Thread: Depressing Fuel Economy... :(

I drive a 1999 GTP. Engine has 105k miles. U-bend delete and CAI are the only modifications. I run conventional oil. I have been paying close attention to my mileage since the fuel prices are on the rise again. My past 5 tanks have been averaging a mere 17.6 MPG. That is 85% highway driving at 60 MPH with very little traffic. The mileage was calculated by dividing the mileage on the trip odometer by fuel used, not using the information center. The engine performs flawlessly and runs great. No rough idle or anything. I have changed the fuel filter, cleaned the throttle body, replaced plugs and wires, scanned for codes, checked for fuel leaks, checked fuel pressure, checked compression, and checked tire pressure. Everything was normal.

With that being said, from the research I have done on this site, the mileage I am averaging is not normal. Does anyone have any suggestions on what I should check next? Last summer I was averaging 25 with the same driving habits. Also, can anyone offer any suggestions to improve fuel economy? I have heard switching to synthetic oil could improve economy. Would probably be a good idea to switch anyways with 105k miles. Thanks!

Weird mine, when i had my '00 GTP, got around 25 average combined. 30 highway. Could have o2 sensor(s) going bad.

Try cleaning your maf sensor? I've read that it can help get a little better mpg's. You also get worse mileage in winter months when it's colder than you would in warmer weather.

Originally Posted by dsmuts

what kind of plugs did you put in it?

^^THIS^^

These motors do not like platinum plugs, and Bosch in particular. Besides cleaning the MAF the only other thing I can think of would be the EGR. Maybe run your tires at a few higher PSI (go by the rating on the tire, not the door sticker). I usually keep mine at around 35 on my Kumho's.

Well, if a PP606 is a Platinum plug, then yes take them back. Most here recommend either AC Delco's (stock replacement), Autolite, or NGK. A good copper or irridium plug will work much better in our type of waste spark ignition system.

Here is some info to help you out....

4) Spark Plugs - Change out those stock spark plugs with new ones to restore any lost power. Brands that most tend to go with are either Copper Core Autolites or NGK Spark Plugs. Get these at your local auto parts store also. They range as follows in order from hottest to coldest plugs...remember...you want the hottest spark possible...so running a super cold plug on a larger pulley to combat knock isn't the best plan...ACD's, AL606's, and AL605's are basically for stock cars or very small bolt ons still running the stock S/C Pulley or modded N/A guys...you want to keep a hot spark. NGKTR55/AL104's for your 3.4" Setup, TR6's are a tad cooler...best used around the 3.25" pulley area, and AL103's...are for the extreme supercharged application...3.0" pulley or lower. Here is a picture to reference the firing order and the correct coil number if yours aren't numbered or you just need a quick reference picture: http://i33.photobucket.com/albums/d6.../plugwires.jpg

AC Delco Factory Replacements: Stock
Autolite 606: Stock
Autolite 605: 1 Heat Range Cooler
NGK TR55: 1 Heat Range Cooler
Autolite 104: 2 Heat Ranges Cooler
NGK TR6: 2 Heat Ranges Cooler
Autolite 103: 3 Heat Rangles Cooler

Yep, take em back if you can. I run the NGK TR55's on my 00 GT and it runs just fine, and the last time I pulled all 6 plugs w/ about 15K on them all they needed was a cleaning and re-gap and they keep on tickin. Nice smooth idle and get about 410 mi. to a full tank, but 85% of my driving is highway at 65mph.

You could try changing the PCV. And maybe checking your vaccuum(spelling) lines for a leak.

I changed the PCV valve today, also changed the spark plugs and wires. I installed NGK iridium. Also changed the oil to mobile 1 synthetic. I cleaned the mass airflow sensor as well. I disconnected the battery for two hours to reset the computer while I changed everything. I will report back with the mileage. Thanks to everyone for the help!

UPDATE

I had new tires installed and had the tires filled with nitrogen. I also switched from NGK iridium plugs to Delco iridium. I had the transmission checked for binding on the shifts. I checked brakes, rotors and wheel hubs. All checked out free of binding. I checked for vacuum leaks. None were detected. My wife and I just got back from a road trip from Norman Oklahoma to Houston Texas. Roughly a 1000 mile round trip @65MPH with the cruise control. Sadly, I barely maintained a 24MPG average. I really don't know what else can be done at tis point. I have tried everything I could think of as well as all of your suggestions. 24MPG is far better than the 17MPG I was getting before. However, a friend of mine who also has a 99 GTP, Bone stock like mine, but over 270k miles. He is averaging 27.5MPG. Just doesn't seem right.