Given I've heard (rumours that they are but prob true) subarus have had loads of problems often caused by maf sensors. If it's not working correctly it could be forcing the car to overfuel.

 

You might just check the air filter in case its all bunged up with crap.

 

I agree with tyres, cant see the warm up effecting things that much.

I usually let my car warm up for 30secs before setting off.

Upon cold startup they do run rich for a short time, the lambda is heated up to get into closed loop more quickly for fuel economy (which i believe to get upto temp is about 10-15 secs).

Which wouldnt take to long, so i cant see why it would take 45mins??

I can understand with a diesel, my grande punto diesel takes an age but not on petrol turbo car.

 

Yeah some of the things that effect the S could effect the subaru.

 

Lambda sensor's knackered.

Common on Imprezas of a certain age and causes over-fuelling.

 

Could it be anything to do with the hot weather?

IIRC the ECU in my car adjusts the boost of the turbo's to compensate for altitude and for hot weather.

Or maybe you are just driving a little quicker to adjust for the hot air.

Air con will also increase fuel usage by 10-20%.

 

Interesting guys thank you - it's back to the dealer soon for new wheels (warranty job) and new brakes, so I might ask them to check the map/lambda sensor - is that the sort of thing that would show up as an error, or are there any other symptoms to confirm that's the issue?

I think the a/c and tyres might contribute, but not to this extent.....

Moff I wish I was driving quicker - around here now the brigade do 28-38mph and no quicker, and there are so many of them it's nigh-on impossible to overtake. I did wonder if that was contributing too because I'm in a lower gear these days than when I'm at a higher speed.

 

Jo i've not read any of the replies but Imprezas are notoriously bad on lambda sensors. I replaced one on every scoob I had.

40 miles per tank is almost exactly what a duff lambda will give you.

Might have also noticed a bit of low down hesitation or the odd idle issue. The ECU masks it more on the later cars though.

Subaru should be able to put the car on the Select Monitor and look at the lambda voltage. It will be stagnant if its fooked. Unfortunately, most of them dont know how to use thi kit though

Moff - you get natually lower boost pressure in hot weather, because the air is less dense to begin with so the compression effect is lowered. I don't think the ECU does anything other than compensate fuelling and timing accordingly - on most cars anyway.

PS - the Impreza has a MAF not a MAP sensor. They break a lot too, but your MPG will actually improve if it breaks, as it causes the car to run lean. Will also feel faster. It was the cause of a LOT of engine failures on the older cars. It will also cut out a lot. Doubt it's that.

 

Thanks MB - will get the dealer to plug in when I take it in for wheels/brakes - def. something is up. It does do the odd shudder on idle but it's always done that, I think it's just a boxer characteristic. Any hesitation I probably just put down to the hotter weather and less dense air, but tbh I haven't really noticed much while sitting behind the blue rinse brigade!

Have done my back in so haven't been out in it much for a couple of weeks Certainly not enough to be driving hard!

 

Lambda isn't used when you are on boost, just for idle and cruising around, which is most of normal driving, hence you are getting bad MPG.