Hi All,

Got a 52 plate WRX the other day. Well chuffed... but for one problem! Engine management light is currently on constantly and AFR guage is bouncing from rich to lean and back over and over again. It very rarely stays still. Car is going back to garage (not Scooby dealer) tomor but I just thought it would be handy to have an idea of what the problem might be.

Garage will prob just take it to Subaru round the corner....

Actual performance of the car doesn't seem to be down any but can smell unburnt fuel so guessing it's running rich. Hi-flow cat installed and there's an Ecutek badge in the engine bay so I'm presuming there's a remapped chip in there. Selling garage didn't know anything about it though.

Any ideas folks? If I can provide any more info about the problem then I'll try my best.

Cheers in advance.

Steve

 

O2 sensor possibly, or the fact that the downpipe has changed could be the cause of the check engine light.

You should expect the afr to fluctuate around the lambda 1.0 or AFR 14.7 mark during closed loop (mainly low revs / light throttle) but it should richen up and stabilise when you accelerate. The accuracy of what the gauge tells you depends on whether you have a narrowband gauge or a wideband gauge.

Usually a duff O2 sensor causes the car to run rich in closed loop and will give the smell of unburnt fuel. However, if the car has been remapped then you are best finding out by who as the mapping could also be the cause.

 

Not sure what AFR gauge you have but once you get used to it you know if it's working right. It can be a bit of a worry if you don't know what to expect! But basically, it SHOULD be bouncing from one end to the other under normal driving conditions, because that's the ECU keeping the mix right by sampling the oxygen concentration in the exhaust, feeding back to the injectors to allow just the right amount of fuel in to either richen or lean out the mix, then going back to square one again and sampling the O2. It does this very fast and hence you get the AFR display bouncing back and forth, which is correct. On a constant wide open throttle you should get a solid steady light showing where your AFR is at. If it bounces under this condition you've got a problem. If it shows a lean reading or even on the lean side of 'stoich' you may have a problem. If it looks like it's running a little richer than 'stoich' on full throttle that's pretty much OK - it's always safer to run too rich than too lean.

There's a lot of people who will swear blind that in-car 'narrowband' AFR meters are no good, or can't be trusted etc etc and the only solution is to have a wideband lambda trailing around all over the place, but the bottom line is that a 'narrowband' AFR gauge simply hooks up to the factory O2 sensor, which is perfectly good for the standard ECU and most aftermarket ECUs and controllers. All lambda sensors give out standard voltages to indicate the oxygen mix and AFR gauges simply display this voltage. It's not rocket science.

 

An NB sensor is only accurate around stoich and is used by the ECU for emmisions purposes, therefore of little use when tuning so I would advocate a wideband however you don't have to have it "trailing around all other the place" Look at Innovate's XD-16 gauge kit for a neat installation.

 

You will need to get a diagnostic check done to confirm the reason for the check engine light. Like I said it could be the exhaust swap but you won't know unless you get it checked. If it is the exhaust swap that has caused the light to come on then that can be fixed / worked around.