Gas mileage

[trainman]

I have a 98 Dodge Quad Cab 1500 4x4 with 5.9 engine. I am only getting 8.8 mpg out of this truck. I have replaced the plugs,wires,rotor,cap and torque convertor. The truck runs fine, just no gas mileage. Any suggestions on what to do now?

[sadlerb_98]

I have on 01 Dodge Quad Cab with a 5.9 and was gitting 9 mpg in town and 11 on the highway. I replaced the exhaust with a cat back flowmaster, changed to synthetic oil, and put a drop in K&N air filter. I now get around 12 in the city and 14.5 to 15 on the highway.

I ordered a power programmer III and it has not arrived yet. That may help a little. I will let you know.

Good luck.

p.s. check your tire pressure at least once a week and make sure your coolant is good.

[rksnc]

Yea love Dodge but never been the one for the best gas mileage out there.

[fredjacksonsan]

Check your O2 sensor. They've been known to go bad and make for bad mileage, causing your truck to run excessively rich. Check the inside of your tailpipe after driving for awhile; if wet and black, it's likely running rich.

My brother's 99 V10 was getting only 6-7 mpg; once he replaced the O2 sensor, he went back up to the (for him) relatively thrifty 12 mpg.

[sadlerb_98]

trainman...how is your MPG doing? The power programmer helped out. 14.75 highway 12 something city. let us know.

[Blylock]

i get 13-14 highway, 7-8 towing.

[BleedDodge]

On this '99 I just bought, I removed the post-cat sensor and the mileage hasn't changed. The check engine light came on but the truck still runs, performs, and burns the same amount of fuel as it did before.

[shogun88888888]

Im reading some of these old treads because sometimes i find some new useful information or trick someone has discovered. Thes forums are great, however, with all these gear heads in here and not one comment on mpg having to do with the obvious. Intake and exhause mods are great but 1) tires, keep them inflated properly, how many people actually know how much air pressure to use for highway, city, or off road driving. The higher your air pressure the better your mileage but dont exceed the tire rating. Lower pressure for offroad for better traction but your mileage will suck. 2) Check the most important factors even if there is no check idle indicators on. Air filters, pcv, crank breathers, o2 sensors, map sensor, crank temp sensor, etc. These tell the computer how much fuel to burn. Even if they are working fine a dirty/old sensor can give false readings and give poor mileage and cause lack of power. Dont forget to check the hose if its pcv valve you are replacing. A new clean pcv valve is great but not if your connecting hose is clogged.

[neon_rt]

The 5.9l has a reputation for the lifter valley gasket to leak, this allows heat from the engine to heat up the intake manifold. This is known to reduce mileage b/c the IAT sensor sees the hot intake temp and the computer retards the ignition timing to prevent knock.

[shogun88888888]

Another great point, hot manifold. There is a sensor in there too, sensor is good but giving incorrect reading for actual enjine condition. Im not positive but i think the haynes or chilton books give a test for most of these sensors and what the normal reading should be. If the sensor is not putting out the normals reading/output that it should, check the sensor and if ok check other factors like gaskets or a part that is too hot that would throw the sensor off whack. Another good one is a crack or rust by the o2 sensor letting fresh air in past the o2 sensor which will cause poor mileage as well because the computer the computer is pumping more gas into the mixture. I believe with the Ram its something like 14:1 air fuel ratio or something like that the computer tries to achieve.

[trainman]

Originally Posted by shogun88888888

Another great point, hot manifold. There is a sensor in there too, sensor is good but giving incorrect reading for actual enjine condition. Im not positive but i think the haynes or chilton books give a test for most of these sensors and what the normal reading should be. If the sensor is not putting out the normals reading/output that it should, check the sensor and if ok check other factors like gaskets or a part that is too hot that would throw the sensor off whack. Another good one is a crack or rust by the o2 sensor letting fresh air in past the o2 sensor which will cause poor mileage as well because the computer the computer is pumping more gas into the mixture. I believe with the Ram its something like 14:1 air fuel ratio or something like that the computer tries to achieve.

I replaced the PCV and now my mileage is great, 14.2 city and 17.8 highway.