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Air/fuel ratio guage
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Old 13 Dec 2004, 09:02 AM   #1 (permalink)
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Air/fuel ratio guage

Morning all
As some of you may know I went to get a service a few days ago ( the proceeds of which have gone a long way towards next years Subaru WRC budget ) and also to fit some gauges.
As the garage did not have an oil temp gauge they gave me an air/fuel ratio dial. Very nice it is too - makes the car feel very festive with all these flashing lights but I'm not convinced this is the primary purpose.
Trouble is , I don;t really understand this thing. As we sat ,bathed in a warm crimson glow, the service guru told me that the reading should always be showing green or amber. But its red now I said - no problem he replied.
So - can anyone give me an idiots guide as to what this thing is connected to and what readings I should be seeing.
With the car running cold everything is very healthy - green lights everywhere. At boost, again all is in the green but, when just cruising down the road at a steady 100 km (which I did for 6 hours yesterday - watching the speed limit you see - so had a lot of time to ponder the flashig lights ) and so around 2,800 revs the readings are manic - redgreenambergreenredgreen etc......
Should I be worried or should I simply rip the dial out and throw it as far away as possible. The car is running fine - no change there but now I find I'm constantly concerned it may be running lean. Too much info can sometimes be a real pain.
Cheers to anyone who can give me their thoughts.
David
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Old 13 Dec 2004, 09:30 AM   #2 (permalink)
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Absolutley nothing to worry about David

When the car is cold then the ECU will run very rich hence the Green display all the time.....however once the car is up to temperature the AFR should show a constant flashing from red to green back to red back up to green (constantly) when using part throttle or as you say cruising. Whats happening here is the ECU is trying to find Stoich - the complete happy medium between the air and fuel mixture. My AFR shos stoich as being Amber. Basically unless you are using half or more throttle and boost the AFR should show the lights swinging back and forth.....as you say very festive The Stoich position is optimum air/fuel for maximum economy. Nothing wrong here at all - however when you use the throttle and Boost then you should see the AFR swing into the green and hold here until you back off the throttle - try it mate. The best way to do is out on the open road with car up to temp. drive along and use the throttle up to say 5-6psi - you should see a few green lights. Then floor it - you should see it light up even more greens !! as soon as you back off it should start the swinging back and forth lark.

As I say nowt to worry about - enjoy 8)
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Old 13 Dec 2004, 10:29 AM   #3 (permalink)
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Thanks for the reassurance Karl - you the man
I am getting exactly the readings you mentioned - boost 5-6 psi all green,back off and swings from rich thru stoich to lean then up again incessantly.
Feel better now so I may just keep it ( or cover up until the festive season next year and then break it out on an unsuspecting public again then!)
Thanks again
Dave
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Old 13 Dec 2004, 01:19 PM   #4 (permalink)
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No probs Dave

Definately worth keeping hold of though cos with our particular year of Scoob MY99/00 they have fragile MAF sensors (mass air flow). These sensors can occassionally fail and can start to lean out the mixture. Now without the AFR in place you could be zipping about impressed with how fast your Scoob seems to be (with the leaner mixture) and then BANG !! This is said rather tongue in cheek as you should suffer other probs with a failing MAF, but you get the idea.

An AFR allows you to monitor this situation a bit - so if you floor the throttle and you don't get a 'rich' reading then at least you know somethings wrong. All I will say mate is you do get used to the flashing after a while - I have to admit that I don't notice mine at all now I do however have a quick glance when I'm on boost just to make sure that everything is ok 8)
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