95 GA getting about 15 mpg

[90supra]

I average about 200 miles to the tank no matter how soft or hard I drive her. Its a 6cyc, automatic with a K&N ram air, new spark plugs, new oil, new brakes, new tranny fluid, and maybe some other stuff. I checked to o2 sensor and I believe it is in working order but not sure. Which wire do I check the voltage on the plug? What else could be the problems, no check engine light even with o2 unpluged.... Help please, my Supra got at least 20 mpg running it hard.

[90supra]

Btw, it has 106,000 miles on it.

[xeroinfinity]

What type of new plugs ?

I would change both O2 sensors, they dont last much longer then 100,000 miles anyways.

Thier are two different types of O2 sensors, a 1 wire and a 2 wire(heated).

For the 1 wire O2, run the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.

Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) and battery ground.

Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.

If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.

If the 02S voltage stabilizes at either end of the specified range, the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a faulty pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.

Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.

Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.

If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.

For the 2 wire O2 sensor, run the engine and bring it to normal operating temperature as above, then run the engine above 1200 rpm for aprrox. two minutes.

Turn the ignition OFF, disengage the H02S harness connector.

Connect a test light between harness terminals A and B. With the ignition switch ON and the engine off, verify that the test light is lit.
- If the test light is not lit, either the supply voltage to the H02S heater or the ground circuit of the H02S heater is faulty. Check the H02S wiring and the fuse.

Next, connect a high impedance ohmmeter between the H02S terminals of the heating element and verify that the resistance is 11.0-16.0 ohms at 68° F (20° C).

If the H02S heater resistance is not as specified, the H02S may be faulty.

Start the engine and bring it to normal operating temperature again, then run the engine above 1200 rpm for two minutes.

* Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) signal wire and battery ground.

Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.

If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.

If the 02S voltage stabilizes at either end of the specified range, the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a faulty fuel pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.

Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.

Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.

If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.


* Backprobe - DO NOT PIERCE THE WIRES! Test from the connectors!
Hope that helps !

[90supra]

The only o2 sensor I have has about four wires going into it. Its on the firewall side of the motor on the exhaust manifold. I was thinking of changing it anyways. I forgot what type of spark plugs but I had this problem before the spark plug change.

[90supra]

I already knew about the backprobing, no cutting wires just checking connector. I have four connectors with four wires. I used a digital voltmeter and the only one that showed something was the third from the left. I think it was the purple wire. And it was between 0.1 and 1.0