Quinton

Hi Chaps.

I have a my99 impreza turbo, with vf28 turbo, decatted exhaust, plus other supporting mods remapped to approx 320bhp.

All of the work has been carried out whilst i have owned the car, it was completely standard when i bought it.

Now my problem is a bit odd, i had the car mapped approx 12 months ago, and all was fine.

It recently started to judder a little before boost, however when on boost it is absolutely fine.

I have tested the maf, and coil pack they are fully working, plus most obvious things like split pipes etc, it had new plugs on remap.

I managed to randomly drop into pole position where my mapper happened to be on that day purely by chance so i asked him to have a look.

He did and he said it was the lambda sensor playing up as he couldnt get a steady reading from it, so he unplugged it and we went for a drive and it ran smooth, fuelling ok etc.

So he advised that i would be fine to drive with it unplugged for a while until I can get another one, as it would run on the fixed map, i.e my ecu learning wouldnt be active.

So i did that for a while and finally got around to getting a new one two days ago and what do u know.... its juddering again, its not kangarooing violently or anything it just feels like it is holding back and releasing etc, annoying as hell because i know it is there!

So does anybody have any ideas what it could be? a dodgy lambda again? i have tried a couple of second hand ones before i bought the new one to try and rule it out, with no luck, but being second hand i thought they may of been shot also.

It definitely runs a lot better unplugged, but i obviously want one active so that my fuelling will adjust as my maf wears etc.

The Lambda i bought from a garage near me is a "blueprint" one

part number ADS77005

Any help / ideas are greatly appreciated.

Cheers

Bob Rawle

A narrow band lambda sensor will only produce a steady reading when the car is on boost, off boost i.e. as you describe its behaviour is to fluctuate up and down as te ecu adds and subtracts injector duty to target stoich afr.

So what your mapper said that is beyond me ... its not a correct diagnosis based on what you have said.

Sounds like you may have a lean spot thats developed, how have you tested the maf? You won't be able to tell what condition its in, a mapper can do that by looking at the engine loads achieved under a variety of conditions, but you will not have any bench mark to measure wether its deteriorating or not, a maf with weakening out put will still "look" ok, but it will be running the car leaner due to its lack of out put causing the ecu to produce the wrong engine load value for the actual load in use.

cheers

bob

Quinton

Thanks for your reply bob,

The way i tested the maf was to swap it for another one which seemed to make no difference at all, so i presumed that wasnt the culprit.

would u know why it seems to run ok with the lambda unplugged then?

what steps would you take?


thanks again for your help

Bob Rawle

You said it was "better" but that implied it was not solved.

Swapping mafs isn't the way to test them, but as a substitution it can help determine a gross fault.

If its the oem ecu unplugging the lambda will turn off the closed loop and then depending on how its been mapped the car should drive still.

So if its more like surging gently that will be the ecu adding and subtracting injector duty as it targets stoich afr, there is not too much you can do, normally the long term fuel learning capability reduces this to a barely noticeable level, but its possible that the fuel map is too far out for that.

If the fuel map has not been set in open loop mode to be very close to stoich then the ecu will not have enough long term trim in it to pull it close enough and so the short term swings will be large which is what you feel.

You have std 440cc injectors fitted ?

Funny this is about the third thread about this sort of thing I have seen today.


cheers

bob

Quinton

hi bob,

sorry for the delay.

yes i have standard injectors fitted, what you have described makes sense to me.

So i will see if the ecu will sort it out over the next week or so, i guess that is all i can do short of letting a mapper have a look again?

are you in the west mids any time soon?

Cheers