Hey guys and gals new to forum.
Have read several posts can't seem to find one that fits my truck.
1999 1/2 ton - 5.9 motor
new map sensor
new o2 sensor
new plugs
new cap and rotor
new plenum gasket
truck has 160000 miles on it
my snap-on scanner shows no codes
Dodge hand held scanner shows no codes
engine has 145 psi compression
scanner shows downstream o2 sensor reading rich running down the road
Truck was getting 16 mph on hwy, 14 mph in town
Now 12 mph on hwy
also running K & N air cleaner
MotorVac- engine (cleaned injectors)
also ran seafoam in fuel

Any guesses?

 

Ant fluid changes?

 

Any fluid changes ?
meaning coolant, oil, etc.
No - last oil change aprox. 4,000 miles ago

 

Yep thats what I mean.

 

How well does the engine warm up? Thermostat working ok? Not warming up enough could lead to a rich condition...

 

death flash maybe

 

temp on scanner on hwy was 88 celsius or 190.4 farenheit

 

I don't think the death flash would have anything to do with it. If anything it would get you more mps.

 

Have you recently changed where you are getting gas?
Possibly your station has started using an ethanol blended gas?
How about your driving habits?



This is going to sound off the wall, but here goes: Just here lately, have you been under stress or anger? I know crazy, but that will affect your driving habits and you might not even notice it...



Also, what kind of plugs did you go to?

 

Clogged catalytic converter?