VR rubbish idle and terrible mpg.....update possibly tps

[timmaaah 0]

Car's been running great - got 33.5mpg on a 30 mile run last Saturday and the idle has been spot on.

 

Went out yesterday and I could barely coax it over 21mpg and the idle is hunting up and down. It keeps almost stalling, then revving then back down again. If I unplug the ISV (the plug leaving the isv still plumbed in as it were) the mpg gets much much better and it doesn't hunt for idle (although it is ticking over a bit higher).

 

I assume the ISV is fubar? It's been cleaned several times in the past, and giving it a blast of carb cleaner yesterday made no difference. Why would a faulty ISV cause crap mpg?

 

Any help gratefully received :)

Edited May 30, 2013 by timmaaah

[Kevin Bacon 5]

The 5 most common things for mpg dropping that much:-

 

MAF

Cam Sensor

Blue Temp Sender

Lambda Sensor

Air leak downstream of the MAF

 

The ISV won't be the cause. It's just struggling to maintain the idle speed because something else is busted.

[timmaaah 0]

Right - a quick update:

 

My long term average mpg is 25 - I do 8 1.5 mile journeys a week dropping little one at nursery combined with longer trips to Norwich/Dereham for shopping and stuff - we're very rural, and the roads are narrow so not a lot of chance to get great mpg. On a 70mph dual carriage way run, she'll do 33-34 driving with gentle acceleration, but I don't do lots of that type of trip.

 

Anyhoo - ages ago I discovered my throttle position sensor *may* be faulty, so I bit the bullet and got one from VW (for eleventy billion quid). I never got round to fitting it as it was much thinner, needing shorter screws. I got off my arse today and swapped them over. I also checked for leaks/splits and nothing. I disconnected the negative battery lead for an hour or so, then reconnected.

 

I started her up and let her get warm - she seemed to be less hesitant blipping the throttle. I then went for a drive to Dereham - a 30 mile round trip on A and B roads. Before I set off, I reset the mpg.

 

Driving with gentle acceleration saw a journey there of 29 mpg. After a reset and a drive back more spiritedly saw 26mpg with a total trip average of 27.5mpg. The hunting is almost gone - just spluttering once on the way back. That would tie in with my long term average.

 

I'll keep an eye on it to see how it goes.

 

TL;DR - it was probably my throttle position sensor.

 

edit - I also found the fuel weep I'd been smelling for the last week or so - the fuel supply to the injector rail had a tiny, weeping split. For ****s and giggles I got a quote from VW - £55 + vat lol lol lol lol. I got a 1m length of hose from Walker Rubber and Plastics on Sweetbriar Industrail Estate for £4.50 including the vat.

 

---------- Post added at 6:55 PM ---------- Previous post was at 6:46 PM ----------

 

Thanks to Justin at VW for suggesting them (I can't re-edit my post :shrug:)

Edited May 30, 2013 by timmaaah