Is it true that when an engines cold a dodgy lambda doesn't show symptoms due to the engine relying on a base map by ecu? I was only wondering as from a cold start my car seems ok yet when it gets to temp that's when it sort of pulses on light throttle. And I know the impreza's aren't renound for their mpg but mine seems to have gone out the window! Lol

Thanks

 

Posting more info on your car will get you a faster response.

I take it you have done a fault code check?

You'll get nothing from a 'cold' lambda either way, it has to be up to temp. When off boost the ECU runs 'closed loop' and fuels directly.
Although a fudged lambda is a prime cause of bad MPG and running your problem could have other causes like a failed Coolant Temp Sensor (CTS) for example.

 

Hi mate, yeah done fault check and nothing. It's a Uk 2000 turbo x reg. Standard apart from cat back exhaust and hks ssqv. It's does seem like it's running rich and does bang and pop a lot considering it's not decatted. The other thing is the sensor itself and the connector plug have a lot of oily residue on them due to an oil leak of some sort I'm guessing!

 

Not quite, but sortof. The reality is that the ECU always picks the fuelling from the base map, but under low load/rpm situations the lookup from the map can be modified up or down by the reading from the O2 sensor (as well as by the various cold start/cold run/etc modifiers) to keep the mixture as close as practically possible to stoich.

And yes, until the O2 sensor has warmed up and stabilised, no closed loop modification is perforned, so you will get whatever fuelling is in the base map plus the modifiers.

That might be caused by a flaky oxygen sensor, or it might be that there's some other problem that is being masked by the cold run enrichment (etc.) when the engine is colder. It might also concievably be that the coolant temperature sensor is duff.

Oxygen sensor is a likely culprit under these circumstances. One thing you can try is to conduct an ECU reset procedure (use the black + green plugs + drive) and then stop and unplug the O2 sensor. Drive around for a bit and see if your "sort of pulsing" noticeably improves.


The HKS (atmospheric venting) dump valve will be causing an element of that "popping and banging". Again there's an imperative to get rid of it and fit a standard one so you can work out what's causing which element of misbehaviour.

Need to work out where that's coming from. Your gearbox dipstick isn't missing, is it?

 

No mate it's there alright, had the driveshaft seals replaced not long back so should be all ok from that front

 

Are you saying the oil is from a previous leak that it has now been fixed, but you still have a lot of residue hanging about??

 

Some of the symptoms certainly sound like a duff Lambda Sensor.

We have the correct ones for you model on the shelf for next day delivery if you find you need one: http://www.superhids.co.uk/lambda_se...u_impreza.html

Tel: 01462 488244

 

you can test the lamba by checking it with a multimeter on the signal wire

 

sounds like the identical problem i have been through in the last few weeks.
you should check out my thread.
to sum it up though,
replaced lambda, solved fuel consumption problem,
took it to scooby clinic, after putting it on the dyno and driving it they felt the hesitation/pulsing, said it was common problem in the ecu, they all do it to some extent, it is the system going in and out of closed loop right at the changeover point.
i asked if there was anything i could replace to help solve it, they said no.
so i am going to live with it.