Hi all. Hopefully someone can assist. Apologies for the long post!

I have a Polo 6n2 1.4 16V with an AUG engine code. This is a rally car and has all started and run OK until one of our last rallies. Here’s an overview of what happened.

We drove and queued for noise test with no issues, as there was a long queue for scrutineering we stopped the engine. When I went to restart it just turned over and over with no hint of a fire. We pushed to scrutineering and luckily they didn’t ask us to start the car to test the cut off switch!! We pushed back towards service and tried again, it then spluttered and fired but seemed to be running on 3 cylinders.

Once the engine cooled down it started OK. We drove stage start and made sure we didn’t stop or stall the engine. It went through the stage with no issues. Back at Service after about 10 minutes tried to start the car and it wasn’t having any of… just turns over but never fired.

Being a rally car we carry a shed full of spares and started swapping things out to no avail. We swap out the following, testing between components:

Coil Pack

Throttle Body

Plugs

Plug Leads

Vagcom did not indicate any specific issues, and the car only started when it cooled down.

On returning home and the next event looming did some research as we needed to sort quickly, there was suggestions it was possibly temperature sensor. I was also concerned that compared to our MK2 Golf the fuel pump seems to run for a very long time when priming. We have now changed the following.

Coolant Temperature Sensor

Fuel Pump (Fuel Pump runs for about 10-15 seconds when pressuring, our golf runs for about 2 seconds)

Fuel Pump Relay

Fuel Pressure Regulator

Fuel Filter

MAP Sensor

(Most sensors have been swapped with spares we had)

Any ideas gratefully accepted. I think we have changed everything but the doors!

Regards Kevin

If I’m stabbing in the dark, wiring issue.

Just go one step at a time though:

Does it work with the AFM unplugged?

Is it sparking?

Are the injectors getting a pulse?

Have you got fuel pressue?

Is the timing still ok?

Has it got compression?

Edited 16 January, 20251 yr by Yeti

Hi, thanks for your response. See replies below.

Does it work with the AFM unplugged? What is the AMF? When I google it, it refers to Diesel. Where is the AMF?

Is it sparking? Yes

Are the injectors getting a pulse? Yes. I also swapped out the injectors as we carry tested spares.

Have you got fuel pressue? Yes, but I would say not to the documented level. Pressure doesn’t vary much hot or cold, nor does it change a great deal when I remove the vacuum from the Fuel Pressure Regulator (and there is definitely vacuum from the pipe)

Is the timing still ok? Yes

Has it got compression? Yes

Once the car cools it runs great, its only when its hot and we have stopped the engine. If I could leave it running all day through service it wouldn’t be an issue. Its only a problem when I stop the car and try to restart. Its as if the fuel was vaporising

AFM would be AirFlowMeter or MAF Mass AirFlow sensor.

But if AUG engine has a Manifold Pressure sensor it won’t need hot wire airflow sensor for ECU.

Tight battery connections? Had a Lupo that died if engine turned off at long traffic light stop after 20 mile commute. That was a slightly loose battery +ve.

Basics. Just need spark, air and fuel.

Clean air filter?

I suppose a leaking injector would mean system didn’t pressurise well, and being too rich in a way that fools narrow band lambda sensor would foul a spark plug or all 4. Carbon coated plugs give poor spark.

Take them out and have a look at the tip when it happens. Cooks’ blow torch will burn it off, or new plugs.

Old race cars used to have warm plugs for starting, then change to cold plugs for the race.

Or, a vapour lock. Bag of frozen peas around the fuel line?

Thank you ‘dvderlm’ for clarifying AFM. We have changed the MAF so don’t think it is that.

I will check out the battery connections. We did swap plugs (and leads, and coil pack!). Though they were old spares, happy to buy new ones.

Injectors have previously been sent away and serviced, I also purchased some new that were swapped out. I have never ran the fuel pump with the injector rail out to see if any injectors are leaking so will take a look at that.

The Air filter is a K@N that we clean and oil after a couple of rallies. Wont start with filter off or on once hot.

It really seems like fuel vaporisation, but why? Ran the car for over 10 years and no problems, now its everytime it gets hot!

As I mentioned I also changed the fuel pump to no avail. I am still very curious about how long you would expect the fuel pump to run with ‘priming’ as our Golf just runs for a couple of seconds, the Polo runs for 8-10 seconds.

I will also check all wiring looms.

Thanks again.

Really don’t know how long it should take to reach pressure. I’m more a carbs guy.

But I found someone with 8 second priming (older model)

More Ethanol in the fuel than a decade ago and it does evaporate quicker.

K&N or nothing flows more air than paper. So a bit leaner when starting hot. But surely not enough to matter.

On hot carb engine reluctant to start I’d floor the throttle and crank about 5 seconds to cool and suck away any inlet manifold fuel pooling. Count to ten then start with light press on the gas pedal.

With flybywire throttle body that probably won’t open the flap.

Edited 19 January, 20251 yr by dvderlm

Some cleaning oil from K&N can affect Mass AirfFlow sensor wire, apparently. But I’ve noticed nothing on a 1.9Tdi with a K&N or a 1.2Tsi I’ve had it.

Bearings could come into play if the issue is related to the alternator or starter motor. Over time, these components can wear, and if the starter motor's bearings are failing, it may struggle when hot but start functioning again when cooled. Although bearings aren't directly related to the no-start issue itself, it’s worth ensuring that components like the starter motor are turning over the engine properly. Sometimes, a starter motor will struggle when the car is hot if the bearings or internals are on their way out.

On 16/04/2025 at 17:01, blair said:

Bearings could come into play if the issue is related to the alternator or starter motor. Over time, these components can wear, and if the starter motor's 203TTF4152 bearings are failing, it may struggle when hot but start functioning again when cooled. Although bearings aren't directly related to the no-start issue itself, it’s worth ensuring that components like the starter motor are turning over the engine properly. Sometimes, a starter motor will struggle when the car is hot if the bearings or internals are on their way out.

Even though bearings aren't the root cause of the electrical signal to start, they do play a role in whether the starter can mechanically do its job. It's subtle but important. A failing starter might still draw power, but not enough torque gets transferred to the flywheel.