1991-1993 240sx: does port wall wetting during deceleration cause a drop in idle

[vancouverbc]

I discovered that the aiv on 91-93 240sx is used to burn up excess fuel from port wall wetting during deceleration and idle. If you dont have the aiv installed, would that cause an idle drop during deceleration? During deceleration you will suddenly become very rich.

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.

[Nismo_Freak]

There is no wall wetting. The injectors are not firing when you have lifted off the throttle on deacceleration, if they were, the engine would produce torque and reduce the effectiveness of the brakes.

The valve opens to inject oxygen into the catalyst to ensure that it does not cool during the deaccel phase since EGT's are extremely low due to a lack of combustion.

The injectors pick back up when the ECU shifts into it's idle map. The emissions equipment does not in any way cause wall wetting. Wall wetting is not a problem in modern cars, the injector sprays onto the back of the inlet valve which causes atomization of the mixture, and that paired with the high relative velocity of the air in the port all of the mixture is crammed into the cylinder.

Wall wetting is an old carburated car issue that involved an overly rich mixture, due to jetting, float levels, etc., or poor manifold design.

[vancouverbc]

Thanks for reply. I noticed that the aiv supplies air downstream of the o2 sensor , etc so I was out to lunch. I did put a new aiv in today and it allowed me to pass emissions at idle. It lowered hydocarbons and carbon monoxide. The exhaust was still rich at the o2 sensor.

For whatever reason, some of the 240sx's are very rich at idle. The article I quoted was for a 1991-1993 nissan high-port engine. They said the injectors were postiioned badly and this created wetting. I dont even know what engine they are referring to. The was SE-R.net which no longer exists.

Thnaks again:)

[Nismo_Freak]

Thanks for reply. I noticed that the aiv supplies air downstream of the o2 sensor , etc so I was out to lunch. I did put a new aiv in today and it allowed me to pass emissions at idle. It lowered hydocarbons and carbon monoxide. The exhaust was still rich at the o2 sensor.

For whatever reason, some of the 240sx's are very rich at idle. The article I quoted was for a 1991-1993 nissan high-port engine. They said the injectors were postiioned badly and this created wetting. I dont even know what engine they are referring to. The was SE-R.net which no longer exists.

Thnaks again:)

They are talking about the SR20DE in the NX2000 / SE-R which has a longer port, but similar intake manifold to the KA24DE. The KA24DE will have a higher intake velocity thanks to it's superior low RPM VE.

If you are running rich at idle, you might want to investigate your coolant temp sensor. You can also do some testing with your MAF ground resistance (common issue), and the TPS voltage at idle. These are two small things that are common to be out of sync on older Nissans.

Just some ideas.

[crzycav86]

There is no wall wetting. The injectors are not firing when you have lifted off the throttle on deacceleration, if they were, the engine would produce torque and reduce the effectiveness of the brakes.

The valve opens to inject oxygen into the catalyst to ensure that it does not cool during the deaccel phase since EGT's are extremely low due to a lack of combustion.

The injectors pick back up when the ECU shifts into it's idle map. The emissions equipment does not in any way cause wall wetting. Wall wetting is not a problem in modern cars, the injector sprays onto the back of the inlet valve which causes atomization of the mixture, and that paired with the high relative velocity of the air in the port all of the mixture is crammed into the cylinder.

Wall wetting is an old carburated car issue that involved an overly rich mixture, due to jetting, float levels, etc., or poor manifold design.

Wall wetting IS an issue on any mpefi system. thats why modern engine management systems have acceleration enrichment. it is extremely important for throttle transients, especially at low rpm where the air's shear stress is low.

i wrote this a while back to help explain:

Quote »here's what happens at constant air flow:the fuel injector injects fuel at a certain rate.most of that fuel hits the back of the valve, evaporates, and gets sent into the engine.a small percentage hits the intake port and accumulates in a puddle with fuel that has accumulatedfrom the previous engine cycles. however, the puddled fuel is also evaporating at the same rate at which new fuel is puddling, so the engine gets 100% of fuel it needs.

heres an example with numbers.you're cruising along the highway and your fuel injectors are flowing 100cc/min.

75% goes into the engine as planned.25% puddles accumulates in the intake port, while 25% is also being evaporated from previous engine cycles.100*.75[from injector] + 100*.25[evaporated from intake port] = 100cc/min, the engine is getting all 100cc/min of fuel.

now you stab the throttle. the fuel requirements double, so now the injector is flowing 200cc/min.75% goes into the engine as planned.25% puddles in the intake port, and 25% of the fuel that was injected at 100cc/min gets evaporated.200*.75[from injector] + 100*.25[evaporated from intake port] = 175cc/min, thats not the 200cc/min that you need based on new fuel demands, so the engine is lean.

if there was no enrichment, eventually the fuel evaporated at the new flow rate would equal the amount of fuel being accumulated at the new fuel rate, so the a/f would eventually reach stoich.

but we dont want to wait. we want the proper a/f right when we stab the gas, so we implement an acceleration enrichment.

in our example we needed 25 more cc/min of fuel. x will be the enrichment factor.

so 200*x*.75 +100*.25 = 200

x = 175/150 = 35/30 = 1.167 or 17% enrichment.

the enrichment reduces with each engine cycle until its no longer required. so 17% is the initial enrichment, and eventually reaches 0%.[/quote]

[AZhitman]

[Nismo_Freak]

Wall wetting IS an issue on any mpefi system. thats why modern engine management systems have acceleration enrichment. it is extremely important for throttle transients, especially at low rpm where the air's shear stress is low.

Sure, but this does not play into account in his situation, hence why I said there is no wall wetting. When you lift off the throttle the injectors are shut off, hence any residual wet fuel is going to be quickly evaporated.

Lightweight flywheels exacerbate the issue as do some driving styles, but for the most part you'd have to be flooring it then rapidly lifting off the throttle and coming down to an idle to produce a wall-wetting issue. That's just unlikely if this is a consistent idle issue.

A gross example of poor fuel control is a rotary engine, and I've never had issues getting one to idle, or stay running coming down off the throttle.

[crzycav86]

Can you hang?

[Nismo_Freak]

Can you hang?

Greg's a pretty sharp guy, I mean he's pretty damn old, but I don't think your balls creating static when you walk on carpet affects brain function.