Will also post in tech forum, but thought you all might could help me...

I just installed a new PLX SM-AFR unit with a SM-AFR gauge, and get weird readings!

The sensor is intalled about 24” away from the engine per PLX recommendations. This is a 90 RX-7 with long headers, no cats. The sensor is in the header, so it’s reading only one rotor, front I think. I have true dual exhaust, no opportunity to read both rotors together.

At startup, idle, cruising: the afr readings rapidly oscillate 19-20-Lean or Air.

Full throttle at low revs reads around 17

Full throttle higher revs it goes to 13-14

No way is this right, I know for a fact the car runs quite rich esp at idle and low revs, can smell the gas. Pugs are fresh and clean, motor runs great, not like one rotor is getting no gas or something.

Any ideas what’s going on? Any feedback would be appreciated. Thanks!

 

do you have any kind of fuel controller?

 

19 to 20 at idle with out any kind of fuel controller is normal.
if you do not have a piggy back or something, 17 AFR at full throttle and low rpms is normal ( N/A ) because your putting the engine under a heavy load. feeding a bunch of air at a low rpm. then, when the engine gets going and builds up your rpms the fuel catches up (secondaries) giving you the air fuel ratio you have. if your car is N/A this is ok (13-14). but if your car is the turbo model, then you will have to get some kinda fuel controller and bigger injectors. 17 AFR for a turbo model is WAY TOO LEAN!

 

I have always heard that the NA runs pig rich. So why are the AF ratio so lean?

Is it because of the air pump?

 

Not sure. All the RX-7 we work on are turbo models and never have any kind of emissions. Besides I dont see how an air pump will pump more gas into the engine to make it richer. I believe the air pump is for the 6 port engine. I guess it acts like a VTEC if you will. When port air solenoid valve opens it sends more air to the port i guess, some one enlighten me on this one.


Unless somehow your ACV valve is bad...not so sure. I know my car at idle is at 21 AFR. And its been that way since the first time i used the wideband. Now even with a neo and 720cc injectors its still there. I believe his car is normal right now. Acroy do you even have emissions??

 

so what can the lowest (or I guess highest AFR) readings be? Can you have 25:1 afr for idle?

 

You misunderstand.

The air pump, pumps more air into the exahust. thus it could make the reading leaner then it really is. It doesn't go into the intake that i know of.

 

I dont wanna give any wrong information here...but whatever AFR the stock N370 ECU throws at me is what im going to get. Even at cruising speeds.
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With my NEO I can adjust low throttle settings. From -50 correction to +50 correction and it tells me 14.9 AFR. Whether I go - 50 or +50 it always tells me 14.9 AFR.
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So i think that at idle if you have a stock computer, your gonna have whatever AFR the computer gives you.
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When you floor it, according to the signal that the air flow meter sends to the ecu, the Neo will change it before it gets to the ecu.
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Now Acroy not having any of this, he really cant do much about it. I do not think his wideband is wrong. I think his car is basicly stock and its gonna get what the ecu gives him.

 

Thanks for the info all

To clarify: No cats, no air pumped into the exhaust. RB true dual exhaust from the motor back. Stock ECU but am installing a SAFC shortly.

2-N-D-pink, yes this is a 6-port n/a car, the 5th and 6th ports are opened by the stock air pump at about 3800rpm. I kept the air pump but it's used only to open the 5/6 ports.

My plan with the WBo2 sensor was to use it and the SAFC to lean the car out 1) for power and 2) to try to pass sniff test emissions.

the car failed the sniff test already, running too rich. The hope was to develop 2 "maps" for the car: one for daily use and one to get the sticker

The limit on unburned HC is around 100ppm as I recall, and the car is running around 10 times that. Sounds like a lot, but that's really not all that much unburned gas. I am *hoping*, with a combination of CDI spark and leaning with the SAFC, to get below the 100ppm mark without cats or air pump.

No about the crazy readings at cruise: c'mon, this thing should read something at 3000rpm cruise besides "lean"! in closed loop the stock ecu is supposed to be aiming for 14.7:1 I thought!!??

 

You are right on that part. Im running 14.9 all the time at cruise. 19-20 AFR at cruise is very lean. The only thing I can tell you is get a bigger fuel pump (walboro 255) or you can find some TII injectors (as long as your harness is for HI impedance injectors) I would say swap the N/A for the TII. The TII are 550cc to your N/A 460cc (I believe).
More gas out of your injectors will give you richer readings.

What year is your car?

 

Do you have an exhaust leak? Even the smallest exhaust leak can cause the signal to get messed up.

 

that's what I'm saying, there's no way the gauge is right! I don't think the engine would even run at 19afr! if it did, it would be a bucking stumbling nightmare. run hot, etc. It does not: in runs rich enough during cruise you can smell gas from the car behind it.

i really think something is wacky with the gauge....

fyi, for fuel i used to have walbro 255, way too much fuel pressure, caused starting issues & smelled like gas all the time. switched to T2 pump, but honestly, from all i've read, the standard n/a pump is more than enough for any n/a setup unless you get into wild bridge or p-porting.

 

Nope, super-solid tight exhaust....

 

so what exactly are you trying to do? are you trying to richen up the car at idle or cruise?

or do you want it leaner for the sniff test?

19-20 is really lean, and your still not passing? I dont know what else to tell you. maybe you should put a stock exhaust for the sniff test, then switch it back once you have your sticker.

 

I'm trying to:
a) find out where the car is at, there's no way the numbers from the gauge are right!
b) tune for performance (likely remove fuel, not add)
c) make a super-lean tune to pass sniff test.

Given that we know n/a cars generally run rich stock, and even richer with free-flowing exhaust, i very much doubt the 19-20 numbers the gauge is unit is giving me. I'm wondering if anyone has any ideas what might be wrong with the gauge or sensor or....

 

and you have the PLX SM-AFR with the O2 sensor. (sold seperate)

 

My mechanic just intalled a PLX wideband SM-AFR with the SM-100 gauge. It reads spot on. He verified it with his own WB.

Do you have another WB available to you?

 

Check all your ground wires. Does the PLX require calibration?

 

Somethings not right. I'd check you wires/leads/ground.
PLX does not require calibration.

"At startup, idle, cruising: the afr readings rapidly oscillate 19-20-Lean or Air. "

Idle should be around 13-14ish. What does the gauge read when you have the PLX powered, but the car not running? (should goto 20) There is a capacitor to be installed w/ the device to make the readings smoother... Could just be a bad sensor...
Check the wires and free air test. Good Luck.

 

gar. Have to check the wiring again...

Yep this is the 2 seperate units you have to buy seperatly: controller (incudes sensor) and gauge.

Thanks Nick and montego for the info. good to know something's actually wrong and i'm not nuts.

i have an email in to plx as well, and will try their tech support line.

 

An RX with WORKING, FUNCTIONAL, AIRPUMP AND ACV will read lean at idle when fully warmed up. I'm talking 15afr-16afr. The REAL afr is actually something like 13-14afr. Real means if you removed or disabled the airpump or ACV, then the 15-16aft will drop down to the approx 13afr. The reason is that air from the airpump is being mixed with the exhaust gas prior to the wideband/narrow band sensors. Pulling the plug on the relief solenoid triggers the ACV to dump the airpump air into the silencer in the right front fender.

On a fully functional NON TURBO RX with airpump etc, all you have to do to see this, is to pull the plug off the RELIEF solenoid with a fully hot engine. The afr will drop from the 15-16afr to the 13afr.

A RX with a 19-20afr and being driven, will buck and fall on its ***. That's toooo lean a mixture. I'm talking a RX with no airpump causing the 19-20 afr. I'm talking REAL DEAL afr coming out the exhaust with no airpump air being mixed with it.

On a RX non turbo with no airpump, ACV etc, the afr should read around the 13afr to 14afr range.

When looking at afr when driving, you have to consider if the 02 sensor is functional or not. At steady crusing with a functional 02 sensor, the afr should be flippiing on either side of the 14AFR. The ECU working with the 02 sensor tries to maintain that 14.7 afr. If the 02 isn't functional, then the mixture will be most likely richer than 14.7 afr.

If you have a complete car i.e airpump/acv and want to know what the afr are, then pull the plug off the Relief solenoid and go drive. That way no air will be mixed with the exhaust gas to confuse you.

Just in case..........the ACV mixes air from the airpump with the exhaust gas as it leaves the engine. The exhaust ports are, the last time I looked. prior to the wideband sensor or the narrow band sensor.

And if you are *tuning* with a SAFC or equiv, then pull the 02 sensors plug so it won't confuse you when crusing along at a steady speed. 02 are not in the loop when WOT, though.

 

great stuff, thanks Hailers, you confirm what I remembered & researched previously about "real deal" afr's on more-or-less stock RX's.

I spoke to PLX tech support and they said it may well be the controller has an issue. It reads "air" like it should in air but all other readings at idle, cruise, etc are wacky.

oddly enough, once the rpm's pass 3800 (secondaries kick in) the sensor reads much more reasonable 12-13.5 range. very strange given that I know the primaries are working fine and the car is in fact running rich.

the sensor is reading the front rotor. just for giggles i unplugged the rear rotor spark plugs and fired it up. The car actually started and ran on just the front rotor, confirming that the primiary injector on that rotor is working fine. if the injector was plugged and not working properly, giving me 19-20afr exhaust mix, I would assume there is no way it would ever start & run on just that rotor.

Found I do have a small exhaust leak on the new bung welded in for the wbo2 sensor. DOH! I will get that fixed and try again.

 

Has anyone ever heard of one rotor leaner than the other?

I believe one rotor can actually be leaner than the other.
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This is going to give you wrong readings. Leaky exhaust right before the sensor

 

Talked to PLX again, they said a small exhaust leak on the bung should not affect readings. Nick, the tech support dude, said a "lack of backpressure" can cause lean readings - he has a Mazdaspeed Protege that reads lean at idle for that reason.

They have a relationship with Gotham Racing and Nick said he'd call them & ask for guidance on if they've ever seen this kind of issue before.

I'm in a small crowd of people investing in a WB for an n/a car... Most widebands are out there for Turbo tuning. Nick though a combination of exhaust leak, low backpressure, a 2nd injector coming on, and the 5/6 ports opening may contribute to what is going on... but maybe just the controller is bad.

The wiring is very solid.

And yes, the rotors can run at different afr's from each other. My long-term plan was to actually install bungs on both rotor exhaust & check they are the same

 

What you are seeing is normal if you still have the airpump in place. The air pump will inject fresh air to your exhaust to help with emission. The air pump operates from idle to 3krpm. From 3k rpm till redline, you should see normal a/f ratio.

hope that helps