• The site migration is complete! Hopefully everything transferred properly from the multiple decades old software we were using before. If you notice any issues please let me know, thanks! Also, I'm still working on things like chatbox, etc so hopefully those will be working in the next week or two.

Depressing Fuel Economy... :(

K

kingjohn1027

New member
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!
 


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.
 
dsmuts said:
what kind of plugs did you put in it?
Click to expand...

^^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.
 
FordMan77 said:
^^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.
Click to expand...

:( I got some PP606 plugs, but I haven't put them in yet. Should I take them back? I'm getting the same milage, but I just found out the connector on my MAF sensor is broken.
 


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.
 
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!
 


You might also consider a fuel injection cleaning and pay attention to the tire PSI. Also try a different gas station as I have found my mpg will fluctuate pending where I got the gas. Around me I get the best mpg with Shell.
 
Does anyone really think that spark plugs will decrease fuel milage by 32%?
I think I'll remove my iridiums and replace them with the cheapest platinums and see if this is true.
 
Last edited:
Rico said:
Does anyone really think that spark plugs will decrease fuel milage by 32%?
I think I'll remove my iridiums and replace them with the cheapest platinums and see if this is true.
Click to expand...

Platinums will work and the mileage difference will not be significant. 1-2mpg either way. But. Platinums get brittle (because of waste spark ignition) and break. In the engine. Where the party's at.
 


And now that I think about it... check your brakes. Make sure they move and groove all sexy. Once had a pad stuck on the rotor. Instant 17mpg highway.
 
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.
 
I don't know if its been said or not but alignment? Mine was out of whack but the car still drove straight and gained some mileage back after everything was in spec.
 
You are running 91+ gas right?

The only thing I can think of is maybe the front 02 sensor is not reading 100% correct but still not enough to trip an SES light.

Right now its getting hard to diagnois stuff over the internet, being there with the car and scan tool would be the best but that cant be the case.
 
You must log in or register to reply here.
Back
Top