I have an MY99 and recently it failed the MOT on emissions.
Tester said it was burning too much fuel, but the car seemed to be running OK to me.
Turned out to be the Oxygen sensor that was the problem.
Now that I have had it replaced by the dealer, the car seems sluggish when its cold and there is a definate hesitation around 2000-3000 rpm.
Any idea why this could be happening?

 

Happened to me also. The cold hesitation is common on MY99/00 Impreza's. I have questioned before why a faulty O2 sensor gets rid of the hesitation but nobody gave me a reason.

 

Don't know if it's relevant but I heard the O2 sensor takes it's time to heat up to it's proper operating temperature. This can easily lead to the car dropping into closed-loop mode when cold and causing the hestitation.

I guess if it's fooked, the ECU would just chuck loads of fuel at the problem and smooth things out.

Of course, I maybe talking complete bo!!ocks

Stefan

 

I have heard of this occurring due to the over sensitivity of the Subaru knock sensor when the cars are cold, they here slight noises in the engine that are only there when cold and retard the timing

 

The car goes closed loop very quickly, that is why it feels sluggish when cold, it could do with staying open loop until its warmed up but in search of emmission friendliness they decided to start the closed loop up when the engine is still stone cold. So its really too lean, the knackered sensor would have been running the car rich which is why it felt better.

Same thing at at 3000 rpm if you are talking about off boost, closed loop is oscillating the fueling making it feel as though it hesitates .... sort of holing back and moving forward in quich succession maybe.

"They all do that sir" is the answer, even remapping won't stop the closed loop messing with the fueling. And an '801 doesn't solve it either.

 

Bob,

Any idea what controls how the car goes into Closed Loop when cold. If a remap or different UK ECU doesn't make any difference, is this because the fixed maps aren't touch by the Ecutek remaps or would it need a different ECU type (Motec, Links, etc.) or is it something completely different?

Stefan

 

The maps are not available in the EcuTek software for these cars, for the new age MY01 on the trip point for off is settable but you can't control when it starts, I assume there are bytes available to adjust, maybe EcuTek will make this available at some point. If you really want to get rid then the only solution is to go aftermarket but its a drastic step unless its really bad. I do now how sensitised people can become over it, one guy I know changed maf/lambda and I flashed it back to 800 code ... made no difference. (ecu wasn't tuned just flashed std.) It was driving him nuts.

 

Thanks Bob.

After 4yrs of ownership I've got used to it now Only happens in the 1st 10mins, so it's no biggie as far as I'm concerned; just interested that's all.

Stefan

 

Bob,

Spoke to the dealer earlier, who has recommended that I fill the car with a fresh tank of Superplus 97RON or Optimax and bring it back for an ECU reset and that should sort the problem.

I have always ran it on 95RON normal unleaded and it has been fine until I replaced the O2 sensor.

The hesitation would seem to be a common problem then by the sound of things, so will an ECU reset sort it ?