1991 240sx: Rich at idle.

[vancouverbc]

As soon as the o2 sensor kicks in around 1200 rpm the mix is optimal but during idle the O2 voltage is around .94 which is very rich. When i had 3 leaking injectors , I was getting similar number. I did a leak test with fuel gauge and did not lose pressure fast. I disconnected the maf, coolant temp sensor, iacv, and tps each in turn and this did not change the rich condition.

I was thinking of disconnecting electrical connection to each injector to see if that changed the rich condition.

[vancouverbc]

1) .870 volts at the o2 sensor2) .870 volts3) .730 volts4) .808 bolts

The biggest drop in richness occurred when I disconnected the electrical connector to third fuel injector. when I disconnected the two worst injectors, the reading was steady at .130 which is lean. When I disconnected the two best injectors, the reading was around .560 and was climbing very slowly.

[vancouverbc]

I guess I will move the injectors to different cylinders to see what happens. Change the o-rings also.

[vancouverbc]

Switched the injectors but the same cylinders are the problem. My new o2 sensor is reading much more lean now so i assume it has become too dirty already. switched sparkplugs and wires too.

[vancouverbc]

Its supposed to be rich during idle. A couple factors cause the 91-93to be normally rich during idle. Only in california where it is closed loop during idle is it not rich.

Figured this out. You definitely need the AIV to pass aircare in British Columbia. You dont need it in California(you only need it for visual inspection) because California ECU's are different. YOu stay in closed loop during idle. ie the O2 sensor controls mixture at idle. British Columbia may be the only place in the world that needs the AIV to pass emissions. If you have an idle test and you live outside California, it is likely that you do need the AIV to pass, however.

non-california 240sx's will be rich at idle. the aiv supplies air to cat so it can burn off the extra fuel. if you measure richness at the o2 sensor you will be rich with or without the aiv.
Modified by vancouverbc at 6:11 PM 4/8/2008

[vancouverbc]

Found somebody saying the aiv is needed for 91-93 to burn off fuel spray during idle. I pulled up the following from a Google search for "PAIR valve." It's pasted from an article by Dan Thompson on SE-R.net:

> What is the PAIR?

Pulsed Air Induction Reed Valve - for all intents and purposes, it's the smog pump. Its job is to introduce fresh air into the exhaust upstream of the cat/con during deceleration and extended periods of idle to help keep the cat "lit off" (at operational temperatures). The PAIRV disappeared from the engine in '94, with Nissan citing engine improvements being responsible for no longer needing it.

The '91 thru '93 high-port engine had problems with "port wall wetting" (not to be confused with "bed wetting", though it's worse in the ARB's eyes), where the injector was far enough from the intake valve that some fuel was sprayed onto the intake port walls. When the throttle was snapped shut, the increase in port vacuum sucked this extra fuel into the combustion chamber and caused hydrocarbon emissions to skyrocket. In addition, low intake port velocity at idle also caused an increase in port wall wetting, leading to a similar condition. The PAIRV was there to introduce enough fresh air into the exhaust for the cat/con to burn off the excessive hydrocarbons. The '94 and newer low-port engine moved the injectors closer to the intake valve and increased intake port velocity at low engine speeds. These changes were enough to allow for the elimination of the PAIRV. They were also responsible for the improvement in the engine's low-end power.

I don't think removing the PAIRV would cause any physical problems such as melting the cat/con, as it would actually cause converter temperatures to drop. Then again, it would offer no performance advantage, aside from maybe a small reduction in weight. In addition, it would increase tailpipe emissions, possibly to the point of failing a smog check. Personally, I left mine intact and functional.

[vancouverbc]

Conclusion: Reading the voltage of o2 sensor does not provide any useful information. The idle is always rich. the aiv supplies air downsteama of aiv at cat to burn off the extra fuel so you pass emissions.