Starting the S3 with low fuel stutters

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Hadders

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Hi guys, so I'm sure this is probably the fuel pump, but thought I'd ask for confirmation.

When my S3 (AMK) starts from cold, the amount of fuel in the tank dictates how well it starts. With a full tank, it virtually switches on, however with the tank running low (less than a quarter remaining) the car seems to stutter when you first start it. It seems to be fine once it gets going, but I'm guessing this is an early indication that the pump is pretty close to needing replacing? If this is the case, could this be affecting the fuelling all the time as well? Also, how difficult (read: expensive!) is it to replace?

Ta muchly.
 
Wouldn't have thought its the pump tbh... you could get the pressure checked on a dyno as this would tell you for sure if the pump is getting tired but if the pump struggling on startup you would be having serious running issues under boost etc... it could be the fuel lines de-pressurising over night and its an initial pressure catches it out... but tbh I think its unlikely pump related especially if the engine is running ok was over it and under boost when warm etc..

Fuel pump failure normally manifests itself as AFR going lean at high rpm under boost as the pressure drops off... seen this on a couple of S3's recently but pretty sure start up was fine...

Maybe injectors are a little dirty and in need of a clean?

<tuffty/>
 
Hmmm, I just find it strange that the amount of fuel in the tank plays such a large part in how well it starts. If it was a drop of pressure in the fuel line, is there a way I can remedy this? I can't imagine the spluttery start is doing the engine much good!

Can I measure the AFR with VCDS-Lite?

I've been meaning to clean the injectors soon anyway, just need to find a local sonic bath *nudge nudge* hehe.
 
Fuel pump should prime when you first turn on the ignition, this should re-pressurise the lines... I don't know for sure but there should be a non return valve on the pump but if there isn't or this is getting tired then the pressure differential between the lines and the tanks fuel level could be syphoning back fuel through the lines as the tank level drops and contributing to the effect you are experiencing...

The block you need for AFR is 031 on VCDS... not sure if the lite version allows that though

<tuffty/>
 
try cycling the ignition twice when the tank is low and see if that helps it start.

Ie ignition on, wait for the pump to whirr then stop, ignition off and then back on, wait for whirr to stop then start. If it helps, its perhaps suggestive of a faulty non-return valve.
 
aragorn said:
try cycling the ignition twice when the tank is low and see if that helps it start.

Ie ignition on, wait for the pump to whirr then stop, ignition off and then back on, wait for whirr to stop then start. If it helps, its perhaps suggestive of a faulty non-return valve.
Click to expand...
Interesting I have had this problem since ive owned the car, and ive done what aragorn as suggested and it works....
I think mine maybe a little advanced than yours, as my car will not start first time...2nd go no problem everytime.
Now the interesting thing is im a skinflint (not that) and i start sweating if i put more than £40 in juice at any one time, however last night i put £60 in.........i was talking to somebody.....lol, and low and behold the car started first time this morning......first time ive put £60 in and first time it as started first time ....for me, so you maybe onto something here Hadders...:icon_thumright:
 
I have an AMK and exactly the same issue! Mine takes a while to start from cold with little juice in it, but with a full tank no problem at all! barely needs to turn over! I though fuel pump so i'd be interested to see how this turns out.
 
Started again first time lol thats twice since i owned the vehicle , so i will be observing closely at what point it starts deteriorating, (only used about 1L yesterday)
So its looking like the gravity of the fuel is helping the pump....
 
I dont recall seeing a non-return valve on the stock pump setup lines.

Does your pump prime (whirrr click) when you open your drivers door.
does your pump prime (whirrr click) when you turn ign on (but not crank)

Does your courtesy light work from the drivers door?
 
badger5 said:
I dont recall seeing a non-return valve on the stock pump setup lines.

Does your pump prime (whirrr click) when you open your drivers door.
does your pump prime (whirrr click) when you turn ign on (but not crank)

Does your courtesy light work from the drivers door?
Click to expand...
On mine i can hear buzzing when the door opens but no click, the interior lights work when the doors open yes
 
Indeed, I tried the two times prime approach and it started fine with a low fuel load.

I've not noticed mine priming when I open my door, but it definitely primes when I turn on the ignition (without crank). I'll have a listen later when I get back to the car.
 
Mine does this too. I've fitted a new pump and filter (because it was going lean @5500 rpm upwards) Still takes longer to start and sometimes stutters into life if the fuel gauge is below a quarter. Don't waste your money on a new pump (unless yours is lean anyway) it probably won't fix the poor starting.
 
Mine has also started doing this over the last 2 months when the tank is below a qtr, sometimes on the verge of stalling but fine after around 3 secs of running
 
Didn't nick have this problem? he changed the temp sender and it was fine? I ordered one of them because i have this problem. I shall see if it cures it, im due to pick it up tomorrow and will fit it too.
 
I've had this problem for quite some time and had the temp sender replaced in that time. And the thermostat.

This non-return valve sounds like it could be the culprit. Once it gets going it doesn't seem to be a problem, I'm just worried about the long term effects of this on other components.
 
I've had this for a while now but it's not the fuel pump as i changed it. Mine's vacuum/boost leak related apparently. But it only happens the next morning on cold starts which i don't understand.
 
I would suggest to replace the fpr, discovered that the injector rail loses pressure as soon as the engine is off and that might drain the pressure line through the return line.
The non return valve is incorporated into the pump.
 
Changed temp sensor and cleaned the injectors ultrasonically and it seems to have cured it? Im waiting for a cold morning so i can try it then..
 
Just thought I update.. I still get bad starts when the fuel is under 1/4 tank. Gonna Change the fuel pump as its 13 years old.
 
Just to add, above quarter tank mine starts every time. Below, it flutters a little, revs and settles. Will try the two turns method! Love AS for this sort of thing.
 
It does help when you prime the pump twice. But that's what Makes me think its the pump. I'm due to change mine
 
Quick bump

Did this ever get resolved? if so what was the cure?

as my car has been doing this on cold starts for a while now and would like to rectify it if possible.

I was tempted to try a new CTS although I have scanned the car today and get no faults whatsoever (which is nice)

Any thoughts?
 
mine does the same i think its the fuel pump. Only since i read something on here did i take fuel levels into account & under 1/4 tank it does struggle. I initially thought it was to do with the temperture.
 
I replaced my fuel pump when the car failed to start one day (about a year or so ago) but luckily I've not had this problem. So, are you guys definitely sure it's pump related? My pump must've been tired before it stopped working yet I never had any issues with cold starting.
 
well its not the pump cos I just put a brand new one in 2 days ago yet this issue is still there.
 
Mine doesn't prime when you open the door, had it since 2003 and it never has.
Have you changed the fuel filter?
I forgot and it ruined the fuel pump 😭
Dave
 
Same issue. It was fine before I done the following:

- Fuel pump (new VDO/Siemens)
- Plugs (BKR7Es - properly gapped)
- Left the car for 3 and a bit weeks

Sometimes it'll go first time if I turn it over for a bit longer than normal.

I was planning on cleaning the TB, Injectors and FPR again (I done them around 18 months ago).

I also have other issues with knock which I think is unrelated (N75 possibly) but I was hoping would be fixed by the pump.
 
My car always starts first try everytime but then the revs drop as if its gonna stall like 300-400rpm and then pick up to say 1200 to finally settle just above 1000 whilst the auto choke is running so this is the stutter I am getting

this whole thing takes no more than 2 or 3 secs and then the car is fine but it never used to do this so it must be a fault somewhere as such. and it only does it when engine is stone cold
 
If mine is low on fuel, it won't start "nicely" first time, so it'll turn over, turn over, turn over, kick in. Cold temps mean maybe three or four seconds. Warm temps, two seconds.

Two turns method means it starts instantly every time, regardless of fuel or temp. Not an issue in my mind.
 
Mine actually died today when I tried to start it and leave it alone started second time but wasn't happy till I gave it a bit of revs above 1000rpm and it was fine.

So I have an old CTS here from my old S3 which I think is ok so am gonna chuck it in and see if it helps the cold start issue. also popped out the FPR and the little green washer has seen better days so I beefed it up with a second smaller o ring as it may of been losing pressure to the fuel rail.

For my car its defo something to do with the auto choke not kicking in properly when its stone cold

Will post back with my results
 
vrbob said:
Mine actually died today when I tried to start it and leave it alone started second time but wasn't happy till I gave it a bit of revs above 1000rpm and it was fine.

So I have an old CTS here from my old S3 which I think is ok so am gonna chuck it in and see if it helps the cold start issue. also popped out the FPR and the little green washer has seen better days so I beefed it up with a second smaller o ring as it may of been losing pressure to the fuel rail.

For my car its defo something to do with the auto choke not kicking in properly when its stone cold

Will post back with my results
Click to expand...

Very interested to see the results to... mine did this before swapping fuel pump, was OK for a few days after the pump swap (guessing cos ambient temp was higher) and now its back again. I seem to have in my mind I've read about this before and someone said camshaft position sensor cured theirs - or I could be going insane :keule:
 
Currently battling with this issue. I fitted a new fuel pump a few month back and I also swapped out the cts which did cure it for a while. The cts was an used genuine part I had lying about so it's possible this has gone bad.

I've decided to leave it for now and wait for it to throw a fault code rather than start replacing bits blindly. It only happens on frosty mornings.
 
I'm less than a 1/4 of tank & tonight after it standing since sunday it did struggle however after popping down the gym it didnt struggle at all. I think its time related, if theres no return valve this would make sense.
 
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