memphis_vette

hey guys i used to get 30-32 mpg. I just took a 4 hour trip and i was getting 24 mpg with ac off and about 20 mpg with AC on. this was at 70 mph. what could have made my mpg drop so drastically?

Bill Curlee

- Lazy or dead O2 sensors
- Dirty MAF sensors. (clean with MAF cleaner)
- Vacuum Leak
- Clogged CATs (effects WOT Power also)
- Clogged/dirty Airfilter (loss of power also)

Data Log your engine and see what the LTFT and O2 sensors are doing.

BC

memphis_vette

ok ill give it a shot

memphis_vette

ok so far i have cleaned my MAF and cleaned my Blackwing intake. can i clean my o2 sensors or would i have to just replace? i looked for vacuum leaks but havent found any. i dont have any codes. how would i find out if my cats are clogged?

87SAM

As Bill mentioned, with clogged cats you should notice a lack of higher RPM performance. To check them I think you would have to measure the before and after cat pressure deference.

memphis_vette

ok thx. i have not noticed any loss in power at all. and the mpg drop basically came out of nowhere

Bill Curlee

You should have someone datalog your car. There are several ways you cn do that.

Use EFI Live, HP Tuners or purchase a aftermarket scanner that has real time data view and it will display O2 sensor output. I have a ACTRON CP9180 for my 08 Jeep and it will read any OBDII or CAN Buss PCM /ECM.

Data logging doesnt need a licsense so if you find someone with EFI Live or HP Tuners to do it, it normaly wont cost you anything. Well, maybe a beer or two.

My scanner was only $130. and well worth the cash. Its a very useful tool. I also have EFI Live for my C5 and C6

You cant not really clean an O2 sensor. Once they wear out, stick a fork in it,,,,Its DONE! If it is not switching properly, thats your issue.

My son purchased a used Pontiac Grand AM yesterday. He had a LEAN BANK 1 DTC after he hammered it. The ACTRON Scanner showed bank 1 O2 sensor hanging out at .450 mv. The other O2 sensor was cycling properly 150-850 mv.



Bill

memphis_vette

pm sent bill

Warp Factor

Bill, how will a dirty air filter cause a loss of fuel economy in MAF sensor controlled engine? The filter would reduce air flow, but I'd think that the MAF would sense the change in air flow, and reduce fuel accordingly.

QCVette

Good question. I have the same thoughts that it would reduce air flow and because of that peak power at WOT, but at part throttle it would just take a little more throttle for the same performance, while the sensors would keep the mixture correct and therefore not changing the mileage.

Is there something we are missing?

FixedRoof

On a 98 if the O2 sensors are original, it's time to replace them. A lazy O2 will cause bad gas mileage. Just the front o2's as the rears are really not important.

memphis_vette

thank you i will switch them out on monday.

FixedRoof

Not saying that it is your problem, but a good possibility and a maintenance item as well with a bunch of miles.

memphis_vette

yup i have never changed them. its needed any ways

Bill Curlee

I drive my car in the winter (when theres no snow) and the filter got damp and froze. Traction in the cold sucks so I wasn't testing WOT. The car had a fresh fill up and the average fuel mileage meter was recently set to zero.
Normally around town mileage in my 98 coupe was 15-17. I wasn't able to get more than 12-13 and throttle response was not as crisp. It was 20 degrees out so I thought it due to the cold but it had never performed like that in the past when it was cold out.

I opened the hood and noticed that the accordion tube between the plastic duct and the TB was sucked in and thats when I checked the filter. It had ICE on it and it was frozen solid.
Thats what happens when you cut open the air box for more air and don't have a hood seal and it rains a couple days before.

You can relate that to a very dirty air filter.

memphis_vette

lol very nice BC

Bill Curlee

Hey,,,My IAT's were very cold!

Jeffwilson34

I noticed a 5MPG drop all of a sudden a few weeks ago.
I also noticed the slight smell of oil.
the 1st thing I checked was the PCV valve hoses and found mine broken where the Valve connects and goes from a large hose to a small one.
I could not find a direct replacement so I jerry rigged one up with fuel line and hose reducers.
My gas mileage picked back up and maybe a bit better.
I averaged just over 30mpg from Tennessee to Daytona.
Plus while I am here in Daytona in stop and go traffic and sitting in the car with the ac on while the wife goes in small shops I am still averaging 23mpg.

memphis_vette

ok ill take a look

dirty-c5

Its more than likely the front 02 sensors. If they are original and the car is a 1998 its definetly time to change them out.