Engine/Transmission (1994 - 1998) AFC settings

[DEddie]

What controls (activates) the AFC? I am experiencing overfueling at low RPM, as soon as the boost builds along with the rpm the black smoke clears up.

This just started a while back, the truck has 235 k on it. Why would the AFC need to adjusted only now??

 

[NHays]

What have you done to the truck, check for leaks in the intake system. you might have a tear in a boost elbow. and what RPM are you talking about. and Boost activates AFC. more boost=more fuel, no boost= little fuel.

 

[pwerwagn]

The AFC has two controls, spring and boost pressure. The Boost pressurizes a diaphragm which has some resistance placed on it by the spring. tightening the spring causes the afc to require more boost to displace the afc foot the same distance. loosening the spring does the opposite.



I personally dont think the afc can come out of adjustment where it would cause more fueling. the only failures I can think of would relate to less fueling. It sounds to me like its not an AFC problem, but a boost leak somewhere, or maybe a wastegate that is partially stuck open and allowing the exhaust energy to bypass the turbine for a while.



Do you have a boost gauge??



--Jeff

 

[bnelson]

I don't know of it happening before but the spring behind the star wheel inside the AFC may have broken. Or the spring loaded indent that keeps the star wheel from moving may have gone away. Remove the plug on the AFC that accesses the star wheel and make sure the spring has at least some tension to it. If it does not the star wheel may still be adjusted to make your truck drivable. To put more pressure on the spring will lessen the smoke.



Any leaks in the boost pressure lines would give you the opposite problem, no smoke and very little power.



Brad Nelson

 

[pwerwagn]

Any leaks in the boost pressure lines would give you the opposite problem, no smoke and very little power.

Unless the AFC is adjusted somewhat loose to begin with, or the foot has been moved foward in the housing. I know on mine when I get boost leaks, it smokes horrendously.

 

[DieselDoghouse]

Now I'M confused - a boost elbow leak (or elsewhere downstream) should air-starve the engine, while the AFC (if its still getting pressure) thinks its ok to load in fuel: black smoke, lower power, NO??

 

[pwerwagn]

I think it all depends on how the AFC is setup.

Im not really sure than the line leaving the afc would cause enough of a leak to make a dramatic diff. I had a boost gauge line come off once and it didnt make a drastic difference with that line just pumping into the atmosphere...



--Jeff

 

[Dieseldemon]

Any leak in the intake sys will cause the engine to smoke and the egts to go sky high the smoke is from unburnt fuel from lack of air (Turbo Boost) Low power = lack of air. Check all your turbo hoses for a cut or blown clamp the one on the pass side on the intercooler for a cut take a pair of water pump pliers and bend the inter metal panel away from the hose