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Model specific boards => Golf mk3 => Topic started by: APinto on 19 January 2010, 00:45

Title: Fuel consumption
Post by: APinto on 19 January 2010, 00:45
Hi there!

I'm the proud owner of a Mk3 GTI 2.0. I use it to commute, driving around 50 miles per day. So, I know pretty well it's fuel consumption habits, which for an economy style of driving sit between 38 and 40 MPG.

The problem is that a few weeks ago it suddenly started to consume more. I usually can reach 400 miles with a full tank, but that time I barely made it over 300 miles. And it's like that since then... Currently, it's in the red zone with only 280 miles...  :undecided:

I took it to a friend mechanic that plugged-in into the computer to check if there was something wrong. But the returned code indicated no errors... He attributed the situation to the automatic-choke, saying that it can cause the car to consume more when the weather is cold. I accepted it, reluctantly, knowing that a 1/4 increase in fuel consumption is way too much...

The weather is warmer outside now but the car is still drinking like mad. Can any of you guys help me? Did any of you ever experienced a similar situation?

Thank you very much for your help, cheers,

AP
Title: Re: Fuel consumption
Post by: Andy-H on 19 January 2010, 08:05
I have just recently had my car through the MOT.

Before it went, we did an emmissions check and it showed it to be overfuelling  :angry:.

A quick check found that the lambda sensor in the exhaust was faulty, sending the wrong information to the ECU.

New lambda in and some tweeking with the fueling and, although I don't completely trust the MFA, it now reads between 8-10MPG more than it ever has. Hopefully  time will tell if it is actually improved.

Other than that, when was the last service, including air filter, plugs etc. Throttle body could be full of crud  :undecided:

Andy.
Title: Re: Fuel consumption
Post by: zdebx on 19 January 2010, 08:07
Not trying to hi-jack the thread, but I've got a similar problem. Mine's not a GTI, it's a 1.8, but I think it "drinks" petrol like GTI, lol

I put £20 of petrol, which is roughly 18 litres, and I drive 50 miles (regular city driving)...Is that normal? Lasts me for about a week, so every week £20 (20x4) - that's a fooooking £80 a month just on petrol. Maybe I'm mistaken and that's ok?
Title: Re: Fuel consumption
Post by: javalin on 19 January 2010, 08:37
worth checking the coolant temperarture sensor - if its not reporting the engine warmed up, the ECU will overfuel

James
Title: Re: Fuel consumption
Post by: oggy on 19 January 2010, 08:58
Yup - Fair comment from your garage man - but only in the time taken it takes for the engine to warm through obviously. Seems like current consumption is excessive.

Agreed that with care I too obtain @ 40mpg - which to be fair is astonishing for a 13 year old 2 litre car

Agree with all other recommendations plus change out the ECU relay itself - they are known to decay a little over time before they finally pop - then the engine fails to start. For £15 or so worth the effort and it's a 5 minute job - ensure you obtain the genuine VW part.  Shove the old one in the glovebox as an emergency spare.

Incidentally Lambda sensor can be a swine to change as it sometimes welds itself to the manifold - do take care here and soak with ease-it fluid or similar first.
Title: Re: Fuel consumption
Post by: APinto on 19 January 2010, 09:12
Thank you very much! I really appreciate your contributions, I'll pass them to the friend mechanic.

One thing that I forgot to mention: yesterday I drove for half a hour, then stopped the car and tried to start it a minute later. It didn't start, I could hear the starter motor and a weird "mushy" sound (sorry, that's the best word I can find). Tried a second time, this time after pressing the immobilizer button in the key chain and nothing again.

I though to myself: this isn't bad, at least now I know that there's definitely a problem! Tried a third time after a few seconds and the car started.

I had to stop the car a few minutes later and it started normally. This morning it didn't give me any problem, either.

Do you guys thing that this might be related with the fuel consumption problem?

Thank you very much again, cheers,

AP
Title: Re: Fuel consumption
Post by: oggy on 19 January 2010, 09:31
Ahem -- see last post ref ECU relay! If not, get garage to test Hall sender - in distrubutor, and change that for starters if it is a dud...
Title: Re: Fuel consumption
Post by: APinto on 19 January 2010, 14:18
Hello!

Based on your contributions and those of a friend of mine, I'll change the following parts:

Coolant temperature sensor
ECU relay
Lamba sensor

Hope this helps, lately when I step on the accelerator it sounds like pound coins hitting the asphalt! :-)

Thank you,

AP
Title: Re: Fuel consumption
Post by: axewielder on 19 January 2010, 14:26
i know ecu realy wont show on diagnostics but wouldnt lambda+ temp sensors be pretty obvious, even if they didnt show as fault codes it should be clear if they are giving dodgy results from reading the relevent figures
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
Title: Re: Fuel consumption
Post by: mac7 on 19 January 2010, 14:46
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.
Title: Re: Fuel consumption
Post by: roger1234 on 19 January 2010, 14:50
Faulty MAF will cause overfeulling too.
Title: Re: Fuel consumption
Post by: axewielder on 19 January 2010, 15:23
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
Title: Re: Fuel consumption
Post by: Larrychips on 19 January 2010, 17:32
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...

Title: Re: Fuel consumption
Post by: axewielder on 19 January 2010, 18:01
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
Title: Re: Fuel consumption
Post by: Khare on 19 January 2010, 18:12
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.
Title: Re: Fuel consumption
Post by: flexiblefish98 on 19 January 2010, 19:51
What company sells V-power? Might give that fuel a try if it adds good fuel eonomy for only few pence more a litre!  :smiley:
AP I have a VW ECU relay for sale if you need one!
Title: Re: Fuel consumption
Post by: zdebx on 19 January 2010, 20:25
Where's this "lambda sensor" and how do you replace it?  :shocked:
Title: Re: Fuel consumption
Post by: roger1234 on 19 January 2010, 20:36
What company sells V-power? Might give that fuel a try if it adds good fuel eonomy for only few pence more a litre!  :smiley:
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?  :shocked:

Usually on the cat with a wire on it.
Title: Re: Fuel consumption
Post by: Khare on 19 January 2010, 21:20
What company sells V-power? Might give that fuel a try if it adds good fuel eonomy for only few pence more a litre!  :smiley:
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?  :shocked:

Located in the cat. It screws into it and is connected by a wire. Usually they are a bastard to screw out.
Title: Re: Fuel consumption
Post by: Larrychips on 19 January 2010, 21:37
Quote
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?
Title: Re: Fuel consumption
Post by: leigh_harty on 19 January 2010, 21:48
take it to a garage.. heat it up and the lambda shudnt be too much of a pain to release
Title: Re: Fuel consumption
Post by: zdebx on 19 January 2010, 21:50
Quote
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?


Thanks for the VERY detailed explanation :) Yeah, it is about £30, so worth a try. Cheers