Oxygen sensor output is mostly LEAN. That normal?

[cmichaelo]

I just installed an air/fuel meter.

I did this because I've destroyed two cats over the last few years, and so I wanted to check on the lean condition while driving. Here's what I noticed:

Coasting:

While coasting along at say 55mph, the reading will fluctuate between the high end of the STOICH (optimal) range and the low end of the LEAN range.

Typically (during coasting) it will stay in STOICH for a few seconds but will then quickly move down to LEAN and stay there for a second or so. In fact, I don't even get a reading.

Then it will move back up to STOICH, and then it cycles like that.

Decelleration:

During decelleration the reading is always LEAN. In fact so much that I don't even get a reading.

WOT:

On WOT it moves into the low end of the RICH range, and it's quite stable during WOT.


Does all this sound normal to you, or should I worry about the tendency towards LEAN condition?


Michael

 

[AutoSHO]

During cruising, it should switch regularly back and forth between rich/lean.

How old are your oxygen sensors?

 

[cmichaelo]

Oxygen sensors are brand new.

Just did the 60k.

It almost switches between RICH and LEAN, except on the A/F meter it's on the last LED in the STOICH range. The next LED would be in the first LED in the RICH range.

Also, there aren't really any LEDs lighting up in the LEAN range, since the voltage is too LEAN.

So it's more like it switches between "ALMOST RICH" and "VERY LEAN" rather than RICH and LEAN.

Do you think I'm still allright?

Michael

 

[shopartsnw]

If this is a stock O2 sensor, then it is a narrow band and is only accurate in a "narrow band" around stoich. It is not a linear output once you are out of that range, so lean and rich are relative terms and it can shift quite quickly.

I put in a PLX wide band unit which is linear across a much wider range, and is more accurate as a rule. www.plxdevices.com

If it makes you feel any better, the PLX unit always goes to lean when decellerating hard (I checked and this is normal as your fuel cuts off at decel). Under WOT, it will go to a richer setting, but this is where you might be actually having a problem affecting your cats. I installed a 255lph fuel pump a year ago and that overpowered the stock fuel regulator. The 190lph is even a little too much for the stock FPR. Did you happen to increase your fuel pump size in the past?

The solution: I bought an aftermarket FPR from SHOnutperformance.com and now I have rock solid differential fuel pressure at all RPMs. FOr more information read the info at SHOnutperformance.com

By the way, I burned out a set of cats on the car before I got the FPR from SHOnut. When you go really rich, the cats get loaded with partially burned fuel and then get VERY hot in the process of finishing the combustion.

Good luck. Let me know if there is anything I can do to help.

- Mike

 

[cmichaelo]

OK. Just read the documentation that came with the A/F meter.

Apparantly the reading I see at WOT and decelleration are quite normal.

And from looking at the graph, since the RICH voltage output drops quite a bit after the sensors heat up, getting a reading a the low end of RICH or high end of STOICH is probably also normal during coasting.

But I'm kind of worried about the fact that during coasting I occasionally don't even get a reading in the LEAN range. I don't think that's normal.

Could something be wrong with the PCM?

Michael

 

[AutoSHO]

I don't think you have anything to worry about. The injectors are turned off during many decelerations, and the oxygen sensor voltages drop to zero, which, in turn, is indicated on the gauge as nothing.

 

[cmichaelo]

Under WOT, it will go to a richer setting, but this is where you might be actually having a problem affecting your cats. I installed a 255lph fuel pump a year ago and that overpowered the stock fuel regulator. The 190lph is even a little too much for the stock FPR. Did you happen to increase your fuel pump size in the past?

Yes, I did upgrade my fuel pump to a 255lph unit about 6y ago.

But shouldn't the PCM or the FPR compensate for this automatically?

Or are you saying that the fuel pressue in the main line is overpowering the FPR, so that in fact the fuel pressure is always too high at the injectors? And then at WOT, where there is no regulation of the duty cycle of the injectors, the injectors will supply too much fuel due to the increased pressure?

So if the 190lph and 255lph are overpowering the stock FPR, wouldn't replacing the fuel pump with an 150lph do the trick? (I'm curious about this since the 255lph I have is starting to whine too much, and I'm gonna replace it soon anyway.)

Michael

 

[K-Dawg]

Or are you saying that the fuel pressue in the main line is overpowering the FPR, so that in fact the fuel pressure is always too high at the injectors? And then at WOT, where there is no regulation of the duty cycle of the injectors, the injectors will supply too much fuel due to the increased pressure?

So if the 190lph and 255lph are overpowering the stock FPR, wouldn't replacing the fuel pump with an 150lph do the trick? (I'm curious about this since the 255lph I have is starting to whine too much, and I'm gonna replace it soon anyway.)

Michael

That's right. The 155 is now generally recommended for most SHOs. I suppose even it is more than is necessary.

 

[shopartsnw]

You can search for the thread, but yes a 190 or 255 will overpower the stock FPR. The PCM tries to compensate for the extra fuel, but ends up running rich at idle and lean on the top end IIRC. It was bad enough that our car had a bad low end stumble due to the extra fuel. You could reset the computer and it would go away, but very quickly it would be learned back.

We stuck with the 255lph and added the FPR because we are going to add a S/C in the the next two months. If you plan on staying normally aspirated with no nitrous, the 155lph should be just fine. You might even see a performance improvement on the low end.

- Mike

Edit: Here is one of the threads to read:

http://www.shoforum.com/showthread.php?t=29488&highlight=FPR

 

[cmichaelo]

Does anyone know how much current the 255lph fuel pump draws?

I was thinking that an easier way to reduce the fuel pressure, aside from replacing the fuel pump with say a 155lph unit, would be to insert a bunch of diodes in series with the 12V supply to the fuel pump.

I believe the fuel pump speed is roughly proportional with the voltage supply. So if inserting ~7 diodes (for a total voltage drop of ~5V) that would reduce the voltage from say 13.5V to about 8.5V which would correspondingly reduce the pump speed from 255lph to 160lph.

Voila! Problem solved...if my assumptions are correct, and the diodes don't blow up due to overheating....

Michael

 

[shopartsnw]

Read the earlier link to a past thread and some of the threads it refers to. Other people have tried slowing down the pump. It works marginally on the 190lph, and there is a write up on how to do it. The 255 lph is just to big. If you get the voltage down lower, it tends to stall out.

- Mike