All Small Six Diagnosing with air/fuel meter

[DON]

I need to get it straight in my head with my a/f meter. I can find it, but there was dialogue a while back about how and what a meter reads. And it was not what I was thinking.
My thinking:
Meter reading of 14 means 14 parts air to one part fuel.
Reading of 10 means 10 parts to 1 and richer.
Reading of 16 means 16 parts to 1 and leaner.
I understand it changes due to throttle position.
Now throw in a bad engine miss under acceleration:
If the meter shows leaner ( substantially higher number) when miss happens, does this show lack of fuel?
What will it show it it is a spark misfire? I would think rich(lower number). But I think this is where I was wrong. Or am I right?

 

[Bronctopia]

AFR is a bad term.
What you actually have is an Oxygen meter.
And as you have realized, AFR is only inferred from it and assumes complete combustion.
Your "AFR" meter has no way of measuring fuel, so a missed combustion event will read as "More Oxygen", and be interpreted as "lean" even with fuel in the exhaust stream.

One way OEM setups can check for this is with TWO oxygen sensors.

Works like this: upstream O2 sensor measures excess oxygen from missed combustion. Has no way of sensing fuel rich exhaust. Signals "lean".

Catalytic converter gonna catalyze. So some of this excess O2 is consumed oxidizing the unburned fuel.

Downstream O2 sensor now shows LESS O2 than the upstream sensor is reporting, suddenly less "lean".

Assuming O2 sensors are both healthy, throw a flag before that fuel and O2 rich exhaust melts down the Cat!

 

[Bronctopia]

Now... Let's try to miniaturize that OEM setup for people who don't want to run a catalytic converter (ahem..racers).

What if we take the two O2 meters, make them really smoll, and wedge a tiny little catalytic converter in between the two!

Now, we can use the difference in the 2 Oxygen meters to guess at some level of "fuel" getting oxidized between them!

Congratulations, you have just invented the "wideband" O2 meter. But you are gonna need a WHOLE lot of "calibration" to make up for(among other things) the fact that 14.7 is an historical number that hasn't really represented the chemistry in road going Otto cycle engines for a third of a century now.

 

[DON]

Ok, I get post#2. Senses o2 only. So I can’t tell from the o2 reading if it is a miss due to spark misfire or a fuel problem ( lean or rich). I understand cat systems as well. 2 sensors, one on each side of the cat. Calculating the o2 before the cat and after the cat, using a figure of how much o2 the cat burns giving a figure of rich or lean.
Post#3…. I think it relates more to me, but I understand it less. My meter does use wideband sensors, not an oem system. So do you mean wideband sensors do calculate rich/ lean? Within the display box? I would think the box would have the calculations programmed in. ?
This is my meter:

Dyno Air/Fuel Meter (Dual Sensor)

Dyno Air/Fuel Meter (Dual Sensor)

www.fuelairspark.com

 

[wsa111]

Depends on your fuel.
Ethanol WOT is 12.2-12.5 Non ethanol is 12.8-13.1
Cruise with ethanol 14.4-14.7, non ethanol 14.5-14.9.

 

[Bronctopia]

think it relates more to me, but I understand it less. My meter does use wideband sensors,

Don't feel bad. Modern Wideband O2 sensors are one of those intersection technologies that took multiple PhD's in at least two disciplines (electrochemistry and electrical engineering) to create.
And the code in your "box" took IT pros to software up all the calibration and compensation.

Basically you have 2 Oxygen electrocells in each of your Wideband sensors. Between them is a chamber where some fairly filtered exhaust gasses "enter" and can contribute Oxygen ,not just free Oxygen, but potentially the oxygen from the CO2 and H2O combustion products (in the case of "fully" combusted rich mixtures).
These two cells operate pretty much like a Narrow Band sensor when there is free diatomic Oxygen (O2) in the exhaust gasses. This would be "lean" mixture conditions, AND mixtures resulting from misfires that contain unreacted O2.

Rich mixtures that have fully combusted all the available O2 are measured by basically attempting to pull Oxygen from the combustion products. How hard this is to do (voltage required) bears some relation to how "reduced" the exhaust gasses are. "Reduced" in chemistry terms basically is a measure of how much Oxygen a mixture Doesn't have!

If you make assumptions about what the original mixture was pre-combustion, and have done a lot of calibration runs on similar mixtures, you can develop a table relating these voltage responses to "what would be expected" as an exhaust gas resulting from combusting various "Air-Fuel mixtures".

But the key is that it is still "back propagating" to what the in chamber mixture was. Change the fuel, or change the efficiency of combustion, and those assumptions can be void, and your "AFR" gauge may start reporting things you know can't be true....

 

[DON]

These two cells operate pretty much like a Narrow Band sensor when there is free diatomic Oxygen (O2) in the exhaust gasses. This would be "lean" mixture conditions, AND mixtures resulting from misfires that contain unreacted O2

So if I understand you (this is pushing my knowledge of combustion chemistry) what I am seeing on the guage, it cannot be determined if the excess o2 is from a “ lean” mixture or a spark misfire. It is just telling me how much o2 there is.
No other parameters have changed to the combustion, well, .. that I know of. Something has changed that is causing the misfire.
wsa111, I will make a note that ethonal free is almost one point “ leaner”. Is the same for idle readings?

 

[Bronctopia]

it cannot be determined if the excess o2 is from a “ lean” mixture or a spark misfire

Right. The sensor alone can't measure, or magically know what your actual pre-combustion mixture is. It is just deducing this from the Oxygen content of the exhaust gasses. Tamper with the "evidence" and the deduction is off.

This is where the human with the data logging comes in.

If the misfire is sporadic and human detectable, run it through a data logging where the misfire happens. You will want some way of marking when this happens. I like blipping the throttle just after to mark the Throttle Position Sensor graph.

A transient misfire should cause the O2 level in the exhaust gas to rise just After the misfire event, and then resolve as correctly combusted exhaust gas from after the misfire event diffuses through the sensor. If your "AFR" meter reads in the traditional "14.7 = stoichiometric", it is probably reporting a rise to 15 - 16 country.

Also, ANY extra source of Oxygen into the exhaust stream will "tamper with the evidence".
An exhaust manifold leak is a prime example. A manifold leak can both eject exhaust AND ingest air into the exhaust as high and low pressure pulses pass through the port and runners. Headers with low back pressure would be more prone to this than say, a restrictive turbo log. This is the one case where "lowest back pressure is always best" runs into real world complications.

And as WSA pointed out, running a load of fuel with more Oxygen content (typically ethanol) will deliver more Oxygen into the exhaust stream. But this should appear as an across the log "leaning".

 

[DON]

That does make sense. I do follow the thought pattern. And how fuel and leaks can affect readings. I have no system that you are referring to, tps or a way to chart the o2 readings. I do understand with what I have, there is no way to determine if the problem is fuel or spark. The readings just verify that some kind of “event” is happening.

 

[Bronctopia]

I have no system that you are referring to, tps or a way to chart the o2 readings.

Gotcha.
So you are probably running a carb and using the "Fast" box to tune it?

All of your 'old school' diagnostic skills are still perfectly valid, and will need to be used to diagnose what is causing your "AFR mystery" (and any perceptible performance issues!)

Reading the spark plugs is a good direct check of in-cylinder mixture conditions, and is still used today to cross check O2 sensor readings. It is just WAY less convenient!

I looked at the "Fast" link, and they claim a variety of "data logging" options. One of which was playback on the box display if I am reading it right?

There was a time before computerized fuel injection where data logging was done in a much more analog way. Typically the recording device was a movie camera, aimed at a variety of human readable gauges.
This may seem a primitive method to us now..... But in the right hands, it was powerful enough to develop nuclear arsenals and land humans on the moon.

Today, thanks to creeping Surveillance Capitalism, pretty much everyone has on their person a miniaturized movie camera with a near instant film development lab, and nearly infinite film.
So you COULD go full "Apollo Mode" and datalog in Gauge and Video!
My suggestion would be a good cell phone mount facing the dash, probably just passenger side of the driver's head.
I'll try to list some of the most common sensors that would be used in an EFI dumped data log, and analog versions:

EFI sensor..,....Analog gauge

Time..... Stopwatch on Dash
RPM ..... Dash Tach
MAP ..... Vacuum gauge
TPS. ..... Locked ankle and knee flag?
O2. ..... That would be your "Fast" display.

There are other EFI only things that have no real analogue. The primary useful one being injector pulse width or duty cycle. It is a real-time measure of how long the fuel injectors are held open.

One of the problems you might run into is time resolution. With just fin de siecle OEM narrow band sensors, the response time I see for O2 readings is about 0.5 seconds for a "rich-lean-rich" transient from say, 0.7V to 0.4V and back. So half of this peak is passing in 250 milliseconds. This is pretty hard for a human eye to register, and your "Fast" display may be doing some smoothing that could completely mask it. It also happens to be about the time frame of a transient misfire/stumble. The kind that you can hear, and maybe just feel if you are under load. Ideally you would want to be able to re-play your video in slo-mo, and somehow do the same with your "Fast" box data play back.

If all of this sounds like a pretty serious High School Physics Lab project....well, that's just exactly what it is!

Before you go this route, I expect you will have run a good vacuum leak check, plug check, timing check, exhaust leak check, and especially O2 bung check and maybe swap out (if your Fast has 2 as pictured?).

You will probably find the culprit before going all mid '60s NASA on your WIP!

 

[DON]

Thanks for the input . I am running carbs and full MSD ignition. I am eliminating things. Have done pretty much complete tune up. I am focusing on ignition. It starts, idles fine, lite acceleration until about 2500 rpm, then a serious shutter and the o2 meter pegs “lean” . I can nurse it to higher rpms. But it is not happy. The next thing I am going to do is swap out my MSD box for the spare. By the way- wired with a weather pac plug and crimped connections. I’ve been under the weather so kinda put it on the back burner. Maybe tomorrow

 

[pmuller9]

Thanks for the input . I am running carbs and full MSD ignition. I am eliminating things. Have done pretty much complete tune up. I am focusing on ignition. It starts, idles fine, lite acceleration until about 2500 rpm, then a serious shutter and the o2 meter pegs “lean” . I can nurse it to higher rpms. But it is not happy. The next thing I am going to do is swap out my MSD box for the spare. By the way- wired with a weather pac plug and crimped connections. I’ve been under the weather so kinda put it on the back burner. Maybe tomorrow

Have you checked the distributor phasing.
If the MSD adapter cable to the distributor has reversed wiring, the distributor rotor phasing will be in between the distributor cap contacts and you will get crossfiring.

 

[DON]

Umm, there is no adapter wire, MSD dizzy. The wires were made by MSD. Two from the box to the dizzy. Plus it has been fine for probably 5k miles. I don’t think phasing would change on its own. I think of phasing as how the wiring is done.

 

[Bronctopia]

If it happens promptly at 2500, that does sound more ignition that fuel.
Especially if you can't see it at say, deep throttle and 1800.
It almost sounds like a rev-limit, and the spark cut would show up just as you are getting on the O2 gauge.

I gotta say, a Spare MSD is probably the fastest troubleshooting tool for this situation that money could buy!

 

[DON]

It happens at very lite load. I had a box fail within probably 6 months of occasional usage. I sent it back to be fixed, and bought the 6al-2. Got it back from MSD and just keep it in the trunk. With the plugs it will be fairly easy to swap. Just have to get to it. To keep the old school look, I hid it under the battery tray. I thought myself it felt like a rev limit being hit. BTY- I have a spare trigger sensor too. Had one of those go bad too

 

[DON]

Well, the backup box changed nothing. Still starts breaking up ( missing, creates a shutter) right at 2000 rpm. No backfiring. I have thought about it and I just can’t bring myself to believe it’s fuel related. Note the backup box is not known good, has been repaired but not tested. Does anyone know of a “ controller” I can try, cheap and temporary, that will work with a MSD distributor/ trigger? The only other thought I had, a long shot, is make sure it’s getting enough amps. Maybe run a headlight or two? Will the light connected across the coil work with a MSD box?
I will check timing too , see if that shows anything . I actually don’t know if the miss will show without a load, I will verify that as well. Will post updates. Any more ideas?

 

[Frank]

No real helpful ideas, but two observations: 1- light load miss can be caused by extreme early timing. Beyond hearing ping. 2) Hooking a timing light to the coil to distributor wire where it sees all 6 sparks, and observing it in the dark will reveal intermittent misfire, as the near constant beam has short dark gaps randomly. Test would assist to verify ignition, not fuel.

 

[DON]

Thanks Frank, I am going to check timing. I will use several, if not all wires. And, will take it up to 2k. This problem developed on its own, no repairs or adjustments have been made that it could have been associated with. New cap and rotor were installed before, but it was running good since then. Since the issue started, new plugs, wires, coil, and back up box. No change.
Had the thought last night about using the module from the DUI distributor in place of the AL-6, but would need wiring guidance. I haven’t changed the trigger in the distributor, but I can’t see that being the problem, it works at idle and it shouldn’t care how fast it’s spinning. I have thought about putting the original distributor in. Can’t remember if I have wires for it

 

[Frank]

Thanks Frank, I am going to check timing. I will use several, if not all wires. And, will take it up to 2k. This problem developed on its own, no repairs or adjustments have been made that it could have been associated with. New cap and rotor were installed before, but it was running good since then. Since the issue started, new plugs, wires, coil, and back up box. No change.
Had the thought last night about using the module from the DUI distributor in place of the AL-6, but would need wiring guidance. I haven’t changed the trigger in the distributor, but I can’t see that being the problem, it works at idle and it shouldn’t care how fast it’s spinning. I have thought about putting the original distributor in. Can’t remember if I have wires for it

Yeah, it's unlikely to be timing. But lighting all 6 at once with the timing light can reveal if it's a secondary voltage misfire. Gaps in the light=misfire. What type of distributor do you have? Magnetic pickup work with GM module without modification to distributor phasing. Points, the rotor has to be customized to change it's phase. Blairsville Ed has done this successfully running points with GM module.

Any aftermarket distributor applying 12V to the coil, the stock coil will not hold up, the resistance is wrong. A can coil from the DS2 era, any can coil labeled "for electronic ignition", or any E coil will be matched to the module/MSD ignitions with full 12V.

With all that firepower from the MSD box, you might have crossfiring inside the cap?? IDK.

 

[DON]

It’s a full MSD ignition: dizzy, wires,al6 and coil. Not a compatiblity problem. I’m not sure of the type of trigger it has. I would call it a Hall effect. It has the tabs on the shaft that rotate past the sensor. I do know you can jump between the wire going to the al6 box and it will fire. A test eliminating the trigger. Is this what you describe as magnetic? Cap and rotor are new and have checked for signs of jumping, clean as a whistle