When I start it cold, it runs great and the wide band AF ratio gauge reads ridiculously lean AF ratios. It will cruise down the road at over 17:1, and will idle at 15.5-16:1. This seems impossible to me. After it warms up, and goes into closed loop, the AF ratio then reads something very close to the AF table in MS-II. I suspect the gauge, in closed loop, is telling the computer to make it too rich as a result.

The gauge is an AEM wide band. I could actually track the duty cycle in MS's and see if the fueling is actually getting richer when it warms up. I guess it is possible the gauge is not reliable when cold. I am considering setting the target AF ratio in the cruise range for something like 17.5:1 and then test the mileage to see if it gets better. If I am right, and the AF ratio meter is reading lean, then the ACTUAL AF ratio in closed loop would be richer than the target 17.5. .

I have driven cars that are fueled too lean in the past, and know what a lean surge feels like. This displays none of that when it is cold and reading the ultra lean AF ratios. It just runs too well for me to believe the gauge.

I am driving down to Lou's this week end, which is about a 300 mile trip one way, and it would be a good time to test this theory. I guess if it gets too lean in actuality, I would the see vacuum drop and longer injector open times.

Thoughts on this? Is it possible for a cold slant to cruise down the road with an AF ratio of 17:1? That sounds crazy to me. It does not stall, it responds to the throttle nicely, and it accelerates smoothly without a miss or hesitation.

For what it is worth, my old narrow band gauge was reading too lean, and was enriching the fuel as a result. Lou suggested setting the crossover point on that one at .7 volts instead of .5, and that made it run better.

Thanks for any thoughts on this. Sam

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A rich misfire will make an A/F meter run lean (excess O2 from 1 or more cylinders)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Ah! So you think it is actually rich when it is cold, and rich enough to fool the gauge? But I feel no misfire, at any rpm. It idles perfectly smoothly. I am open to your theory.

Last time I pulled plugs, which was recent, they looked good with light brown/whitish color.

Sam

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Does your AF sensor heater work correctly? - Those sensors WON"T work properly until they are "up to operating temps". Every one I see on late model OEMs has a built in heater.

Also make sure that you have absolutely NO exhaust leaks upstream or w/in 12" downstream of the sensor. Even a tiny (inaudible) leak can let air get sucked in on negative exhaust pressure pulse & the O2 will make the AF read falsely lean. Often this type of leak can seal (mostly) when the exhaust gets hot.

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63 Valiant Wagon
225 - 4 bbl

Bingo! There is a major leak at the exhaust manifold to head gasket. Until the is changed, the O2 sensor is not going to read correctly. I suspect once I change the gasket, I will have to go back and reset the AF values in the AF table.

Thanks. Sam

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