Engine a/f ratio gauge...interesting observations...

a/f ratio gauge...interesting observations...

Ever since I've had my '03 Corolla 5-spd, the one thing that has made me curious is how it behaves on the highway.

For instance:

I'm cruising anywhere between 55-80+mph, I let off of the gas, and the car coasts...just like in an automatic transmission. I push the clutch in and the rpms stay high (whatever rpm it happened to be at for the speed I was going) for a good 10 seconds or so, then very slowly drops to idle. It's like I'm still hitting the gas...but I'm NOT. It doesn't do it ALL the time, but a majority of the time it does on the highway.

Sometimes I let off the gas and it obviously slows down. I can push the clutch in and the engine immediately falls to idle....SOMETIMES.

So, today I installed an air/fuel ratio gauge. For the 3-way catalyst to work, the fuel system has to fluctuate rich-lean-rich-lean-rich-lean, etc. for the catalyst to do it's job properly. The LED's on the gauge indicate that is exactly what is going on. Anytime I had my foot on the gas, the LED's would rapidly fluctuate between lean-rich-lean-rich-lean-rich, etc. If I released the gas, the mixture immediately went lean....EXCEPT those times that it did the whole "coasting" thing. When I let off the gas and it was doing the "coasting" thing, rather than outright decelerating, the mixture continued to fluctuate rich-lean-rich-lean-rich-lean. It's like the idle air control valve was opening up to prevent the engine from slowing down.

Anyone have an explanation?? I know it's for emissions purposes...but what exactly is going on here? Anyone know for sure?

Late,
Trav

It's because your using a narrowband AFR, which is nothing more than eye candy. If you want a real reading of your AFR, drop $300 on a Plextor M-300 from UnderDogRacing.

listen to derrick when he speaks

I know it's a narrow-band. Works in a narrow range of around 12:1 to 16:1 and that's it. That's obvious when it goes lean and then NO lights are lit up.

However, that's not my concern....I can tell it's doing what it is suppose to by alternating the exhaust from stoich-lean-stoich-lean, etc. That is what the fuel system is suppose to do and the gauge, even though it is narrow band, still shows that to be the case.

What I want to know is what is happening when I'm driving at highway speeds and I let off the gas and it feels like I am still hitting the gas, even though I'm not. I know it does this to maintain the lean-rich alternation so the 3-way catalyst can do it's job. I.e., I KNOW it's for emissions that it does this on the highway because that is what the A/F ratio gauge is showing me. BUT, what I want to know is how it does this...

Does it open the idle air control valve, does it alter the VVT, what's going on??

Late,
Trav

Youre not getting reliable readings on the highway with that guage, to reiterate what Vamp said.

Are you saying when you press in the clutch your engine stays at RPM? Sounds more like a throttle problem. Has the dealership ever heard of this? Does anyone else have this problem?

Just so that I'm sure you understand this fully, a narrow-band a/f guage is fine for making "observations" as you are doing, but is 100% useless for any type of tuning. Trust me, I know, if I could have gotten away with a $30 narrow-band guage then I wouldn't have spent $450 on an LM-1 kit.

I disagree. The a/f ratio gauge, even though it's narrow-band is accurate as any other. It's just not precise. It tells you whether it's lean, rich or stoich, just not HOW lean or rich. That gauge is as accurate as the O2 sensor is.

The rpms staying high....as said, it's not a problem with the throttle. The throttle is fine. IF it were a PROBLEM, then it would do it more often than it does. This is something that ONLY happens at 55+mph and ONLY in 5th gear. How would that have anything to do with the problem? I'm 99% sure the ECU is programmed to do this....it's for emissions purposes (ULEV status). During decelerating, the fuel mixture goes lean and NO2 emissions go way up. This is a way to reduce NO2 emissions...just keeping the lean-rich-lean-rich alternation so the 3-way catalyst can do its job.

Late,
Trav

oh i never knew that

I 100% agree. I'm not tuning, see. The problem, of course, is the OE O2 sensor is narrow-band and you have no way to quantify what the LED's mean - i.e., who knows which LED corresponds to a 13:1 ratio, or a 15:1 or whatever.

I agree with you, though. It is fine for observing general trends and characteristics of the a/f ratio if you're not tuned.

Late,
Trav