Intermittent starting issue

[skahigh]

The only electrical connection to the HPFP is the fuel metering valve.

Thanks, is that something that can be logged at all to help see if it's working correctly?

 

[DXX]

Thanks, is that something that can be logged at all to help see if it's working correctly?

One for @mmi I reckon, I have no idea. I don’t remember reading of any previous cases of fuel metering valve failure.
I haven’t looked at the logs in too much detail, age related thing, more spanners than ‘puters!
If the rail pressure is not rising when the HPFP is turning it seems there are not many possibilities.
1. No low pressure fuel delivery - disconnecting the feed to the HPFP from the filter should show flow.
2. Catastrophic failure of HPFP / drive mechanism - not the case as you have intermittent starting.
3. HPFP is delivering fuel but pressure regulating valve is dumping the fuel back to tank - disconnect the drain line and check for flow. There have been cases of these valves failing.

 

[skahigh]

Thanks, that's really helpful. I've just been studying the fuel system diagrams provided by @Dellmassive and I can see exactly what you mean.

Given the intermittent nature, you think it's safe to assume the HPFP is good? No potential issues with the drive mechanism that could be causing it?

Once it's running, it runs fine until the next time it has to try and start.

 

[DXX]

Thanks, that's really helpful. I've just been studying the fuel system diagrams provided by @Dellmassive and I can see exactly what you mean.

Given the intermittent nature, you think it's safe to assume the HPFP is good? No potential issues with the drive mechanism that could be causing it?

Once it's running, it runs fine until the next time it has to try and start.

The rail should stay pressurised when the engine is stopped.
If it doesn’t there will be a slight delay in reaching pressure to enable a start.
If it’s loosing pressure it could be;
1. Leaking pressure regulator valve, known issue.
2. Leaking injectors, known issue and will create another world of pain if not attended to quickly.
3. External leak, should be obvious!

 

[skahigh]

I thought it was odd in the logs to see the fuel pressure value drop to 0, is that even a valid value?!?:



And I noticed in the first log I did that evening, there was a slight increase in the fuel pressure not seen in the later logs:



Is the fuel pressure value relative to atmospheric pressure? If so, zero would make sense as a value in itself but I gather from previous logs provided by @mmi and @Dellmassive that 300 kPa is the expected value when the engine is stopped.

So this would start to look like a leak somewhere rather than the lift pump not operating? Something like a return valve sticking open (Fuel pressure regulating valve N276)?

Maybe I'm running away with this too much.

 

[Dellmassive]

A leaky or sticking injector could cause the no start.

If one was leaking derv into a cylinder, that would cause low rail pressure.

But probably cause a lumpy crank due to fuel in cylinder.

Possibly white smoke at start from extra fuel?

 

[DXX]

800 kPa is 8 bar, not adequate fuel pressure for running as far as I’m aware.
The HPFP can deliver up to 2000 bar dependant on engine load.

 

[skahigh]

800 kPa is 8 bar, not adequate fuel pressure for running as far as I’m aware.
The HPFP can deliver up to 2000 bar dependant on engine load.

I understand, I was more commenting that it was interesting that there was some rise above 300 kPa not seen in later logs (only a minute or 2 later), as if the problem became worse.

 

[skahigh]

A leaky or sticking injector could cause the no start.

If one was leaking derv into a cylinder, that would cause low rail pressure.

But probably cause a lumpy crank due to fuel in cylinder.

Possibly white smoke at start from extra fuel?

I would assume you'd have to see some increase in rail pressure if this was the issue also? There would have to be pressure in order for that pressure to then be dumped through the injector?

 

[Dellmassive]

Yes. A slight increase.

But pressure is created by resistance to flow. The hpfp just creates the flow.

So if an injector is stuck open the pressure won't build as the fuel will bleed into a cylinder.

 

[mmi]

I thought it was odd in the logs to see the fuel pressure value drop to 0, is that even a valid value?!?:

Is the fuel pressure value relative to atmospheric pressure? If so, zero would make sense as a value in itself but I gather from previous logs provided by @mmi and @Dellmassive that 300 kPa is the expected value when the engine is stopped.

When engine is stopped the pressure reading is simply next to none - e.g. in post #14 the pressure reading was just fluctuating from 300 to 900 kPa. I'd say none of them is exactly accurate - just sensor's imperfection. Actually in the full log there were also value 0 recorded.

So if an injector is stuck open the pressure won't build as the fuel will bleed into a cylinder.

Injectors can bleed also on the "other" side - returning fuel back to tank. E.g. here

[T6_measured] Injector failure

Keyword “loud knocking metallic noise from engine”. VW has published in September a service note which can be found with vehicle's VIN. The sound clips from my engine (150PS, EU6) with comments on 21 Oct 2017 in thread Tick over noise EDIT: audio files from the thread copied here as...

www.t6forum.com

 

[skahigh]

Just thinking out loud...

My best argument against most of these potential causes, leaky injectors, electrical issue with lift pump, etc. is that when it starts there are no running issues at all. I would think any intermittent issue like that would at least occasionally show itself when running (misfires, low fuel pressure warning, etc.).

Earlier today I was monitoring the fuel pressure when the stop start kicked in and I noticed that the fuel pressure didn't drop.

It got me thinking, one of the return valves must open when the engine is manually switched off in order to dump the high pressure fuel back into the tank. If this valve were sticky, it would open on switch off and from time to time get itself stuck open preventing any build up of fuel pressure on next start. Once unstuck and operating again it wouldn't necessarily show itself again until another cold start.

Still waiting for it to fail again in order to test the pump though!

 

[Dellmassive]

T6 - 4-cylinder common rail engine CXEB CXFA CXGA CXGB CXHA CXGC CXHB

Workshop manual for the 4-cylinder common rail engine CXEB CXFA CXGA CXGB CXHA CXGC CXHB

www.t6forum.com

 

[Dellmassive]

Look at the N276 - rail pressure metering valve.

You should be able to monitor the valve via VCDS.

..


..


..

 

[mmi]

Earlier today I was monitoring the fuel pressure when the stop start kicked in and I noticed that the fuel pressure didn't drop.

That's interesting...

Could you please make a log with the parameters below - to catch the "non-dropping" fuel pressure at stop/start engine shutdown.

Please Tick "Group UDS requests" - by 7 - this greatly enhances sampling rate.​

​

IDE00021 Engine RPM​

IDE00407 Rail pressure regulation: status​

IDE00589 Fuel pressure​

IDE01378 Fuel high pressure: control deviation​

IDE05632 Fuel pressure regulator valve actual value​

IDE05633 Valve for fuel metering: feedback value measured​

IDE07821 Fuel metering valve: activation​

IDE07822 Fuel pressure regulator valve: activation​

IDE07824 Tank-internal presupply pump 1 bank 1: activation​

IDE09340 Primary injection: quantity setpoint​

---

I have observed that the values below jump up when engine stops (by stop/start) and fuel pressure drops. Have thought the commanded purge "pulse" is shown in these.

IDE05632 Fuel pressure regulator valve actual value (from 350mA to 1000mA)​

IDE07822 Fuel pressure regulator valve: activation (from 10% to 25%)​

 

[skahigh]

Hmmm, maybe I just didn't leave it long enough then!

I've just made a log of most of those values, I think there's too many to log all at the same time though because after a certain number all the values start returning NA.

Unfortunately, the stop start was refusing to kick in (automatically disabled symbol on the dash) even after a decent length drive (45 minutes), so I'll have to try again another time.

 

[Howzat]

Having the heater set to max can disable SS so the next time you stop just turn the heating right down to see if it then kicks in.

 

[skahigh]

Having the heater set to max can disable SS so the next time you stop just turn the heating right down to see if it then kicks in.

Thanks, I had turned the heating off completely and also cleared engine fault codes as I believe sometimes it won't switch off if there are existing faults. Best guess it's something to do with the DPF regen or possibly battery condition (though I'm sure the latter is fine)?

I tried manually disabling and then re-enabling stop start with the button on the dash but no dice.

Anyway, I'll try again tomorrow.

 

[Howzat]

When you say off, do you mean the blower? It's the temperature that needs to be set to minimum just to be clear.

 

[skahigh]

Thanks, I didn't realise that, I had turned the blower off and assumed that achieved the objective!! Will try again tomorrow.