Sluggish Cold Starts

[f00glee]

For the last year or so, my Mk5 GTI tends to struggle a bit when starting after being left sitting for ~4-5 hours or more.

Ive scoured the forums and found plenty of posts where people have the same issue and offer potential solutions, but have yet to find a post where someone actually cured the problem.

I've heard of:
-Battery (just changed mine)
-Lpfp
-Plugs and coils
-Fuel sensor on hpfp
-Fuel check valve between hpfp and lpfp (does this even exist?)
-fuel type (I use only v-power, though)

When opening the drivers door, I do hear the fuel pump priming. I also let the dash lights go out before starting, but this doesn't always make a difference. Also worth noting the car is completely free of any fault codes.

The car kind of sputters into life over a few seconds rather than starting positively like it should. My suspicion is a lack of fuel pressure after being sat for a while, but this is just a suspicion!

The car never actually fails to start and runs flawlessly otherwise. Has anyone had a similar issue and actually fixed it? Most seem to just live with it as a "feature" but I find it frustrating! This is the last thing after a series of small issues I want to fix to make the car "perfect" in my eyes.






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[pudding]

The LPFP does indeed have an anti-drain back check valve built into it.  These can start weeping over time and cause sluggish starts.  As you say, the engine should fire up and settle to a smooth idle immediately.  Mine does at 114K, but I replaced the LPFP a couple of years ago for the updated version.

This guy is a VW diagnostics expert, so check this out for a better explanation.   It may not be your exact issue, but it's certainly an avenue to explore -

[MIJ_JAGGER]

Fuel pump relay maybe?

[f00glee]

Hi Pudding, thanks for the reply. Interesting video! Perhaps I could perform a similar set of tests using VCDS? I believe block 103 is the low pressure fuel sensor reading. Perhaps this will be enough to confirm a low-pressure side fuel pressure issue at start up.

I had my fuel filter replaced 6 months ago, so unlikely to be that as the issue existed before this.

Also interesting that turning the key to the last click before engine cranking caused further priming of the low pressure side. Didn't realise this was the case. Perhaps my sitting and letting the lights go it is allowing pressure to leak back, worsening my issue.

After work I'll try a couple turns like this before cranking the engine - could be a stop gap measure until I can diagnose the root cause. 

Thanks for the help, as always. Will report back as I learn more.

MIJ - if I am hearing the fuel pump, I guess I can assume the relay is okay?


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[pudding]

Not sure mate.  I haven't had to diagnose fuel supply issues yet, but it would be interesting see what that block says after 5 hours of sitting.  You'll have to pull the relay before you lock the car though, otherwise it will fire up the pump when you open the door, so will skew the readings.  I think it's no. 167 under the bonnet.

I replaced my original LPFP as I was getting fuel cuts on corners when the tank was 1/4 full.  Apparently the early pumps suffered over-heating, so they revised it 5 or so years ago iirc.  Not had that problem or starting issues since.

Fuel filters should be replaced annually imo as petrol is filthy!

Yep, toggling the ignition to position 2 also pulses the LPFP, so yeah, if you toggle it 4 or 5 times and it starts on the button......I think you've found your culprit!

[Patrick2691]

Mine suffers from this too and interested to see a resolution. Interestingly though if you put the ignition on then leave it for 30-40 seconds before starting it won't sputter into life but start instantly

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[pudding]

How many miles on these cars?  Another cause is a leaking injector(s).  It dribbles when the car is off, gradually bleeding away pressure from the rail.

If that is the case, the HPFP needs to run for a few seconds to restore the lost pressure, hence the sluggish starts maybe? 

Leaving the ignition on for 30-40 seconds is an odd one though!   

[f00glee]

35k miles here. I'll spend a few days trying different things when starting and poking around in VCDS and see what I can see!


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[pudding]

Unlikely to be injectors in your case then mate    They're pretty much done by 80K, especially on remapped cars.

[Patrick2691]

66k on mine. Found the leaving it a while thing very odd too. Nothing is happening in that time?

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[pudding]

66K again shouldn't be dead injector territory, but getting close to needing a clean/replacement.  That is very strange with the ignition.  Every time that happens?

Going back to earlier analysing about measuring block 103, I saw 7.5 bar on mine when left for an hour, engine off, if that helps at all.

[f00glee]

Just had a go on VCDS with mine. Had been left for around 2 hours so possibly not in sluggish start territory. After opening the drivers door and turning the ignition to position 2 as usual I measured ~7BAR (possibly really 6 bar+1bar atmospheric pressure which I'd assume the sensor would read even is the fuel lines were empty) at both the low pressure side (block 103) and the fuel rail (block 106). Cranked and started smoothly, so perhaps I at least have a log of a decent start up.

After start up the fuel rail held nicely at 50BAR (normal I think) and the lpfp maintained a duty cycle of 46-48% on idle (again, this is normal from what I have read).

Sitting with the ignition in position 2 and engine off after running for a couple minutes, the low pressure reading held steady at 7 bar again, though perhaps this is to be expected as with ignition on the and engine off the lpfp duty cycle was 5.9%. Fuel rail pressure at this point was dropping from 50bar very slowly ~0.01bar every 10-20 seconds.

So no real answers, but no surprises either. Ill repeat the test tomorrow after the car has been sat overnight and see if anything appears to be different.

[pudding]

That is all normal 

I think the main test will be to remove relay 167 and do the same readings in the morning, because opening the door will obviously prime the pump if the relay is still in place.  The fuel filter has a 5.5 bar regulator built in, so not sure what bearing that would have on the readings.  It should never fall below that in theory!

[Dave J]

This is strangely all great reading. Thought I am going mad with my ED30 not seeming to start the same way a few times in the morning. When cold at home (on a slanted drive) it either splutters to life, or starts keenly. Does it too at work on a flat surface...

Mine was perky as you like for a while on turning over after fitting the highly rated Varta by Bonelorry's thread (https://www.tayna.co.uk/E44-Varta-Silver-Dynamic-Car-Battery-P7726.html). Better cranking power, and no issues for ages.

As per f00glee's post, (apart from the battery) mine has had the following new parts as improvements in the time I have owned it,

Loba HPFP
New R8 coils
New VW LPFP
New sensor on HPFP

At my next (looming service) I want the fuel filter replaced for the newer 6.6bar version anyway as part of good practice (never been changed).

Mine is on 86k now (bought on 47k in 2012, and always run on VPower in my ownership). Could this simply be down to a carboned up head? I know the injectors can get a bit tired, but mine doesn't show any issues when running in terms of missing, etc...

I have Vagcom, but am no master when it comes to logging stuff...

I do have an intermittent ignition switch fault logged though. Never had an issue with the car not starting, etc.

Address 16: Steering wheel        Labels: 1K0-953-549-MY8.lbl
   Part No SW: 1K0 953 549 AQ    HW: 1K0 953 549 AQ
   Component: J0527           036 0070 
   Coding: 0013142
   Shop #: WSC 00581 210 83422
   VCID: 010232F958614249E6-8054

   Subsystem 1 - Part No: XXXXXXXXXXX
   Component: E0221           002 0010

1 Fault Found:
00883 - Ignition Switch (D) Terminal S
            012 - Electrical Fault in Circuit - Intermittent

Any one have the same?

[rich83]

Cant say mine starts perfectly.... but providing it starts (eventually) its not something I have been bothered about trying to sort out. on 110K