i have a 99 3500 dually 4x4 5.9l 360 (yes underpowered i know) . i can afford the gas but it is just rediculous. things is know..
1. needs plenum repaired bad( have done three times so no problems there ittle get done)
2. just bought it and no regular maintaince has been done what so ever.


so my question is how much is it going to improve the mpg if i do the plugs, wires, cap, and rotor fix the plenum change oil, air filter, psi in tires da da da..... also what else can i do to improve the mpg? any help would be appreciated!!!

 

If your going to fix the plenum you should replace the plate as well, so you dont have a re-occurring problem

 

Check your CAT converter and your O2 sensors.

 

There's the problem right there.

 

ya ill get the aluminum plate for hughes. how do i actually check converter and o2 sensors to see if there working? how many mgps will this add?

 

Don't check the o2 or the cat, replace them, simple as that. The blown plenum ruins them both. Gut the cat or get a Magnaflow direct fit and replace the precat o2 with an NTK from Rock Auto.

 

A 1500 with that engine will get anywhere from 12-14 MPG in town, typically. On the highway, 14-16 MPG typically.

Since you have a larger, heavier truck than a 1500, you can never expect more than that (and it will most likely always be less).

How many miles on the truck? If over 60k, I'd replace the pre-cat sensor(s). NGK or Denso brand sensors (cheapest place to find them is Rock Auto), don't use Bosch or Standard brand.

Also, use a Felpro gasket with that Hughes plate. Very beefy, much higher quality gasket than the Hughes gasket or the stock unit. Can be found at most auto parts stores.

Hit the cat with a rubber mallet. If you hear rattling inside, the cat is bad. Whether or not it is bad now, the blown plenum will have hurt its life expectancy pretty badly, so go ahead and replace it regardless or have one handy. Magnaflow high-flow cats are the best for our trucks.

 

You dissin' Standard Motor Products brand? ; And just for a FYI, the cat does not have to rattle to be bad.

 

how much will doing all this help?

 

maybe 2mpg if you can keep your foot out of it with all the new found power.