WD21 Emissions for dummies!

[scaramoche]

Like many of you, I love my 1989 Nissan pathfinder, also Like many of you the nightmare of failing emissions gave me nightmares. Because im on disability, with limited income, taking it to a "mechanic" was not possible. With this in mind I focused on learning everything about passing emissions, and thought i would share this with you.

First, my pathy is a 1989 nissan pathfinder, with the vg30i enginer(has the throttlebody with 2 injectors). Much of what im going to share is common on all cars in one form or another, so understanding what they do, and learning how to replace will save you tons of cash, headaches, and in my case improved my miles per gallon.

Befor i start i want to state, im not a mechanic, and even though ive researched this a great deal, I will try to explain everything in simple terms, but forgive me if i make some mistakes. If you do find some, please let me know and ill try and update.


so first, what are emissions?

simply put when fuel is burned in your engine, the byproduct is gas's. These gas's are very volitile, cause global warming etc. In a nuttshell what "Emission" systems do is minimize the amount of gas's released into the atmosphere.

So how do they minimize the gas vapors?

there are 2 primary ways to do this.
1st-recycle gas vapors back into the engine to be burned again
2nd-adding oxegen(air) to the vapors which allows them to be "burned" in your catalytic converter


so lets' look at the parts

OXEGEN SENSOR
how it works
O2 sensor-oxegen sensor; This device is one of the most important devices in modern cars/trucks. It's basic function is to detect how much oxegen in in your exhaust. Now by itself it doesnt do anything, BUT it's importance comes in when you find out that your fuel/air mixture(which is the ratio, or mix) needed for the engine to efficiently "burn" the fuel is controlled by the information this sensor gives.. The o2 sensor connects to your ecu(the computer in your car/truck) the information given to your ECU by the sensor allows the computer to adjust how your engine not only performs, but how much gas vapor is released.

To replace
basically if you look under your car/pathy, you will see your exhaust pipe, near the engine, usually before the catalytic converter you will see a plug screwed into your exhaust. This plug has (in my case) 3 wires connected to it. Using some bolt penetrator(used to make removing easier), and a wrench, you unscrew, and replace with a new one. It should be noted that the wires run up into the engine bay. you will find the connecters on the passanger side, near the firewall.


Exhaust Gas Recirculation(EGR) system.
The purpose of this system is to take exhaust gas's from the engine and direct them back into the combustion chamber.

How it works.

Behind your intake manifold, is the EGR valve. This valve is bolted to the intake manifold. It has 2 connections.
1-connection one is from your exhaust(it's the big connection)
2-is a vacuum line connection, that is on top of EGR.

When there is no vacuum pressure applied to the EGR, the recirculated gas IN NOT entering the intake manifold.

From the EGR Valve vacuum line, you connect to the Thermal Vacuum Valve(TVV). This valves function is to keep the vacuum off, until the engine is at a temp of 134-145 F. At which point it opens allowing the EGR control solenoid to apply a vacuum. There are 2 connections to this device.
1-vacuum line from EGR Control Valve
2-Vacuum line from EGR Control Solenoid

The EGR Control solenoid has 3 visible connections.
1-Vacuum line from TVV(and ultimately EGR Valve)
2-Vacuum line that connects to the air cleaner.
3-Wiring harness that controls open/closing of the EGR control Solenoid.


CATALYTIC CONVERTER
What it does
This device is basically bolted or welded on your exhaust pipe(not the muffler, should be right before it). It takes the exhaust vapors from the engine and through a chemical reaction creates CO2(carbon dioxide) and Water(H2O).

To replace, you either need a bolt on replacement(very expensive) or take it to a mechanic and have them weld a new one on.

PCV system

The pcv system(positive crankcase ventilation) was created to decrease sludge, and unburned hydrocarbons. This system is based of the fact that when the engine is running a small amount of gas escapes the piston rings and gets pumped into the crankcase. The pcv systems purpose is take these gas's and pump them back into the combustion chamber.

How it works.

From the Air cleaner/filter assembly clean air is directed into the valve covers.

From here it enters the crankcase where the air combines with the gas's in the crankcase.

Once the gas's are combined, they exit on the alternate valve cover.

From this valve cover, the gas is directed to the PCV valve which is connected to the intake manifold, to be combusted.

NOTE: at low speeds, the valve is partially open, as speed increases, so does the opening of the valve.

NOTE:if PCV is closed, broken etc, it can cause oil leaks at gaskets, oil pan, etc.


Evaporative Emission Control system

The purpose of this system is to resolve the problem caused by escaping gas(from tank) fumes. The parts involved are the GAS TANK, VAPOR/LIQUID SEPARATOR(ALSO KNOWN AS FUEL CHECK VALVE), VAPOR VENT LINE, CABON CANISTER, VACUUM SIGNAL LINE, AND CANISTER PURGE LINE.

How it works.

From the gas tank a vent line runs to the FUEL CHECK VALVE.

The FUEL CHECK VALVE has 2 inputs. The fuel tank runs one end of the valve, while the other connects to the carbon canister side. Inside the valve, liquid gas funnels back down the input of the fuel tank, while the vapors continue on to the canister.

Once the vapor has left the fuel check valve, it goes to the charcoal canister.

While stopped or at idle, the canister holds the captured vapors. Then as rpm(throttle) increases, this causes a vacuum which opens the purge control valve, cleaning out trapped vapors.

These Vapors coming from the purge valve, go directly to the intake manifold, to be burned.

NOTE: the canister has 3 lines.
1-vapors coming from the Fuel check valve
2-Vacuum line that connects to the throttle body(top connector)
3-purge line, which goes to the intake manifold.(bottom connector)

NOTE: to test the Fuel check valve
1-blow through gas input, you should have a lot of resistance.
2-blow through the canister side, you should have no resistance.


Automatic temperature controlled(ATC) air cleaner.

This system deals with how temperature effects air/fuel mixture, and ultimately the atomization of fuel. For example when the intake air is cold, as well as the engine has a more difficult time atomizing. The result is more hydrocarbons in the exhaust. So this system basically does the following, when the engine is cold, it causes hot air to be drawn in, when the vehicle reaches operating temperature, it draws cool air.


How it works.

Inside the air cleaner there is the ATC temp sensor. When the vehicle is “cold”, it causes a vacuum, as it heats up there is less vacuum.

From the ATC sensor a vacuum line runs to the ATC vacuum motor.

The ATC opens or closes(ie directing cold or hot air) altering the intake air locations.

The 2 air intake locations are from the manifold(there is a shroud where the snorkel connects), or from outside the vehicle.

Air induction Valve(AIV) system.

The purpose of this system is to allow air into the exhaust. When these hot exhaust gas's are introduced to Air(oxygen), additional “burning” of the exhaust gas's will occur, reducing your emissions. What makes this system seem complex is what occurs in the air induction valve.

NOTE: The air induction valve(round clam/shell looking device). While it seems relatively easy to “Push” air into the exhaust, but because of physics, there is a chance you can get “blowback”, ie exhaust gas's not going out your tailpipe, but rather exiting at the AIV. So the AIV has 2 reed valves to prevent this.





thats it! now keep in mind other things can cause issues with emissions, such as leaky injectors, or tired engine, or your heads are going. Either way hope this helps

I would also like to thank all the nissan guys at NPORA, because most of this information comes from them.

Scara

[asnorton44]

One of the reasons I am glad my state has zero emissions testing.