Gas mileage TERRIBLE - F-Body Online

copekyle · 02-01-2008, 01:37 PM

ok... here's my problem. I've had this car 6 months and I've always gotten ok gas mileage. Usually about 280 to 350 on a tank. And that's pretty standard city driving. Well the I started noticing about two fills up ago. I only got 250 on that tank. This one I'll be lucky to get 220 out of it. The spark plugs and coil were replaced the day I bought the car. K&N filter has only been in for about 4000 miles. Other than that... I'm not sure what else would cause such a great decrease in gas mileage... any ideas???

Thanks in advance!!
Kyle

Phate · 02-01-2008, 02:09 PM

Lead foot.

FBC Performance · 02-01-2008, 02:55 PM

[quote=copekyle;12140]ok... here's my problem. I've had this car 6 months and I've always gotten ok gas mileage. Usually about 280 to 350 on a tank. And that's pretty standard city driving. Well the I started noticing about two fills up ago. I only got 250 on that tank. This one I'll be lucky to get 220 out of it. The spark plugs and coil were replaced the day I bought the car. K&N filter has only been in for about 4000 miles. Other than that... I'm not sure what else would cause such a great decrease in gas mileage... any ideas???

Thanks in advance!!
Kyle[/quote
This is from one of my techs. How many miles have you put on the car since you bought it and put new wires and plugs on it? Check the coolant temperature gauge. How long has your fuel filter been on the car? GM says you shouldn't need to change spark plugs and wires till you have 100K on the car. My tech says that's not reliable. You may want to consider a tune up. Have you changed fuel types? 87 to 91 or 91 to 87? That could have a small effect, but not big enough to cause the problem. Or maybe it's a little of everything.

Phate · 02-01-2008, 03:01 PM

Quote:

Originally Posted by FBC Performance

Quote:

Originally Posted by copekyle

ok... here's my problem. I've had this car 6 months and I've always gotten ok gas mileage. Usually about 280 to 350 on a tank. And that's pretty standard city driving. Well the I started noticing about two fills up ago. I only got 250 on that tank. This one I'll be lucky to get 220 out of it. The spark plugs and coil were replaced the day I bought the car. K&N filter has only been in for about 4000 miles. Other than that... I'm not sure what else would cause such a great decrease in gas mileage... any ideas???

Thanks in advance!!
Kyle

This is from one of my techs. How many miles have you put on the car since you bought it and put new wires and plugs on it? Check the coolant temperature gauge. How long has your fuel filter been on the car? GM says you shouldn't need to change spark plugs and wires till you have 100K on the car. My tech says that's not reliable. You may want to consider a tune up. Have you changed fuel types? 87 to 91 or 91 to 87? That could have a small effect, but not big enough to cause the problem. Or maybe it's a little of everything.

Maybe a fuel filter, but I doubt anything making that kind of difference would be spark related. Otherwise you'd have P0300's out the ass.

FBC Performance · 02-01-2008, 03:22 PM

Quote:

Originally Posted by Phate

Maybe a fuel filter, but I doubt anything making that kind of difference would be spark related. Otherwise you'd have P0300's out the ass.

What about coolant temp gauge?

FBC Performance · 02-01-2008, 03:31 PM

Quote:

Originally Posted by Phate

Maybe a fuel filter, but I doubt anything making that kind of difference would be spark related. Otherwise you'd have P0300's out the ass.

Ok again from a tech who is a v6er. Take it to a shop and scan coolant temp sensor and O2s checked.

Phate · 02-01-2008, 04:37 PM

I can see coolant temp being probable, but how would 02's throw off gas mileage that much? If the 02 is out even the smallest amount, yet again, your going to throw codes. If the car isnt throwing codes (wasnt mentioned), then fuel filter is still my only idea. Other than the typical Lead Foot

copekyle · 02-01-2008, 05:24 PM

ya ya.... lead foot! I always have a lead foot though... so that's not the problem! I was under the impression that if the fuel filter needed changed then it wouldn't put enough gas into the engine, not too much? It is throwing no codes and doesn't seem to be running any differently. Can someone tell me how the coolant temp would affect gas mileage?

Kyle

Phate · 02-02-2008, 05:32 PM

Quote:

Originally Posted by copekyle

I was under the impression that if the fuel filter needed changed then it wouldn't put enough gas into the engine, not too much?

Thats correct, but its not running rich. If it was running rich, you'd have VERY smelly and cloudy exhaust, and it would misfire. If its running lean and the filter isnt letting enough gas through, the computer will not know it, and continue to feed alot of air to very little fuel. Best way to check a fuel filter, is to take it off (takes 10 mins tops, its located under the car on the fuel line right in front of the drivers side rear wheel) and blow through it, if you get any restriction, then its time for a new one.

FBC Performance · 02-05-2008, 10:31 AM

Quote:

Originally Posted by Phate

I can see coolant temp being probable, but how would 02's throw off gas mileage that much? If the 02 is out even the smallest amount, yet again, your going to throw codes. If the car isnt throwing codes (wasnt mentioned), then fuel filter is still my only idea. Other than the typical Lead Foot

An O2 sensor can get "lazy" which its slow to read from lean to rich. If this is the case its not always going to throw a code since for the most part the PCM is just looking for the voltage to cross over the .450mv threshold. So if its taking longer to make the transitions from lean to rich or vice versa the pcm is throwing extra fuel to make it get to the rich side. They don't really give the PCM a time frame for the switching to occur.