Why has my gas mileage dropped?

[nogard]

I was getting 28-30 mpg with my '99 5S-FE this summer. It has been cooling off a little and just before that is was really humid and the air was heavy. I know this can have something to do with it. But I just had to redo the spark plug tube seals. So I put new plugs in the car and wires. I did this at the end of the summer, but it seems like my mpg when do shortly after this. I have nippodenso Toyota spark plugs and the wires are both from the dealer. I know the plugs are a twin electrode style, but I forget if the gap is adjustable.

Any tips on what might be causing the 21-24 mpg? I checked the air in the tires, and the air filter, all the normal stuff that has a big affect on mpg.

[ihartmacz]

Check the fuel filter and the PCV. Try a seafoam. Does it seem like the car is down on power?

[ghettosled]

colder temperatures = more dense air = higher fuel consumption.

i drop about 3-4 mpg every winter, then it goes back up in the spring.

[Mike Gerber]

First of all, 28-30 is too high IMHO, unless you are doing almost 90% highway driving. I averaged about 22 MPG in my 98 5SFE engine in city driving. That is in line with what you are now getting. That said, it really isn't cold enough yet to cause the kind of drop in fuel mileage you are experiencing. As the temperature goes down just a bit at this time of year, you are probably driving without the a/c off most of the time as opposed to on just a bit earlier in the Summer months. That should offset the slightly longer warmup times this time of the year. It will drop a bit more when the ambient air temperature really drops as we move more toward Winter.

If you really think your mileage has dropped that much there are things you can check. First check the air filter for being extrememly dirty; that one is easy. Check your tire pressures. Each drop of 10 degress in the ambient air temperture will drop the air pressure in your tires approximately 1 pound. Next check the engine coolant temperature sensor for being out of spec. It could be telling the ECU that the engine is constantly cold, when it is actually has reached normal operating temperature. That would result in a constantly richer mixture than is necessary, increasing fuel consumption. You will need a DVM and the specs from any manual to do this. The next thing to do would be to replace the thermostat. It could be opening slowly or it could be stuck in the open position. That could result in an engine that is warming up slowly and again result in a richer than needed fuel mixture for an extended length of time. Next check the brakes for dragging at all 4 wheels. Jack up the car (1 wheel at a time is ok if you don't have a floor jack and jackstands) and rotate each wheel by hand (with the e-brake off) to see if any wheel feels like it is dragging. The O2 sensors (you should have 2 of them) can also be out of range, but that most likely would throw a check engine light. I would leave checking the O2 sensors as a last thing to check.

These are the most common things that can cause decreased fuel economy, without noticing any decrease in engine performance.

Mike