ben.harris

iTrader: (2)

I've got a 1997 UK Turbo 2000 with PPP fitted.

When I first start the car, it behaves nicely for the first 60 seconds (or so) and then I notice that under light acceleration/maintaining constant speed (say 30mph) there becomes a lumpy pulsing kind of feeling almost like continually tapping the accelerator slightly. If I accelerate harder, it pulls away fine, but resumes as soon as I start to maintain a constant speed at low-ish revs. Once the car has properly warmed up (say 10 minutes later) the pulsing is no longer noticable at any revs and the car behaves perfectly until I next come to drive it again.

Now - I'm thinking that when I'm seeing these symptoms, the ECU is likely to be running in closed loop mode, and using the O2/lambda sensor to adjust fuel trims. I suspect the corrections don't start straight away, which is explains the 60 seconds before I notice it. Also, after a while, I assume the O2/lambda sensor has become properly hot and is now working correctly (I've read they need a lot of heat to work properly).

Would it sound feasible that the sensor itself, or the heater element on the sensor is at fault and would exhibit these symptoms or would this throw a CEL code? I've checked and there are currently no codes stored. I'm getting pretty good fuel economy (for an Impreza) so this is partly why I'm questioning the O2 sensor. I think it's probably the original sensor, so 17 years old and 95,000 miles.

Does any of this sound likely, or should I be looking elsewhere? Any advice would be appreciated.

ben.harris

iTrader: (2)

OK - been doing a bit more reading and it seems that lambda sensors are only good for about 100,000 miles - so it's likely to be due for replacement anyway. Then I remembered that I've got a brand new 4-wire Bosch Zirconia lambda sensor that I bought for my MX5 and never got round to fitting.

So... next question - My Scoob currently has a 3-wire sensor. It seems that the only difference between a 3-wire and 4-wire sensor is just an extra earth connection (instead of using the exhaust as an earth). Presumably, if I find a convenient earth point and wire up the 4th wire on the 4-wire lambda sensor to this, it should work fine?

ben.harris

iTrader: (2)

OK - even though no-one replied, I just thought I'd update this thread in case anyone is searching for anything similar in the future, as I hate it when you find something in the search results and then there is no answer!

So - I replaced the lambda sensor at the weekend with the 4-wire Bosch sensor I already had (Bosch 13257 - I think it was intended for a Chrysler). I wired the extra wire to a nearby earth point (the original sensor only had 3 wires - 1 signal and 2 heater). The replacement sensor wasn't quite the same - it had less openings cut in the metalwork for the gasses to pass through, so I'm not sure if that means it will be less responsive, but figured it was worth trying still anyway.

Anyway, the new sensor, combined with an ECU reset, has resulted in a car which is so much smoother to drive. I really can't believe how much of a difference it has made. The slight lumpyness during gentle acceleration and steady holding of speed has now completely gone. If I'm honest, beforehand it wasn't really that bad, and a lot of people probably would have thought it normal for an older car. However, it just didn't feel quite right to me. I'd also noticed quite a bit more popping from the exhaust, but I'd initially put that down to a decat centre section I'd recently fitted. The exhaust pops seemed to be increasing more and more though over the last couple of weeks which had made me start considering other options.

So... If you have a similar combination of symptoms to these, I'd definitely recommend replacing the Lambda sensor, even if you're not getting a CEL on the dash.