Weird AFR Readings

[lucittm]

I installed my "In Your Face Gauge" PLX AFR sensor last weekend to hopefully help me diagnose my lack of idle (I had to unscrew the air screw until it almost fell out to keep the engine running). So I was thinking I had a rich situation...

Nope. My AFRs are running about 14 unless I stand on the throttle. On boost, the AFRs drop to a respectable 11-12 and stay there for a few seconds after I am forced to let up because the telephone poles start to look like picket fence posts.

Anyway, while I am cruising my AFRs cycle from about 13-16 constantly. The needle looks like a metronome at the rate of a sweep every second. I am wondering if this is normal. It almost seems like it starts to run rich and the CIS "catches" it and it comes back down to around 16. It overshoots the 14.7 and then it goes back rich again, over and over. Maybe I can get a video of it from my blackberry.

Oh, and when I downshift and lift my foot the AFRs can shoot up to 18 momentarily with the throttle plate closed, I guess that is normal?

Opinions are welcome.

Thanks,
Mark

[dsiegel360]

Downshifts will be lean. 18 is fine. My EFI is programmed to shut off fuel on decel.

When I first got my "in your face" gauge, it was very difficult to figure out whether I was instinctually pulling my foot off the gas pedal while trying to hold steady to do a reading of acceleration and cruise. I was pulling my foot up every time I looked at the gauge.

It is easier to get readings of acceleration and deceleration because at cruise we're usually not holding the gas steady.

Does it hold steady when accelerating without boost from 2k rpm to 4500 rpm?

[mark houghton]

Possibly a partially clogged injector(s)? Would be noticable at steady throttle position with AFR's swinging momentarily lean. I was having similar symptoms with fluctuations at steady throttle but good steady boost AFR's....until I heard what may have been detonation. Just waiting for my injectors to return from being cleaned to see if it solves the problem.

[beepbeep]

Is the car equipped with O2 sensor? If yes, this is completely normal. Narrow-band O2 sensor has no means of detecting actual fuel ratio, it only flips from low to high when mixture passes 14.7 (stoichiometric). Thus, ECU will oscillate around this high-low threshold in order to keep AFR's near 14.7.

[lucittm]

Goran,
Yes, I have an O2 sensor. I was going to test it but how? Do you just disconnect the O2 sensor connector an look for oscillating voltage around .5v or is it .5mv?

Mark H. please let us know how your clean injectors turn out. Even though yours are slightly different than mine the principle is the same. Where did you send them?

It does hold steady on acceleration and today I noticed that the oscillations are closer together at higher RPMs.

Thanks,
Mark

[Speedy Squirrel]

I think the O2 has three wires, is that correct? If so, warm up the engine by driving for 20 minutes or so. Then separate the O2 sensor connector and check the voltage from terminal 1 to ground. It should be 0.1 to 0.95 volts for a good sensor. This is the procedure in the manual. Terminal 1 is on the raised part of the connector, all by itself.

Your A/F's sound completely normal to me. CIS has to "kick" the mixture rich with the frequency valve and then wait for the O2 sensor voltage to go up to signal it to stop. Then it waits for a bit and kicks it again. This is how the control works. It just keeps the O2 sensor voltage oscillating around about 0.61 volts. That voltage represents something near 14.7, A/F which is the desired result.