07 pontiac wave help! (gas mileage has dropped)

[stanzz]

i've been getting really lousy gas mileage for a while now, can't figure it out. i get around 350km per tank! iv'e done the plugs, then a cai, and then the wires, all for nothing because my mileage remained the same. there is no cel, but for kicks had the guys at a local garage hook it up to a code reader..... and nothing. i'm banging my head around trying to think what could be wrong? are there any problems that can occur without tiggering a cel? any suggestions? oh, i did notice that my car had burned or leaked a few quarts of oil from my last oil change, about 3 months ago. there is no leak at the drain plug. help!

[Pickles]

When you say you get 350Km per tank, is that from filled to the brim to gas light? Because thats still 21mpg and isnt unheard of if you drive only city and are a leadfoot

[stanzz]

well, from full to about 1/8 of a tank. i drive city and highway, and i pamper it like it was my baby. btw i used to get around 450km, and with a cai, it should be around 500. ???????

[jayman]

colder weather may play a role in this also. mine dropped a little when the temp dropped.

[Pickles]

I don't know why you think a cai will get you better mileage either. It actually gets a little worse...

[avguy]

... oh, i did notice that my car had burned or leaked a few quarts of oil from my last oil change, about 3 months ago. there is no leak at the drain plug. help!

Losing a "few" quarts of oil in 3 months would normally be a lot bigger problem than poor gas mileage (although the 2 could very well be related symptoms from the same problem). Do you see any oil on the floor, in the engine compartment, or is there any blue smoke? What's the history of the oil loss (sudden, or slowly increasing over time), and also the full maintenance history of the vehicle.

[MetroMPG]

Pickles had a really good point: it would help a lot if you were more accurate with your fuel consumption "data".

It's not very accurate to say "X km to the 1/8th mark" on your gauge. Ideally you should keep a fuel log where you completely fill the tank (every time - until the pump shuts off), and make a note of your exact distance covered for that amount of gas on the receipt so you can keep track of your L/100 km or MPG.

Keep a record on your phone, spreadsheet or track it with a site like Fuelly or EcoModder.

Jayman is right too: onset of winter can play havoc with fuel economy. It's normal for it to fall 10 or even 20% compared to summer mileage, depending on conditions & your specific usage.

[MetroMPG]

Aside from points above...

are there any problems that can occur without tiggering a cel?

Off the top of my head:

Low tire pressure (been keeping an eye on them as the ambient temps drop?)

Alignment?

I once had a vacuum leak that caused a rich condition (much worse mileage) and it didn't throw a CEL. Soot was visible accumulating at the exhaust tip.

[Scanman]

I once had a vacuum leak that caused a rich condition (much worse mileage) and it didn't throw a CEL. Soot was visible accumulating at the exhaust tip.

This happened to me, also. A small vacuum line on the intake manifold had broken. It was costing me about 10MPG.

[stanzz]

i do sometimes get a burnt oil or burning wax smell coming through my vents when the car is it idle after driving.

could cam timing be the problem?