From: "Daniel Patrick Bliley"  dpbliley@eos.ncsu.edu
Date: Wed, 8 Oct 1997 00:05:35 -0500
Subject: all What does oxygen sensor really do?

The oxygen sensor actually senses the differential in atmospheric oxygen content and exhaust oxygen content. The sensor is hollow and both the inside and outside are coated with platinum. The inside is indexed to the atmosphere. The differential causes a potential (or voltage) to be created by the sensor. The greater the differential, the greater the potential. An oxygen sensor puts out anywhere in the neighbourhood of 100 to 900+ millivolts. 450 mV is considered the ideal point (the perfect stoichiometric ratio). Anything below indicates a lean condition; anything above indicates a rich condition. This signals the computer to make adjustments to the mixture.

You'll notice that many catalytic converter manufacturers recommend that you make sure your O2 sensor is functioning properly before replacing a clogged or melted-down converter. Simply put, the converter may overheat if it has to catalyse too many raw hydrocarbons. Its monolithic ceramic core may even reach melting temperature. In other words, the O2 sensor is vital to several emissions control aspects of your car.

You can check your O2 sensor's output with a simple digital multimeter set on the smallest voltage increments (NOT analog, as they are generally not sensitive enough and may even use excessive amounts of voltage that may render the sensor useless). You can see how the O2 sensor's output changes with varying throttle positions, and you can even introduce a little propane gas into the intake to richen the mixture. The O2 sensor should respond quickly to varying conditions; one that doesn't may be excessively carbon fouled. Check it out; it's sorta fun!

Dan Bliley--dpbliley@eos.ncsu.edu
Undergrad, Mechanical Engineering
North Carolina State University

IZCC #1003, Triangle Z #37
'79 Datsun 280ZX Coupe
'87 Acura Integra -- pocket rocket!

Another potential source of difficulty is the fuel evaporative emission control system, also found on earlier Zs. This consists of the fuel cap which contains a vacuum relief valve, a canister filled with activated carbon, a fuel check valve between the fuel tank and the carbon canister, and 2 pipes to the inlet manifold. The general idea here is that when the car is not in use, fuel vapour is captured by the carbon canister, then sucked from the canister when the engine operates.

Although not stated anywhere in the manual, I think what can happen is this: if there is a problem in the fuel evaporative emission control system, you can get too much vacuum in the tank which then prevents the fuel pump from delivering full fuel pressure to the injectors.

It won't help either if your fuel filter is clogged, or if there are problems with your pump, and when your Z gets to over 20 years old, rust inside the fuel lines can come away from the surface, and find its way into the filter and block it up.

Fuel Filter (LHS):

When I changed my fuel filter, it was slightly clogged ( a black deposit) , and reduced the fuel pressure by about 2 psi across it. Fuel pressure is around 43 psi, more about that later.

To test for this condition, disconnect the canister purge hose & plug it.

I recommend undoing the fuel cap and releasing the pressure in the fuel tank too. This hose is located in the area of the throttle position sensor, on the front left hand side of the intake manifold. Its easy to pull off, and its only a little hose.

Go for a test drive & see if the hesitation disappears.

If you want to remove the carbon canister for checking (there's a valve in it), allow for a bit of cursing if your car is a TT. To get at mine, I had to remove the front bumper. It's located next to the left hand intercooler.