Back in August I had my STI 6 mapped by Andy Forrest. He commented that my lambda sensor had failed which would help explain the comparatively poor fuel economy compared to my previous UK classic. The car drove great until the VF28 died about 40 miles post mapping...a 4hr ride home in a recovery truck was rather frustrating having waited months to get the car all set up and ready for its mapping slot!

I have since had a VF35 fitted and also replaced the lambda sensor with the (couple of years old) one off my old UK car. While waiting to get a map tweak done for the new turbo I have put a couple of tanks of fuel through the car and yet the economy is still only 20-21mpg. I have not been caning the car at all given it's not set up correctly, just half throttle and no going above 4-5k rpm or so. I was assured by someone knowledgeable that this is perfectly safe to do given the VF28 and VF35 are not too dissimilar.

To my question: would you suspect I've got another dud lambda sensor or can the poor economy be explained by the fact the mapping hasn't been adjusted for the new turbo? I am perfectly willing to accept 20mpg if that's the norm for a c.340bhp classic with JDM gearing however I keep feeling there's no reason it shouldn't be a bit better given I used to get 24-25mpg from my (albeit 100bhp less!) UK car.

Thoughts appreciated!

 

Im no expert on these things but a mate of mine had his blob sti mapped with a dud lambda sensor and experienced the same sort of fuel economy. When he asked about it the mapper said something about putting the sensor in a loop (cant remember open or closed) so that mapping could take place. He's now put a new lambda in and is going back to get the map adjusted for fuelling so you might need to do the same.

 

Thanks for the reply. Andy F said last time that it didn't affect the mapping because the failed lambda only resulted in over-fueling on closed loop (ie before his map applies if that makes sense). My map tweak for the VF35 is on Thurs so really hoping that another lambda fitted following that wouldn't need another map adjustment...

 

20mpg is good for a 340bhp classic jdm.
You got 24mpg from a UK car running 240bhp,that's only 4 miles per gallon difference.
So I'm surprised you're shocked?

 

It's 20% more fuel though when most of the time I was barely exploiting the extra 100bhp or so.

I've seen people saying 20mpg is perfectly normal yet others saying there must be something wrong if not getting 23-25mpg.

 

The sti closer ratio is defiantly going to make a difference,are you running standard size wheels? The tacho can be out with different wheels,have you checked your speedo with a gps reading?
Even the weather conditions will play a small part in fuel economy,especially on a turbo car,different day,different baro.
I still say 20mpg is good for a tuned Scooby(just my opinion tho)
Don't suppose you have an AFR gauge do you?

 

I am on 17s with 215/40/R17 tyres but switching back to 16s I've decided.

Not sure how running 17s would have any impact on fuel economy though...even if they did make the speedo over-read?

No AFR gauge unfortunately.

I'd understand 20mpg with a heavy right foot but I have not been hammering the car at all

Will just have to wait till Andy F can tell me if the current Lambda sensor is functioning properly and go from there I suppose...fingers crossed the map tweak may even improve matters.

 

wrong turbo and map with duff sensor is never gonna be right. Untill its sorted and mapped correctly dont worry about it.

Also keep it off boost till its sorted.

 

If your speedo is out(which it is on 17's) then your tachometer is out so how can you gauge mpg correctly

 

Ah I'm with you now! The rolling circumference is lower with the 17s so if anything that would lead to an over-estimation of mpg. That said there should only be a 1-2% discrepancy.

 

Haha ok I'm grasping at straws here mate,is a pretty close rolling circumference tbh.
See what AndyF has to say,let us know how you get on

 

Update:

Lambda sensor was functioning fine however the car was running rather rich following the previous mapping session. Andy Forrest offered up a very good explanation:

When he mapped my car before here was possibly a small leak in the up-pipe which messed with the AFR and led to him adding extra fuel to the map. Not long after mapping, the up-pipe blew at one of the welds which possibly resulted in my VF28 ingesting a small amount of metal which knackered the blades.

With a new up-pipe and turbo in place the map was correspondingly rich owing to the lack of a leak and consequently Andy was able to take some fuel out when he mapped the car again. The result has been approx 330-340bhp whilst now achieving 24-25mpg...delighted!

 

Nice one

 

Mapped by Andy on my Blob STI

Vf35 355bhp - Worst 120- Best 180 per tank

Sc42 427bhp - Worst 110 - 160 per tank

Never done a full tank on motorway tho

 

lol you must hammer it a bit , sti ver 7 with a map on jdm close ratio box i'm between 22.5 and 25 combined , the fsti 27.5 combined to 35 on a long run , i do give it the odd boot or what's the point of having the car , the secret is to keep your foot very still on the throttle once you are up to speed , the smallest increase change and you burn a load of extra fuel , that's based on 70 mph , even at 80 mph the same idea works but not as efficient , on a funny note my 5.7 litre v8 on a long run with about 440 bhp at 70 mph returned 35 mpg which i was surprised by