Intermittent performance problem

[Jammin]

I bought a 2005 GTi earlier this year and for as long as I've had the car I have found that on some journeys acceleration feels flat, whereas on others it feels much more aggressive and involving. I recently replaced the DV with the latest version (06H 145 710 D) which definitely improved things higher up the rev range, but still on some journeys, off the line and into mid range revs, acceleration is flat. I am looking to replace the PCV next, but wanted to ensure that I'm not looking in the wrong direction first.

It *seems* as though the problem occurs when ambient temperature is 18degs+ (although this could be coincidental) and as well as flat acceleration, I notice that it's more difficult to obtain smooth gear changes and throttle response in general seems lethargic.

Should I be looking at the PCV next, or at something else?

[Hurdy]

I'd do the PCV fix anyway.....see below!

FSI engine after 19k miles!!!!!! 

http://i10.photobucket.com/albums/a140/pippyrips/DSCN3441-1.jpg

http://www.clubgti.com/forum/showthread.php?t=189147

I'll be looking at mine with interest as it has 18k miles on it at the moment!

A turbocharged car will feel a little more lethargic in hotter conditions anyway due to heat soak, but this doesn't sound like your problem.

Is the engine idle nice and stable or uneven and lumpy?

[ub7rm]

I don't know what your mileage is but this sounds a lot like MAF trouble.  I had the same trouble on my mk4 1.8T and this was ultimately the cause of the problem but like yours some days it would be fine other days flat.  Coupled with that occasionally but not always the revs would jump around a bit at idle especially after a long drive.  This doesn't always generate a fault code so the best way of seeing if its at fault is to log the MAF output using VAG COM (VCDS). 

Another test is to actually disconnect the sensor.  This forces the ECU to use preprogrammed values and although the car won't run as it should, if the MAF is at fault it will run better than with the MAF connected.  However, if the problem is intermittant and its only starting to fail this won't be conclusive. 

Another mk4 issue that could cause this sort of problem was the temp sender for the ECU (not the gauge one) but I haven't heard of this being a problem with mk5's. 

You could go in for expensive part swapping type of diagnostic but the best way of finding out is to get it on a diagnostic machine.

[Jammin]

Thanks for the replies. The car has 32k on the clock and as of yet the idle doesn't seem to be affected. Unfortunately I don't know anyone with VAG COM to be able to log MAF output, but if the problem becomes consistent, I'll disconnect the MAF to see if there's a noticeable improvement. Any idea how much a replacement sensor might be?

[ub7rm]

I would guesstimate around £100 (going by mk4 prices) but you can exchange your old one for a new one which takes about £25 of the price.  Don't be tempted by cheaper ones, this is one parts that needs to be OEM.

At 32k its a bit early for the MAF to fail but I have heard of it happening... 

[Jammin]

Well, long story short, I replaced the DV, front PCV, MAF sensor and air filter (which appeared to have never been changed). When the car was behaving, it shot off down the road like a scolded cat much quicker than ever before, but would soon revert back to feeling flat.

On holiday recently I had to fill up with BP 97 ron as there was no Shell garage nearby for VPower and the car definately immediately improved. Having stuck with BP for a few tanks, it's now consistently much more drivable and responsive, although there are still times when it really pulls like a train, which highlights the fact that it's not doing so all the time.

Is there any logic to this development, does it point to a particular component fault? Alternatively, does it cast doubts on the software that the car has onboard, should I be investigating what map it's running?

[Hurdy]

Sounds like the car could be mapped incorrectly

[Teutonic_Tamer]

Or could it be that the Shell garage was dodgy - putting normal unleaded in its' V-Power tanks? 

Jammin, what is it like on Tesco 99, or any of the other super unleadeds?  Total Excillium is probably one of the best 97 ron brews. 

[Jammin]

Or could it be that the Shell garage was dodgy - putting normal unleaded in its' V-Power tanks? 

Jammin, what is it like on Tesco 99, or any of the other super unleadeds?  Total Excillium is probably one of the best 97 ron brews. 

I've used 3 different Shell garages and all with the same result, so it's *probably* not incorrectly labelled fuel (although you never know).

I've not tried either Tesco or Total, but on the next fill up I'll put a tank of Total in and see how it is.

I suppose the other sensible action would be to get the car into VW for them to re-flash with a standard map, just to make sure.

[Teutonic_Tamer]

Or could it be that the Shell garage was dodgy - putting normal unleaded in its' V-Power tanks? 

Jammin, what is it like on Tesco 99, or any of the other super unleadeds?  Total Excillium is probably one of the best 97 ron brews. 

I've used 3 different Shell garages and all with the same result, so it's *probably* not incorrectly labelled fuel (although you never know).

Maybe those three are all owned by the same dodgy family business?  Shell petrol stations do seem to be the worst for allowing illegal activites.  Credit card fraud is well known to be the worst at Shell garages.  I nearly got shafted for credit card fraud at a Shell garage - but thankfully, I was on the ball, and noticed it happening on the spot, so called in the old bill and my credit card company (who said mobiles can't be used at petrol stations ).

Don't forget, virtually all petrol stations, except supermarket ones, are just franchisees of the actual brand of fuel - and maybe the 'quality control' of Shell franchising is worse than say Total or Esso or BP?

I've not tried either Tesco or Total, but on the next fill up I'll put a tank of Total in and see how it is.

Try Tesco 99 - it has been repeatedly proven to be the best fuel in the UK, and is actually better than V-Power.

I suppose the other sensible action would be to get the car into VW for them to re-flash with a standard map, just to make sure.

Good idea.  Did you ever get it scanned for any stored fault codes?