99 4R V6 Ltd - Sudden drop in MPG

I bought my 4R (V6Ltd) back in 2003 and I have meticulously kept track of the MPG since I purchased it. Since then I have consistently averaged around 19 to 20. Some tanks getting 22 some getting as low as 18.

Recently my MPG has dropped to around 16MPG. Today’s tank even came in at 15MPG! I am wondering what may be the culprit for this sudden drop. My driving habits haven’t changed drastically, though I don’t drive as far (2 miles each way) and the weather has gotten cold recently (40s and 30s at night). In winters past my mileage didn’t drop this much. The power output also seems fine, i.e. no change.

I am wondering if it is “stuck” in some kind of warm-up mode the whole time? Could the plugs have been fouled by a bad tank of gas and it isn’t igniting right? I have no warning lamps (no check engine lights) or anything that seems odd except for the drop in MPG. I don't smell gas fumes so I don't think that there is a fuel leak...(I keep it garaged).

Any theories or ideas of where to start looking would be great!

Regards,


Jimbo3san

Particulars like, last tune up, dirty air cleaner, bad gas, dirty injectors, is it not gong into overdrive, a new thermostat will fix the overdrive issue. Different tires, but not the cause for the drastic drop of late. Low air pressure also.
BUT these are all things that proper maintenance will avoid.
Does it idle smoothly? Rough idle is usually indicative of failing secondary ignition components.

Either a marginal HO2 sensor (Sensors in front of catalytic converter) or bad engine coolant temperature sensor could do this to you.

I keep the oil changed, air cleaner changed, and other fluids checked regularly and recently, in fact. Tire pressure is good all around and checked regularly too.

- Plugs...somethin' I do need to check. I am just going to put a new set and see if that fixes it or at least helps it.

- Couldn't say on the injectors. If the plugs don't work then this may be my next thing I will check into. Any easy tests that would indicate plugged or bad ones?

- I don't think it is an Overdrive issue because the RPMs at cruising speeds are about 2,200 as it has always been.

- Engine seems to idle smooth and not stumble or sputter at all. Then again maybe I don't have the feel for this engine. There are no funny vibrations that are out of the ordinary when it is at idle.

- As far as O2 sensors, would a faulty one cause idle and power issues as well? In a previous vehicle (non-Toyota) when the O2 sensor went bad the car had some serious power issues at higher speeds and would suddenly cut out...not to say that this would behave the same way at all...

- Coolant temp sensor...would this give me some funky temp readings? The temp is normal. Same as it has been since I bought it. Or would it give me a "check engine" light if it were bad? Unsure of what conditions cause the light to come on.

Thanks for the input. I will try changing plugs. I bought OE equivalents for it today. I will run a tank and see.

Regards,


Jim B.

As far as O2 sensors, would a faulty one cause idle and power issues as well? In a previous vehicle (non-Toyota) when the O2 sensor went bad the car had some serious power issues at higher speeds and would suddenly cut out...not to say that this would behave the same way at all...

I don't know of all the possible symptoms a sluggish HO2 (or A/F ratio) sensor can cause, but anything affecting accurate fuel metering could be at fault.

Coolant temp sensor...would this give me some funky temp readings? The temp is normal. Same as it has been since I bought it. Or would it give me a "check engine" light if it were bad? Unsure of what conditions cause the light to come on.

The temperature readout would not be affected. Check the manual for the expected resistances at various temperatures and test it with a multimeter. Since the engine doesn't have an independent measurement of temperature, it has no way of knowing that the sensor is bad. The engine coolant temperature sensor is on the front top of the intake manifold. (The gauge sensor is at the back of the engine).

Both the HO2 sensor and the ECT sensor are know to go out of spec, causing problems with fuel mixture. The HO2 sensor usually causes the Check Engine light to come on with a characteristic fault code, but you never know.