Oxygen Sensor Failures on Innovate Motorsports MTX-L AFR Gauge

[VWLover77]

I bought an Innovate Motorsports MTX-L Air-Fuel Ratio gauge about a year ago to help with the tuning of the AFM on my stock '78 Bus (2.0L, hydraulic lifters). It's hooked up for continuous operation when the engine is running.

The bung for the O2 sensor is located on top of the stock exhaust elbow so it's seeing exhaust from all 4 cylinders before it flows into the CAT and muffler (stock CA exhaust setup for 1978).

The first Bosch wideband O2 sensor that came with the unit lasted about 6 months / 5000 miles before it started failing. The gauge readings were OK until things got hot and then the readings would get really erratic until finally the gauge would error out and stop working.

I thought maybe the bung was too short, thus exposing the O2 sensor tip to too much hot exhaust blast, so I had a friend weld on a 1-inch bung extension to protect the sensor tip from the exhaust blast and bought a new genuine Bosch O2 sensor.

Everything worked perfectly for another 6 months / 3000 miles, but now the erratic readings and error code are happening again.

The range of readings I typically see on the gauge are from around 10.0 while accelerating a cold engine on a cool morning to 22.0 when the throttle is snapped shut during high speed deceleration. Steady highway speed cruise is around 13.3.

Any thoughts as to why I'm killing O2 sensors?
_________________
Don

1978 Westy
1971 Super Beetle Convertible


"If more of us valued food and cheer and song above hoarded gold, it would be a merrier world." - J.R.R. Tolkien

[mikedjames]

Running too rich, soot damages sensor.


Its quite tricky on a Bay - in my experience spending too long around 10:0 will reduce the lifetime of the sensor, but it is hard to get it up to near 12:0

My first genuine Bosch sensor lasted about 60k miles, the Chinese LSU4.2 replacements are not lasting anything like as long - with the rich running maybe 5000 miles. . I have been through a piston melting experience so I set the carburettor up to run rich, accidentally spent a couple of days with the choke heater disabled, then the sensor failed ...

(the piston melt was over advance on a "pre-timed" distributor where the timing light was broken, added to a vacuum stat on the air cleaner that decided to let in hot cylinder head air to the intake again if it got over about 110F.
Caused cylinder head temperature runaway. )
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[W1K1]

[SGKent]

10:1 is way too rich for any reasonable production of power. It is like throwing water on a flame. Fuel cannot burn when it is denied Oxygen. 13.3 is fine for highway when warm.

Also you said the O2 sensor is on the top facing down. That is the correct way. If it is on the bottom facing up then it gets wet with condensation and that kills it.
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[VWLover77]

It's only at 10:1 for less than a minute until the engine starts to warm up.
_________________
Don

1978 Westy
1971 Super Beetle Convertible


"If more of us valued food and cheer and song above hoarded gold, it would be a merrier world." - J.R.R. Tolkien

[skills@eurocarsplus]

the o2 probe needs to be in the flow, not taken out of it

either you have a batch of bad sensors or something is up with the unit itself

by having the probe too far out of the flow, you aren't getting proper 'burn off' if you will from the soot

all of that said, Bosch is fucking shit today. Try a Denso if you can and get that probe back in the airflow
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[KTPhil]

Is it getting too much voltage? Checked your gen/alt/reg lately?

[raygreenwood]

[VWLover77]

Thank you Skills.... I'll see about getting that bung extension taken off and getting a Denso sensor. I'm also thinking I'll only install the O2 sensor for tuning sessions and keep it on the shelf otherwise.

Ray, the engine is a fresh rebuild (about 2,500 miles now) and is not burning oil. The sensor is heated with power supplied by the AFR gauge.
_________________
Don

1978 Westy
1971 Super Beetle Convertible


"If more of us valued food and cheer and song above hoarded gold, it would be a merrier world." - J.R.R. Tolkien

[Wildthings]

I have had the same sensor for a dozen or so years now, installed in the tail pipe as close to the muffler as I can get it, so about 16" from the end of the pipe. I run about 13.5 at cruise and it drops to around 12 at full throttle, this is at sea level, and the numbers noticeable go down as I go up in elevation.

No feel for the whys and wherefore.

[skills@eurocarsplus]

[Clatter]

You also don't want to run leaded fuel as it'll kill the sensor as well.

I only ever used them for tuning purposes.
Seems asking to kill a sensor leaving it in all the time though a lot of people do.
The people who do, however, usually run water-cooled engines with stable temps and accurate modern engine management.

Getting another Bosch sensor off of Amazon for my old Innovate LM-1 didn't fix it.
Might try getting a sensor from Innovate themselves but not a guarantee.

Sucks to have to be buying replacements,
But using this precise new tech on an old dinosaur is kinda asking for it..
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Bus Motor Build

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[KTPhil]

On my Type 3 with stock exhaust, I installed the bung at the muffler end of the resonator. The resonator can be replaced with three bolts in ten minutes. So I have my "test resonator" and then once adjusted, I use my "regular resonator" to avoid hours on the O2 sensor.

Where in your Bus muffler did you install yours? You want it to get all four cylinder's gas, but it shouldn't be so close to the tip that it can be affected by outside air oxygen.

EDIT: I reread your first post and this installation is not practical. Cats, oh my!

I guess you should try to remove it once adjusted and use a stainless steel plug with generous anti-seize on it.

[skills@eurocarsplus]

his placement is fine...if i read it right...

if it's in the collector for all 4 cyls and it's before the cat, that is no different than a modern car and the pre-cat o2 location (or 'front' o2 as it were)
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[raygreenwood]

[Wildthings]

I don't know that the amount of zinc has ever been regulated in motor oil, but phosphorus has been and is proportional to the amount of zinc in ZDDP. ZDDP is not a single compound but a group of them, some more volatile than others. It is the volatile ones that are the problem, they disappear from the motor oil with time potentially causing a loss of protection and a certain percentage of them end up in the catalytic converter reducing its effectiveness. Phosphorus is regulated in SM and later oils to be a maximum of 800 ppm in 10w30 and thinner motor oils. Thicker oils have no restriction, though the manufacturer may reduce the level of ZDDP anyway.

The way to reduce the problems with the volatile ZDDP compounds is to eliminate them from the oil to start with and use only the less volatile compounds. This means that less ZDDP is needed in fresh oil as it doesn't disappear very readily and since it largely stays in the oil it does not do the same amount of damage to the catalytic converter.

[raygreenwood]

[airschooled]

If you're actually getting 10:1 under any kind of acceleration, I would start experimenting with a resistor in parallel with TS2. Something like 1,400 ohms would be my starting guess. Then let the TS2 take over adjustment once the engine has more heat in it.

If you have the TS2 spacer, remove it.
Robbie
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[raygreenwood]

[VWLover77]

Ahhhhhh..... OK, I knew the high-zinc oils were not good for the cat (mine is pretty much "dead" anyway, but not clogged), but I did not know it also killed O2 sensors. That may very well be the cause, as I have been running the Castrol GTX Classic oil with high zinc for the last 3,000 miles.

As far as that 10:1 AFR... again, it only ever happens within the first minute of the cold engine running, when I pull out of my neighborhood and immediately have to get on the throttle pretty hard to climb a steep hill. It doesn't smoke or bog, so I'm convinced that's just what the cold engine wants and needs under those conditions. What AFR does a cold, carbureted engine run with full choke?

The lowest AFR I've ever seen with the engine warm is 12.0 under hard, full-throttle acceleration.

Oh, I also learned yesterday that soaking your O2 sensor in Berry's Chem Dip parts cleaner for a couple of hours will definitely kill it dead dead dead. It now reads full lean under all conditions. Oh well, it was going to need replaced anyway. "You can't hurt junk" as my father-in-law always tells me!

Yellow arrow in the photo below shows where the bung and O2 sensor are located (not my exhaust system in the photo).

_________________
Don

1978 Westy
1971 Super Beetle Convertible


"If more of us valued food and cheer and song above hoarded gold, it would be a merrier world." - J.R.R. Tolkien