O2 sensor display, w/video

Diasplay source

I got it from a fellow Vanagon owner, Ken Lewis, kdlewis*northstate.net He may or may not still be making them.
 
Not assuming...

I think the effective range is very small, which is why I think the AFM adjustments are not a good idea. Once you tweak enough to be outside of the range of the system's ability to compensate back to Lambda, you might as well run without the O2 sensor and CAT. If you use the O2 sensor the system will correct it within certain limits, as it's designed to do.

You can modify an engine so that it can use more intake charge, but the stock system will provide it within certain limits. An engine that draws more air will open the AFM flap farther/sooner. Outside those limits you would need to upgrade AFM, injectors etc. Each of these improvements come at a cost. A larger AFM will flow more air at high rpm, but be less effective at low rpm. The signal is still linear, so a larger AFM is really only needed for a larger motor.

Larger injectors can flow more fuel, but economy and low rpm atomization will suffer. Higher fuel pressure is also possible, but fuel system safety and control at lower rpms will suffer.
 
Capped bypass screw

Erwin the capped "bypass" screw is for adjusting the idle CO%, or mixture, which is the normal way to set the base mixture for this L-Jet system. This is used because at idle the un-heated O2 sensor is considered too cool to be accurate. Above idle the sensor's output is used to control the mixture, and the base idle % is how the baseline for above-idle range is set.

It's capped to prevent fiddling of this sort of course. And most folks don't have a CO% meter or know how to use a VOM to set the mixture (although this is fairly simple).

BTW, determining the limits of the range of adjustment of the system is possible by tuning it (say, adjusting the AFM flap tension) and comparing the O2 sensor voltage (or engine sound/rpm) both connected and disconnected. If it doesn't change when connected, it's safe to assume the tweak is beyond the system's ability to compensate. This is referred to in my book as a set of tests, the Rich Stop test and the Lean Stop test. The Rich Stop test tests the system's ability to compensate with the FPR hose disconnected and capped, and the Lean Stop test introduces a vacuum leak (unmeasured air) via the dipstick seal purposely left unseated. In both cases the system should correct once the O2 sensor is reconnected.

Hope this helps.
 
Pete,

My experience with other L-Jet cars indicates that you are on the right track with your questions/comments. The typical range of adjustment for the O2 sensor is pretty narrow, and I suspect that it is similar (or even identical) across the range of L-Jet installations.

As I have stated here several times before, tweaking the AFM simply adds fuel relative to the position of the flap in the AFM, but it does nothing to address the mixture at WOT. So from that perspective I agree with Gregory that altering the AFM in an attempt to compensate for engine mods is a half measure at best.

I agree with you that the easiest way to substantially change the mixture on the stock FI system is through the use of an adjustable FPR. This is pretty commonly done on a variety of cars, even today.

Gregory, I don't share your concerns over the use of higher fuel pressure to compensate for minor to moderate engine mods. The system is speced to operate at and beyond the pressure that the pump is capable of delivering. FPRs fail on occasion, delivering full pressure to the injectors-- the engine runs like carp but it doesn't exactly hemorrhage fuel. Again, higher fuel pressure is commonly used to compensate for other mods and it works just fine. I do share your concerns about going to larger injectors (unless truly warranted), which is why I recommend higher fuel pressure.

I also do not understand your logic regarding the use of an O2 sensor in conjunction with injection mods like an adjustable FPR. I think we all agree that the range of adjustment offered by the O2 sensor is small, so it obviously cannot compensate for a large deviation in mixture (assuming the ECU is even operating in closed loop mode). But that doesn't mean the O2 sensor is not still useful...

Let's use the mild interim engine I'm building for my '85 as an example-- I'm raising the compression, polishing the intake and ports (the *inside* of the intake and ports, Bob), installing 37.5 mm valves, changing cams, and installing a header and free-flow exhaust system. The engine will flow more air, and will therefore need more fuel. I intend to use an adjustable FPR, as I think it'll be sufficient to compensate for these mods until I can upgrade the EFI itself.

Now, let's say I raise the fuel pressure enough to provide a slightly richer mixture than stoichiometric. So maybe the engine will be closer to the 12:1 A/F ratio that's desirable for max power. But in cruise situations when the ECU is in closed loop mode it'll lean the mixture out a bit, getting it back closer to stoichiometric. Will the L-Jet ECU take a 12:1 ratio all the way to 14.7:1 based on the O2 sensor data? I doubt it'd get anywhere close, but in that case every little bit helps...

Note that my engine will not have a catalytic converter, but it will have an adjustable FPR and an O2 sensor. Fuel mixture would be much more critical if a catalyst was planned, in order to avoid overheating the converter with an overly rich mixture. But that doesn't mean the O2 sensor data won't be useful, even if it won't fully compensate for an major variation in mixture.

Think about it this way-- let's say your X overheats in stop and go traffic unless it has two electric fans fitted. Does that mean that a single fan is useless? Heck no-- it may not be moving sufficient air but it's keeping the engine from running even hotter than it would with no fan at all. Same thing with an O2 sensor-- the ECU may not use its data to compensate for a major mixture anomaly, but it's better than nuthin'.

Cheers,

///Mike
 
Dan, that is indeed a better video but the videographer still gets inexplicably distracted....

....by the car.

I do like the sleek, modern lines a lot. The Italians do a nice job with the car unveilings. :)

///Mike

PS: the car is interesting as well.
 
Mostly...

Mostly I wouldn't want folks thinking they can tweak the AFM and make a small difference with the O2 sensor still in the loop.

If you have some way of providing extra fuel during WOT that doesn't only come into play at WOT, and so affects the mix at cruise or idle, the O2 sensor would try to correct that, that's all. If you know what you're doing and still want the system to try and correct for cruise, well it might but it's not covered in any of the literature I have. I'm also not sure if it's possible to damage an O2 sensor with an overly rich mixture or not.

I think you'd have to have a very special engine like you're planning to need the changes, and for most of us it's best to leave it alone.

One thing to look at is a slightly different system, Digi-Jet as used on my former '84 VW Vanagon. The Digi-Jet system is remarkably similar to L-Jet, with some differences. No cold start valve, instead it enriches through injector pulse via the ECU when cold, and two throttle switches; one is the closed-throttle one like our except it also interrupts the O2 sensor signal at idle, and a WOT switch to provide full-throttle enrichment (continuous injector pulse). Might be adaptable to the X. Also has separate ignition module similar to the Strada and uses a separate digital idle stabilizer.

I'm thinking the WOT enrichment is of particular interest. Without knowing, I'm also wondering if the circuitry for WOT enrichment is in the ECU and maybe the switch and wiring can be added to the Fiat ECU? No idea if it's possible.

You could also use vacuum switches like the Fiat Spider Turbo used to add fuel under boost. It's a band-aid fix but another option.
 
Yes, you can damage an O2 sensor by contamination as a result of an overly rich mixture. In my case I'm hoping to get the overall mixture close enough to stoichiometric by using the adjustable FPR that the O2 sensor will be able to work normally. Guess I'll find out-- we'll tune the car using a WBO2 sensor and watch the mixture across the RPM and temperature ranges.

As an aside, you can tweak the overall mixture by adding resistance to the engine temp sensor circuit, fooling the ECU into thinking that the engine is colder than it is.

Of course, the best solution is still a programmable EMS-- especially since that would facilitate the removal of the restrictive, power robbing AFM. But a combination of some of the tweaks mentioned here should result in a well behaved engine, even if it has some moderate modifications done.

Cheers,

///Mike
 
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