Why Is My Fuel Economy Getting Poorer

[chocolate_o_brian 0]

I've had the car for approx 6 months now, and recently I have noticed how poor the fuel economy is. Now I know these are 20 odd year old cars so won't be exactly 'VW Bluemotion' efficient, but it seems to be worsening. I used to be able to get around 250 miles to 35-40 litres, which for the mixture of town driving on 30/40/60 roads I thought was ok.

 

Last week I paid particular attention to work it out. I don't have a heavy right foot and same roads I drive every week. I did 208 miles, and it took just over 40 litres to fill up. Now I work this out at 23.6mpg , even my old UK Scoob managed 24-25 around town and that was a 2.0l turbo. I see some on here getting 300 odd miles, and I hazard a guess thats with a similar driving style to myself.

 

This week is another example. I have so far done 195 miles, but 120 of those have been motorway miles, at 60mph with very gentle throttle. I wanted to see the difference it made. Well I have under 1/4 tank left, so doubt I'll get another 40 miles if I'm lucky! I mean, come on....

 

The car is a 1.9 '88 model, no engine mods apart from a K&N induction kit / Magnex cat-back exhaust, standard 1.9 gearbox so 3,000 rpm @ 60mph. Something needs to get done, otherwise I'm getting rid of the damn thing.

 

When it had it's M.O.T. in July, the fueling was very lean, and I had the garage (very reputable) set up the air fuel mixture and gnerally get her runnign sweet. Admittedly I have replaced a couple of corroded breather hoses in the time since, and thus had to fiddle a small bit with the screw near the air flow meter to get a correct idle at approx 900rpm. Is there something I'm missing, as I assumed if the idle is correct and not too high/low the air fuel mixture is bob on. Or is it something else causing my poor economy? AFAIK there aren't any fuel leaks as I can't smell anything under the car or in the engine bay.

 

Sounds silly, but would it just be a case of taking it to my preferred garage and having them set up the car again?

 

Any advice or experience on similar grounds would really be appreciated, as it's really doing my swede in

 

 

Andy

Edited December 24, 2008 by chocolate_o_brian

[Flix 0]

Get the fueling set upagain Andy. As the two hoses have been replaced it could now be running rich.

Also the colder weather will effect economy, especially on short trps as it takes longer to warm up.

180 miles on a tank, I only get about 80 in mine.

[nick 3]

It's just the weather. Longer warm up times, along with lights, heater, demister, etc... all sucking up power. My Mi is doing about 230 to a tank which is way down on the 280-300 that it used to do in warmer times.

 

Nick

[welshpug 1,657]

you need to make sure you are calculating your MPG as accurately as possible, i.e fill up, zero the Odometer, drive, fill up and see how many litres used to Mileage recorded.

 

going off "I have 1/4 of a tank left" isn't accurate enough, I get about a 100 miles from a quarter.

 

then, check the coolant temperature sender's resistance, data shown in the Haynes manual for this, then check the AFM.

 

you adjusted the wrong screw to correct the idle speed, you should use the air bypass screw on the throttle body for that, the one on the AFM is for idle speed mixture only.

[richsmells 2]

Fresh oil usually improves mine a little.

[Baz 421]

Mine's getting around 300 miles to a tank atm.

 

Make sure all the sensors are in good nick, coolant temp sensor, TPS is shutting off fuel at idle, and the afm is decent, working well, can all add up to economy.

[chocolate_o_brian 0]

Thanks for the replies thus far guys...

 

Chris, I'll be in touch about the fueling etc.

 

Welshpug, read my original post again. I said I have been calculating the mpg correctly, i.e. brimming the tank, noting mileage on that tank then brimming again a week later etc. This is how I figured out my mpg. You mis read what I said about this weeks mileage.

 

Baz, will check the sensors when I get the fueling looked at. I'm hoping it may be a combination of the colder weather, replaced hoses and over fueling. The car has a manual switch for the fans so I assume the temp sensor is buggered. Been like it since I bought the car, and the mpg was better in the warmer months.

 

Richsmells (do you?), had a recent service which me and Flix did. Oil and filter etc. Had new spark plugs just after I bought it, and other odds/sods too.

 

The a.f.m. is something I will have to get checked too for piece of mind. It would be so much simpler if one thing was wrong, but as usual, it seems a collective of niggly things. Either way, I'm not settling for under 24mpg, as I may as well buy another Impreza which is both more comfortable and faster lol.

 

Thanks so far guys, it is appreciated.

[whizzer71 0]

My std 1.9 does around 270 - 280 miles to a brimmed tank,If I drive it REALLY hard itll go as low as 230 per tank.

Cant remember the last time I serviced it though

 

[Cameron 16]

Not going to acknowledge what Nick said then? Probably the most accurate reply so far.

[chocolate_o_brian 0]

Not going to acknowledge what Nick said then? Probably the most accurate reply so far.

 

No need for sarcasm, I missed Nicks post. Hope it is the weather, but doubt it. Car runs horrible, and is lumpy on take off. It's got to be a fueling issue as the car has run an awful lot better. When cold I can understand, but fully warmed up - somethings a miss.

[Baz 421]

Have to agree, that's still a little low even for the weather IMO.

[damien 22 1 Cars]

i get around 300 miles to a tank normal everyday driving but i service her every month (new oil/plugs/leads etc).

i would do a service, clean the AFM/throttle body/breather pipes, see if your tyres are inflated to the right amount (it should say on the drivers door what this is) get some injector cleaner (not that redX rubbish) and run a tank of ron 99 fuel

[Cameron 16]

How much do you drive it when its fully warmed up though? That's the killer this time of year, you finally get it warmed up and then you arrive at work / wherever you're going and turn it off. I had a 207 HDI as a courtesy car last week and it was getting less than 20mpg for the first 10 mins or so and that was taking it very easy!

[chocolate_o_brian 0]

i get around 300 miles to a tank normal everyday driving but i service her every month (new oil/plugs/leads etc).

i would do a service, clean the AFM/throttle body/breather pipes, see if your tyres are inflated to the right amount (it should say on the drivers door what this is) get some injector cleaner (not that redX rubbish) and run a tank of ron 99 fuel

 

Tell you what, I dream of 300 miles to a tank

 

The car has been serviced recently, and all the breather pipes have been checked recently too, as a couple were replaced due to cracking and general corrosion. Tyres are always run at 29psi and checked weekly. Also only ever run the car on Shell V-power, as I remember seeing you run the 1.9 GTi models on 97+ ron fuel.

 

Cameron, hoping what you say could be true regards the warming up. I may try an oil change more frequently than 6000 miles and see if it makes a difference. I currently have 10w 40 in there for the colder weather, and will swap for 15w 40 in warmer climes.

 

The car is driven on average for 10-12 miles on a morning dropping the Mrs off and the same again in the afternoon picking her up. Usually do about 180-200 miles a week basic commuting, and by then the tank usually needs filling, hense my concerns.

 

Think I may book the bloody thing in with my M.O.T. garage and get the fueling checked for leaks, contamination etc.

[CaptainK 11]

Cameron, hoping what you say could be true regards the warming up. I may try an oil change more frequently than 6000 miles and see if it makes a difference. I currently have 10w 40 in there for the colder weather, and will swap for 15w 40 in warmer climes.

Out of interest, why? a 10e40 and a 15w40 is the same viscosity at "operating temperatures" (the "40" bit) but only differ at "cold" viscosity with the 10w being slightly thinner at cold start than the 15w. So why change it in the summer time? If your engine ran super hot then maybe go for a 10w50 for better viscosity at high temperatures, but why up the "cold start" in summertime?

 

But back to the original problem, have you tried looking at the SAD (Supplementary Air Device - i.e. the cold start valve on the 205 GTi) at the back of the engine block. Maybe the SAD isn't closing properly when your car is up to "operating temperature". Thus its still providing too much air (being stuck in cold start mode) and thus the ECU is adding more fuel. That'd make it run a bit lumpy when up to operating temperatures.

[mattmk1 0]

Out of interest, why? a 10e40 and a 15w40 is the same viscosity at "operating temperatures" (the "40" bit) but only differ at "cold" viscosity with the 10w being slightly thinner at cold start than the 15w. So why change it in the summer time? If your engine ran super hot then maybe go for a 10w50 for better viscosity at high temperatures, but why up the "cold start" in summertime?

 

But back to the original problem, have you tried looking at the SAD (Supplementary Air Device - i.e. the cold start valve on the 205 GTi) at the back of the engine block. Maybe the SAD isn't closing properly when your car is up to "operating temperature". Thus its still providing too much air (being stuck in cold start mode) and thus the ECU is adding more fuel. That'd make it run a bit lumpy when up to operating temperatures.

 

 

If the SAD was sticking the car would be holding a noticeably high idle. Or not idling at all.

 

Nicks right though, my MI was doing 300 plus miles a tank through summer, now its down to around 250. Surprising how much demisters, heaters, lights etc load the engine.

 

The only way you'll know for sure is to get the car on a gas emissions snifffer. It'll show wether your mixtures out which'll indicate either poor set up or possibly an ECU temp sender fault.

[chocolate_o_brian 0]

If the SAD was sticking the car would be holding a noticeably high idle. Or not idling at all.

 

Nicks right though, my MI was doing 300 plus miles a tank through summer, now its down to around 250. Surprising how much demisters, heaters, lights etc load the engine.

 

The only way you'll know for sure is to get the car on a gas emissions snifffer. It'll show wether your mixtures out which'll indicate either poor set up or possibly an ECU temp sender fault.

 

Well I've got the car booked in Saturday at my local garage. He's gonna have a fiddle with it and only wants a few beer tokens - good like that. Usually he's far too busy on Saturdays due to weekend rally competitions, so luckily I'm in!!

 

Re. the oil change, the car in the summer ran fine on 15w 40. That was what the previous owner used, and I put 10w 40 in under advice off here for winter so... The s.a.d. is fine as it was swapped and gave the same results. The problem is the fueling, as cracked breather hoses have been replaced which would be sucking in air, being compensated by extra fuel. The previous owner seemed to have the mentality of "fixing" cracked hoses with tape rather than replacing them.

 

The air filter has been thoroughly cleaned today (K&N) and will be going back on tomorrow after drying out and being re-oiled. This and the fueling/timing getting sorted Sat, I hope will cure my issues. If not I would expect my mechanic to find something else to replace