Fuel consumption

[roger1234]

Faulty MAF will cause overfeulling too.

[axewielder]

your recent sound is either the usual mk3 exhaust blowing/rattly heat shield or could be the destructive knocking/detonation, hopefully not though especially if its overfuelling

If the manifold gasket was blowing, it might allow air into the exhaust stream. The lambda sensor will measure more oxygen in the exhaust gas and tell the ECU to pump more fuel in as a result - hence the high fuel consumption.

Just a theory.

good thinking batman its another possibility, although mine has quite a leak at the mani+ no obvious probs, but again that should show up in the figures when looked at in vagcom

[Larrychips]

On the topic of fuel consumption, I thought I'd add that the PowerFlow exhaust I recently had fitted made a HUGE impact on my fuel economy.  Before, I had a Supersprint manifold with a "sports" cat...and standard exhaust from there back.  I was getting 22-25mpg round town, and 32mpg at most on the motorway...was costing me a small fortune (60-80 quid a week!) as I live around 55miles from work.  After I had the rest of the exhaust system done (cat back), that jumped up to 38-42mpg on the motorway. 

As for your problem, I have to agree with what a few people have said already...the lambda sensor.  That is the main sensor the ECU uses to maintain the fuel/air mixture, if its blocked or nacked, it will cause your ECU make decisions based on false info...   They are pretty cheap, and very easy to replace...maybe worth replacing it.  But 1st sort out your manifold...a leak in your manifold will for sure make a big difference to the lambda sensors readings...

[axewielder]

i agree but if you can get it on a computer first to check those things it might save time + money changing things that arent necisarily broken

[Khare]

I can get 42mpg on my cammed gti if I take it real easy and steady. On average I get about 30-35mpg though...A tenner of v-power (119.9 down my way) is about 8l and it can get me around 50-60 miles or so. My dad owned mk3 gti's throughout his life and said if you drive them carefully they are rpetty decent on fuel, but as soon as you start overtaking and driving heavy they are extremly thirsty. I guess its just normal dude, depends how you drive.

[flexiblefish98]

What company sells V-power? Might give that fuel a try if it adds good fuel eonomy for only few pence more a litre! 
AP I have a VW ECU relay for sale if you need one!

[zdebx]

Where's this "lambda sensor" and how do you replace it? 

[roger1234]

What company sells V-power? Might give that fuel a try if it adds good fuel eonomy for only few pence more a litre! 
AP I have a VW ECU relay for sale if you need one!

Shell it's higher octane.

Where's this "lambda sensor" and how do you replace it? 

Usually on the cat with a wire on it.

[Khare]

What company sells V-power? Might give that fuel a try if it adds good fuel eonomy for only few pence more a litre! 
AP I have a VW ECU relay for sale if you need one!

V power costs 7p/l more than normal unleaded in my area, sometimes it can cost less if the fuel prices are lower.

Engine runs smoother and better with it + the cleaning addictives in it de-coke the internals and help protect the seals and gaskets.

Where's this "lambda sensor" and how do you replace it? 

Located in the cat. It screws into it and is connected by a wire. Usually they are a bastard to screw out.

[Larrychips]

Usually on the cat with a wire on it.

Yep its between the cat and the down pipe from the manifold, sticking out of the exhaust at a 45 degree angle with a wire connected to it.  All you need is firstly to get under the car (safely!) and a 17 spanner...don't for Gods sake use a shifting spanner!  Remember exhausts go through hideous temperature changes so the metal is fairly brittle...if you mess up the thread etc...well...lets just say it will start to get expensive.  Unplug the wire, unscrew the sensor from the pipe and swap it for a new one.  If you have an easy way to get under your car, it might be worth just taking it out and giving it a very good clean, replace it and test your MPG for a few days.  There are some posts on this forum from members who have had very positive results from replacing it.  If I recall they are only around £30...hardly breaks the bank. 

But going back to your manifold leak...do you know where it is and how bad it is?  Possibly just needs a new seal (under £10 from a VW dealer), or is it cracked?