problems on my sons Fabia with strip fuses near battery. There were two that had hairline invisible cracks that when warm broke contact. Not sure on the Octavia or if they would stop car but worth a look. If VCDS isnt finding any codes its an odd one and may be electrical in some way. It doesnt have an aftermarket alarm/imobilser?

sounds like power is being cut from the ecu before a code can be recorded

1 hour ago, Stonekeeper said:

Does the VCDS cut out at the same time the engine stalls

Also does the rev counter move whilst cranking

Usually when cranking the communication between tester and ECU is limited. But when it runs, it works fine. When it stops, I still can communicate with the engine ECU. So ECU is under voltage. It probably just cuts out the fuel and close the injectors, but for unknown reason.

And also yes, the rev counter is moving when cranking

No extra alarm or immo

Edited 3 February3 Feb by eMvoc

Try unplugging the EGR valve to see if the car stays running longer than 5 minutes

Got the below from AI

Critical Components to Check

1. Relay 458 (E-Box): In the engine bay fuse box, Relay 458 is the main power supply relay for the ECU. If this relay has high resistance or burnt contacts, the ECU will have enough power to "talk" to a tester but not enough "current" to actually fire injectors or hold relays closed.

2. The "5V Reference" Collapse: If a sensor like the Camshaft Sensor or EGR Valve has an internal short that only occurs when hot, it can pull down the ECU's internal 5V rail. This causes the ECU to cut fuel and "glitch" without storing a code, as the fault occurs in the hardware power management layer before the software can register an error.

3. Injector Loom (Common 1.9 TDI BXE Fault): The wiring loom inside the cylinder head (submerged in oil) is a notorious weak point. While it usually causes a misfire code, a short-to-ground in this loom can occasionally "trip" the ECU's safety shutdown, causing it to kill the fuel pump and injectors simultaneously to prevent damage.

Recommended Next Steps

• The Heat Test: While the car is running, use a hair dryer to carefully heat the ECU case. If the car dies much faster than 1–5 minutes, you have confirmed an internal ECU hardware failure.

• The Cold Start Bypass: If you can immediately restart by jumping the fuel pump relay but the engine still won't fire (even if the pump is running), it confirms the ECU is refusing to pulse the injectors.

• Scope the Crank Signal: Even though the sensor is new, check the signal at the ECU pins (not the sensor) when the stall happens. If the signal vanishes there but the sensor is fine, the issue is the ECU's internal processing of that signal.

I have tried AI too. Most of the time it knows ****, to be honest. But thanks