Why has my MPG dropped???

GeneralWest · May 25, 2011

I've got a 2001 S3 with the AMK engine. About 2 weeks ago I removed the fuel injectors and sent them away to be cleaned and flow tested. I also changed the spark plugs at the same time and took the inlet manifold off to clean it up and spray it black.

I hoped that doing the injectors and plugs would improve my MPG which had been sitting at 24-25mpg on the DIS but since re-fitting everything I'm only getting 22! I've covered about 250 miles since and it's not improving, I if I give it some beans it drops even further. Is this normal and will it improve once I cover some more miles? Even when I've disconnected the battery in the past I've never had an average MPG that low after a couple of hundred miles.

The car is fine to drive and there's no apparent loss of power. No improvement in power either unfortunately but the report that came back with the injectors only showed a 2 to 4% improvement in flow rate per injector so I wasn't expecting much.

I've re-checked everything that I'd messed around with, I can't find any vacuum or fuel leaks and all the electrical connections look good. I've scanned the car for fault codes after every drive but there are none stored in the ECU. I did get an error code of 18010 to do with low voltage when I first started it but I understand that's normal when the battery has been disconnected for a while, I cleared it and it hasn't come back.

Should I just wait and see whether the MPG gets better over time or is there anything else I can check?

Any ideas would be appreciated.

Alex C · May 25, 2011

does it get up to temperature? 90Â°C?

S3Lol · May 25, 2011

when I cleaned my injectors, I went from 30.5 up to 34 mpg so doubt it is that.

s3dave · May 25, 2011

S1LFH said:
when I cleaned my injectors, I went from 30.5 up to 34 mpg so doubt it is that.

If you want anything doing right do it yourself....... :whistle2:

Dani_B19 · May 25, 2011

Are you driving the car any different?

jojo · May 26, 2011

Have you started to flick your aircon on during the hotter weather? That would reduce your mpg by about 10%.

Syvo · May 26, 2011

i would wip the injectors back out and have a look over them make sure there nothing blocking them.

fuel filter could be one may need changing.

check under the car when running make sure you not leaking any where had this one before caught my fuel lines on old car when the put it on the ramps and put small split in line.

clean the sensor for the dis i think iirc its under the rear seats on the drivers side

tom792 · May 26, 2011

tyre pressures? sounds stupid but even small changes can make big impacts on mpg

Welly · May 26, 2011

S1LFH said:
when I cleaned my injectors, I went from 30.5 up to 34 mpg so doubt it is that.

34 MPG? Holy hell, I wish I got that :-(

My DIS claims on the motorway it's getting ~45 MPG. But that is just an outright lie.

Dane · May 26, 2011

MPG's over rated. I've been getting better mpg than all of you this past few weeks but its BORING and I'd happily give it up!

OP - How olds your MAF?

Alex C · May 26, 2011

Welly said:
My DIS claims on the motorway it's getting ~45 MPG. But that is just an outright lie.

I'm testing my DIS ave trip MPG against the fuelly.com calculated trip mpg. Only done two tanks, so far the DIS mpg is over by 2.5% and 1.2%...

Wish i could average 34 mpg

Welly · May 26, 2011

Cookstein said:
I'm testing my DIS ave trip MPG against the fuelly.com calculated trip mpg. Only done two tanks, so far the DIS mpg is over by 2.5% and 1.2%...

Wish i could average 34 mpg

I've registered on it - but I am too scared to actually do it.... lol. T'would be too depressing.

GeneralWest · May 26, 2011

Wow, thanks for all the suggestions, I'll try and tackle then one by one:

Water temp: The car has always got up to 90 degrees nicely and stayed there, it still does so I don't think that's the problem.

Driving style: When I first put the car back together I did put my foot down a bit more than usual, it's the done thing when you've modded something. I'm sure painting the inlet manifold should have gained me 50bhp ;-). For the last week though I've been driving like a granny and the MPG is still super low.

Aircon: Ever since I've had the car
I've left the climate control to do it's own thing all year round. Maybe that's why my MPG was only 25ish before, I'll turn it off for the next few days and see if there's any improvement.

Fuel filter: Good idea, I'm getting this changed today, I've asked the mechanic to check all the fuel lines for leaks as well.

DIS sensor: I wasn't aware of this, I'll check it out. Anyone taken their's out before?

Tyre pressures: I had new 4 tyres fitted a month ago, they inflated them to 34psi. I checked them at the weekend and they were all the same. Is 34psi about right for Toyo Proxes on the standard Avus rims?

Cheers for the help guys.

IrishDave · May 26, 2011

Cmon register and see. It cant be that bad. My last car I was getting on average 7 miles/gallon. It was costing me an absolute fortune as it was my daily driver also.

GeneralWest · May 26, 2011

OP - How olds your MAF?

At least 1.5 years which is how long I've had the car. Maybe it is time for a new one. Is there any way of testing them?

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Broken Byzan · May 26, 2011

you need to log the MAF at some point and see what she pulls. My guess is a sub par MAF as stated.

Dane · May 26, 2011

I run 36 psi rear and 34 psi front on my Avus'

GeneralWest · May 26, 2011

I don't have VAGCOM yet, although I want to buy it soon. I've got a Liquid boost gauge and a handheld Sealey VAG scanner, can I test the MAF with the liquid?

Edited: Reason fat fingers, little iPhone.

s3dave · May 26, 2011

GeneralWest said:
I don't have VAGCOM yet, although I want to buy it soon. I've got a Liquid boost gauge and a handheld Sealey VAG scanner, can I test the MAF with the liquid?

Edited: Reason fat fingers, little iPhone.

Yes you can.... mass air flow on the liquid and get it as close to the redline as safe/possible it will hold the peak value post back the result.

GeneralWest · May 26, 2011

s3dave said:
Yes you can.... mass air flow on the liquid and get it as close to the redline as safe/possible it will hold the peak value post back the result.

Nice one. I'll go for a drive tonight and check that. I'll post the result up later. Just so I know, what kind of number should I expect if the MAF is good? My engine is pretty much standard, no remap (yet!).

Alex C · May 26, 2011

s3dave · May 26, 2011

yeah ^^ 165 - 180 G/S ish depending if you are 210 or 225 originally

GeneralWest · May 26, 2011

Cheers lads.

GeneralWest · May 26, 2011

I think I may have found the problem: there is a breather hose which runs from under the manifold and goes up to the jumble of hoses on the top of the engine. It wasn't sealing properly at the manifold end. I must have missed it when I rechecked everything the other day, doh. Would this cause crappy MPG but no noticeable loss in power?

I've had the fuel filter changed anyway and I'll test the airflow tonight just to make sure.

Alex C · May 26, 2011

maybe, im guessing it the hose from the manifold that goes to the N249 and then onto the DV. Would have been a boost leak, engine (MAF) would have thought there was more air than there actually was and subsequently overfuelled

GeneralWest · May 26, 2011

Cookstein said:
im guessing it the hose from the manifold that goes to the N249 and then onto the DV.

Yep, that sounds like the one. Hopefully it was the cause of my problem.

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GeneralWest · May 26, 2011

Right, thanks for the help everyone. It seems the loose hose was the cause of my problem. Just driven about 30 miles an managed an average of 29mpg, the best I've ever had!

I stilI want to test my MAF just to be sure but I didn't get a chance to log the airflow because of traffic. My favourite stretch of private road should be pretty much deserted on my way home tonight, so I'll do that then.

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MingeBlue · May 27, 2011

GeneralWest said:
I think I may have found the problem: there is a breather hose which runs from under the manifold and goes up to the jumble of hoses on the top of the engine. It wasn't sealing properly at the manifold end. I must have missed it when I rechecked everything the other day, doh. Would this cause crappy MPG but no noticeable loss in power?

I've had the fuel filter changed anyway and I'll test the airflow tonight just to make sure.

My MPG bombed due to the top breather pipe being split went down to 22mpg approx from about 27mpgs (around town), replaced it and its back up to 27mpg again.

GeneralWest · May 27, 2011

MingeBlue said:
My MPG bombed due to the top breather pipe being split went down to 22mpg approx from about 27mpgs (around town), replaced it and its back up to 27mpg again.

Thanks for the suggestion. I've already been through the nightmare of dodgy breather hoses, I replaced them all last month after they started breaking down and blocked my oil pickup with melted rubber. I'm glad I caught it in time, it came very close to writing my engine off.

I gave my car a good run last night and could only get a maximum airflow of 157 g/s even at the redline. Does this mean my MAF is on the way out?

Just out of interest, what does g/s actually mean, is it grams per second?

colicabcadam · May 27, 2011

jojo said:
Have you started to flick your aircon on during the hotter weather? That would reduce your mpg by about 10%.

10% are you sure, i very much doubt this!

sportstractor · May 27, 2011

Is say 10% is very accurate, aircon put a substantial load on the engine. Your g/s reading doesn't seem to bad, not worth changing IMO. 168g/s is ment to be exactly 210 bhp...

sportstractor · May 27, 2011

Ops just seen yours is 157, that's not great, did you test it in 4th? As that's when max load occurs

GeneralWest · May 27, 2011

sportstractor said:
Ops just seen yours is 157, that's not great, did you test it in 4th? As that's when max load occurs

Yes mate, I tried in 3rd and 4th. I'll give it another go tonight just to make sure, but to be honest I think I'll change it tomorrow morning anyway and see if it makes any difference to the reading.

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A3 T · May 30, 2011

What is everyone using to measure their mpg ?

I looked at the on-board computer and it says that I'm getting 11.2mpg.......lol

GeneralWest · May 30, 2011

A3 T said:
What is everyone using to measure their mpg ?

I looked at the on-board computer and it says that I'm getting 11.2mpg.......lol

I'm using the on-board computer too, although it's not 100% accurate it does give you a rough idea. Are you sure you sure that you are looking at the average comsumption rather than the instantaneous? The average has a symbol before the numbers like a circle with a diagonal line through it.

A3 T · May 30, 2011

GeneralWest said:
I'm using the on-board computer too, although it's not 100% accurate it does give you a rough idea. Are you sure you sure that you are looking at the average comsumption rather than the instantaneous? The average has a symbol before the numbers like a circle with a diagonal line through it.

This is the one I'm using mate (looks like its gone up a bit....lol ) :

Syvo · May 31, 2011

owch 11.5 think mine goes around that when booting the car and 4mpg when flat out normal driving around 25mpg

Alex C · May 31, 2011

yeah, got to be something wrong there....? surely. Have you got any lights blinking at you? Fault codes?

TDIQ · May 31, 2011

If you calculate your true MPG by brimming the tank and compare it against the DIS figure few times and take an average you can change the DIS correction value using VAG-COM so it gives a you the actual MPG you're getting.

A3 T · Jun 1, 2011

Cookstein said:
yeah, got to be something wrong there....? surely. Have you got any lights blinking at you? Fault codes?

What sort of lights.....lol

And nope no fault codes.

Aar0n said:
If you calculate your true MPG by brimming the tank and compare it against the DIS figure few times and take an average you can change the DIS correction value using VAG-COM so it gives a you the actual MPG you're getting.

So could it be that the on board computer is just out of sync ?

Why has my MPG dropped???

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