Bad gas mileage lately...
 

Hey guys my 97 C5 just turned 100k miles... I used to average like 30mpg on the highway @ 75mph, lately I've only been getting around 24mpg or so!!! Do you think the cat could be clogged, its stock im assuming.
I have a cold air intake so I know the filters are pretty much clean as well.
Could it possibly be our gas as it contains more E10 Ethanol? Thanks guys...

Start by replacing the fuel filter.:thumbs:

Most fuels now have 10% etoh in. Stations dont have to advertise or post this on the pumps

That is a big contributor to lower fuel milage

Clean the MAF (simple to do, run a search) first. Changing the filter is a great idea as well if its never been done. :cheers:

true, but we have had 10% ethanol here for ever and we are still averaging 27-30mpg on the highway. o2 sensors could be a big thing, dirty fuel injectors, clogged cats like you said, but you should have a smell i think if those went bad? any DIC codes

:iagree:

I would try the really good gas stations - Like Mobile, Shell, Texaco etc.

Yes they will be a little more but the fuel mileage is much better, I tried both and Mobile seems to be the best for my Vette:D

Plus all I can get in California is 91 Octane and that is why I run Methanol too while under B@@ST keeps the Octane higher and cools the intake charge:D

Thanks,Matt

what is MAF?

Mass Air Flow, do a search on cleaning procedure. :cheers:

I have descreened my MAF about a few years ago, I will do a search on the cleaning process though. I know the two wires that connect to the intake temp sensor don't have the best connection, but its not throwing any codes at the moment... The fuel filter is brand new, and it seems like its not getting anywhere near the gas mileage it should. Within a short 10 mile drive i used an 1/8th tank on the highway, that doesn't seem right!

Could be 10% alcohol in your gas. Try a different station.

10% ethanol shouldn't take you from 30 to 24. I hardly notice a difference. Does your instant MPG also read lower than it did before? And, dumb question, but are you actually figuring out your real MPG by miles driven/gallons filled up or are you just reading the DIC?

Great suggestions made above. Also make sure to check your tire pressure.

Thats my mpg just going by the dic. It seems as if it is doing even worse than that...
I did remove the intake and clean the filters along with the maf. It looks pretty clean on the inside, so I'm not sure what the problem is... The tires all have 30 psi in them as well. Not to mention I seafoam the top end once every year as well to clean out the gunk!

Another suggestion:

Don't know when you last changed plugs but I think the originals are platinum 100k plugs so now would be the time if you still have the original plugs installed.

..rickko..

thanks rikko, i put platinum +4's in it around 75k miles as well!

I'm having the same problem. I've done a search on how to clean the MAF, come up with nothing. I'm sure its very simply. But, can someone tell me the exact procedure or post a link for cleaning the MAF

Thanks, Allen

:iagree:

Plugs about evey 50,000 :thumbs:

There's a thread on this somewhere in the C5 section

Check your plugs anyway with 25,000 miles...

All the above but most important

KEEP YOUR FOOT OFF THE FLOOR !!

My gas mileage went down drastically recently, 0n my 'o2 vert... not a lot of miles on it, but still 6 years old.... replaced the plugs/wires, and it runs like a new car again! Gas mileage right back up, and the kind of power it had when new.....

My mileage has also dropped. I put headers on it and right after that I notice the mileage drop. Tried new plugs and wires. The air filter is clean. Fuel filter would not reduce mileage.

I had Corvettes of Houston install the headers and they adjusted the computer. The car did not run quite right for a while and when I start up in the morning, the exhaust does have a smell. It seems to me my proformance has not really improved much, but I do get less mileage. I'm wonder if Headers and exhaust was a wise $3,500 purchase.

Live and learn, not every bit of advise you get, is not good advise.

:(