I bought my 99 feb. 2002. at that time i was getting 20 - 22 mpg by nov of that year my mileage had dropped to about 18. i had read posts on this site that winter fuel could be the problem. however by summer the problem still existed and got worse. i have had the lift pump replaced and also about 4-5 months ago had the crank shaft pos. sensor replaced. as of right now im getting approx. 16 mpg. (reg fuel & oil filter changes all stock). dealer says no codes come up that might indicate a problem. i would really like some Ideas here Thanks.

 

WDS41,

I bought my '99 used in November of 2002 and the milage was 17 highway and 11 city. I found this site and everyone said check the lift pump with a fuel pressure guage. My readings were 7 psi at idle, 5 cruising and 0 WOT. I put a new lift pump on and now I get 19.5 highway and 14 city. Made a huge difference for me.

 

Replace your IAT sensor. They're really cheap. Or you could try cleaning it.

I typically get 15 mpg since I bought the truck with 99K miles on it, 16 tops. This was AFTER a new Lift pump and high flow banjos.

I just got finished changing my injector pump though and I'm hoping after the $1500 price tag I at least get 17 ???

If you don't have a fuel pressure gauge get one asap and make sure LP pressure never drops below 8psi.

 

Buying an IAT is not a bad idea, but you can clean the one that's in there and it will help if that's the problem. Take it out and using Qtips and brake cleaner, clean the thermistor inside the casing thru the casing vents. Make sure that the thermistor (the little blue bulb) is not touching the casing both before and after cleaning.

Also, make sure all of the intake pressure connections are good and snug (18ft/lbs if you have a torque wrench). That's all the hoses from the turbo outlet, to the intercooler and from the intercooler to the intake air horn plenum.

Finally, make sure the bolts holding the top of the intake manifold are torqued to 18 ft/lbs. Check them all. Any leaks in the charge air system will cause lower mileage.

Winter driving does come with lower mileage but mileage should return with warm weather. That's why I would check all of the above.

 

Before my engine went out my mileage kept slipping for the last two months. During the repair I replaced the manifold gasket and also replaced the IAT which was clean. I did that just sop with 240,000 on it. For the brief 1000 miles I drove it, the mileage was way up from the previous mileage. There was nothing internal that would have affected mileage, in fact after the repair the rings had not yet really seated and it should have been lower. I can only guess at which affected it the most or if both were below par.

 

You could also try resetting the APPS, that may help economy. The procedure is detailed in the tech facts section on this site.

 

[Also, make sure all of the intake pressure connections are good and snug (18ft/lbs if you have a torque wrench). That's all the hoses from the turbo outlet, to the intercooler and from the intercooler to the intake air horn plenum. ]



Did this on our "97, it made a huge difference in power. I don't drive that one so I can't tell you about the mileage but it has to have made some difference.

 

The trouble could also be that your local fuel supplier has dropped the cetane rating of the fuel he's ordering.
Is your state one that requires the Cetane to be labeled on the pump?.
Virginia stations require a minimum rating to be labeled on each diesel pump.
I notice a reduction in MPG when running 40 cetane vs 45 cetane.