Gas milage suddenly dropped

[Whiskey]

Hello all!

I have a 1998 Toyota Corolla that is my daily driver. I was getting 31-33 Miles per gallon,.. and this suddenly dropped to 24 MPG. It is a well cared for, one owner vehicle with almost 200,000 miles on it, all the matenince was done it's entire life, including things like transmission flushes.

The first thing I did is look for leaks, I found the fuel injector pulse dampener was leaking some, so I replaced it, but it made no significant difference - I guess it wasn't leaking that much.

I've replaced the plugs (they were due anyway), this also made no difference, the old plugs were not badly carboned up.

The vehicle is not running badly - the check engine light is not on, and I'm at a loss as to what could be the problem. Please help with some suggestions of what to check, and how to troubleshoot.

Oh,.. btw - I don't race around in it, I drive it conservatively - I have a rock crawler trail truck I use to get my kicks

Thanks all!
Whiskey

[Kiwi-Corolla]

Welcome to the forums! Start by checking how clean your air filter is and how inflated your tires are (probably not the cause of such a significant drop in MPG but worth checking). Have a look at the temperature gauge next time you're driving the car, it should sit on half or just under. If it's lower than that then there's a chance that your thermostat needs replacing, which can play a big role when it comes to fuel consumption.

There are a lot of other factors which could contribute to the poor fuel mileage, such as dirty or leaking injectors, vacuum leaks, exhaust leaks, sticking EGR valve, incorrect timing, malfunctioning oxygen sensor, worn valve guides, bad fuel pressure regulator or fuel filter, clogged PCV valve etc etc. Start by checking the simple things first and go from there

[Whiskey]

Thanks for the welcome!

Tires are at 35 PSI, the car warms up quickly then holds just under half in all kinds of driving conditions, the air filter has 10,000 miles on it since last replacement and this vehicle is not driven in dusty or dirty conditions.

I don't hear any vacuum leaks, but I could try spraying some carb cleaner around and see if it revvs up when I spray anywhere.

Dirty injectors is a good thought - when this tank get's low I'll pickup some injector cleaner for the tank.

The exhaust is not leaking. Timing is all computer controlled, but I can check it.

The 02 sensor would always trip the check engine light right? Is there any way I can confirm it's operation?

How can I test my EGR valve?

I will check the PCV valve - that I can do before I leave work today. I can pick up a fuel system pressure tester as well.

Thanks for the post!
Whiskey

[Kiwi-Corolla]

Originally Posted by Whiskey

The 02 sensor would always trip the check engine light right? Is there any way I can confirm it's operation?

Yeah typically it would cause the CEL to come on, but not always. You can check it with a digital multimeter. Here are the instructions from my Haynes manual for the 7th Gen Corolla, should be the same as the 8th Gen Corolla:

1. Locate the oxygen sensor electrical connector and backprobe the black wire on the harness side of the oxygen sensor connector. To properly backprobe the connector insert a long straight pin (a T-pin is preferred) alongside the wire until the pin contacts the metal wire terminal inside the connector. Connect the positive probe of the multimeter onto the pin and the negative probe to ground.

2. Start the engine and monitor the voltage signal (millivolts) of the main oxygen sensor as the engine warms-up. The oxygen sensor will produce a steady voltage signal at first (open loop) of approximately 0.1 to 0.2 volts with the engine cold. After a period of approximately two minutes, the engine will reach operating temperature and the oxygen sensor voltage will fluctuate between 0.1 to 0.9 volts (closed loop). If the oxygen sensor fails to operate as described, replace it.

Originally Posted by Whiskey

How can I test my EGR valve?

Here some more instructions from my Haynes manual for the 7th Gen Corolla:

1. Disconnect the harness connector for the EGR temperature sensor and measure the resistance of the sensor at various temperatures.

It should be:

112-degrees F at 69 to 89 k-ohms
212-degrees F at 11 to 15 k-ohms
302-degrees F at 2 to 4 k-ohms

Best of luck!

[Whiskey]

This is going to sound stupid,.. but how do I use this fuel pressure gauge?

I've looked everywhere, and I can't find the normal pressure testing port,.. and the fuel lines are all connected with these funky connectors rather than hose clamps so I can't tap in using the "T" fitting that was supplied.

I'm pretty sure that the problem is the 02 sensor now,.. but I'd still like to verify my fuel pump operation since I have the gauge

Thanks for the great post Kiwi!
Whiskey

[Kiwi-Corolla]

Originally Posted by Whiskey

This is going to sound stupid,.. but how do I use this fuel pressure gauge?

I've looked everywhere, and I can't find the normal pressure testing port,.. and the fuel lines are all connected with these funky connectors rather than hose clamps so I can't tap in using the "T" fitting that was supplied.

I'm pretty sure that the problem is the 02 sensor now,.. but I'd still like to verify my fuel pump operation since I have the gauge

Thanks for the great post Kiwi!
Whiskey

No problem, you're more than welcome to click the 'thanks' button below my posts if you like . As for using the fuel pressure gauge, this thread should come in helpful for you:

Where is the connection for connecting a fuel pressure gauge?

[Buurin]

Originally Posted by Kiwi-Corolla

Here some more instructions from my Haynes manual for the 7th Gen Corolla:

1. Disconnect the harness connector for the EGR temperature sensor and measure the resistance of the sensor at various temperatures.

It should be:

112-degrees F at 69 to 89 k-ohms
212-degrees F at 11 to 15 k-ohms
302-degrees F at 2 to 4 k-ohms

Just a friendly reminder that our 8th gen have no EGR at all.

[Kiwi-Corolla]

Originally Posted by Buurin

Just a friendly reminder that our 8th gen have no EGR at all.

Cheers. Good to know . None of the Corollas here in NZ have EGR so I wasn't sure which overseas models did and didn't come equipped with it.

[Whiskey]

Oh,.. that "Thanks" thing means something here? Cool cool - done

That's funny that the 1998 Corolla has no EGR, when my 1984 Tercel with the 3A-C engine does. It even has an O2 sensor! On a carberated car - I'm pretty sure it's years past shot - I can't imagine what it could possibly be doing.

Looks like I need to buy myself some fittings, though it's not such a huge problem anymore since I replaced the O2 sensor today, and jumped from 24 MPG back to 36 for a 80 mile freeway test drive, with the AC on That's what I'm used to

Whiskey

[TiloToy]

MAF sensor on airbox dirty? Search for cleaning diy on here somewhere.

[teppista881]

Originally Posted by TiloToy

MAF sensor on airbox dirty? Search for cleaning diy on here somewhere.

1998 has MAP sensor, not MAF, which I believe was introduced in 2000.

[NutsNBolts]

The o2 sensors don't always log a fault code as you might get one thats partially failed or slow to react.They are easily checked on live data with a scan tool or multimeter

[Kiwi-Corolla]

^ +1. In addition to what I said in post #4, you can simply back-probe the signal wire (should be black in colour) from the oxygen sensor with a digital multimeter when the car is at normal operating temperature. To achieve normal operating temperature, either drive the vehicle or run the engine at 2500rpm under no load for 90-seconds.

Rev the engine to 4000rpm and the voltage should fluctuate between 0.1-0.9 volts (it'll be about 0.3 volts at idle). If the multimeter reading doesn't rise above 0.7 volts, a lean mixture is indicated. If the multimeter reading doesn't fall below 0.35 volts, a rich mixture is indicated. If the multimeter reading shows 0 volts however, then check the wiring between the oxygen sensor, diagnosis connector and ECU. If no fault can be found, replace the oxygen sensor.

[bail_w]

Did you replace both o2 sensors?