Outer Limits 4x4 Board

What are the symptoms of underfueling on a diesel?

When i put my turbo on 6 months ago i havent screwed up the fuel, i left the settings as N/A. It doesnt blow much smoke, mabe a little white sometimes, but not much, and never black. The turbo pumps 7-8 psi max with no aneroid. Egts normally sit at 350c @110kph and rise to 450c max when pushing up a hill. 95 percent of my driving is at 110 on the freeway, so i dont want to screw the fuel up if i dont need to, however, sometimes it feels like it surges at full power (very slightly).

Oh, and its a 3.9 isuzu diesel (4bd1) in a 85 110.

Andy

You can't underfuel a diesel.

Well I guess you can, not enough fuel and it stalls.

Bingo.

"Correct" air fuel in a diesel for maximum power is 14.5:1, at which point, I am reliably informed the cars following you will crash due to the smoke you would be putting out, so , yes, all diesels run "lean" to a certain extent.

If ou tipped more fuel in you would make more power but your EGT's will go up.

You can't hurt a diesel by running it too lean, it will just make less power.

Steve.

Deisel definition - Complete combustion in an excess of oxygen.
They always run lean. Not a bit lean, LEAN. Foget the concpet of petrol motors and mixture.

if runnign stoichiometric or rich - the black smoke will kill others.

its possible to run too lean. one of the garages had a vechile come in with a stuck waste gate. anything over half throttle it just blew clouds of white smoke. your proberly looking 20lb+ boost to do it, plus some vechiles reduce the fuel when they overboost.

with 7-8 pound boost i wouldn't bother changing the fuel settings.

tweak'e wrote:its possible to run too lean. one of the garages had a vechile come in with a stuck waste gate. anything over half throttle it just blew clouds of white smoke. your proberly looking 20lb+ boost to do it, plus some vechiles reduce the fuel when they overboost.

with 7-8 pound boost i wouldn't bother changing the fuel settings.

There is something else wrong there. When diesels run really lean they can't build that much boost, there's not enough heat in the exhaust to drive the turbo.

yes/no. i was wondering about that.

its still getting fuel to build boost, it was making enough power to drive at open road speed to the workshop.i remember he said they found the wastegate acututor diaphram had split.

i think it was getting enough fuel to build boost due to the drivers foot , its just going lean to huge amount of boost.....mayby execess inlet temps due to turbo over revving ?? possibly EGR would have been on as well ??

i can't remeber what vechile, it was some sort of people mover.

If I'm right, white smoke = unburnt diesel = way beyond just "rich"

My guess is an electroniclly controlled diesel or a diesel with boost enrichment (fuel aneroid) with the wastegate fully shut, the computer is seeing a fault condition and doing somthing stupid.

In any case YOU CAN'T RUN A DIESEL TOO LEAN. they just rev slower if you pull fuel out and eventually stop. That is how diesels are controlled - they are fuel limited not air limited like a petrol.

Generally, diesels don't have butterflies in the intake and the throttle cable is hooked to the fuel pump - more fuel=more revs less fuel=less revs.

With a turbo, less fuel till equals less revs (and boost.)

(Yes, I know there are exceptions with both modern petrols running throttleless and diesels running butterflies, but stay with me on this)

Steve.

proberly right, i wonder if its white smoke due to way to much boost (compression) rather than being lean.
mind you this was a long time ago, i proberly have my wores crossed somewhere.

afaik white is not rich but rather imporoper combustion ie to cold, incorrect timing.

tweak'e wrote:proberly right, i wonder if its white smoke due to way to much boost (compression) rather than being lean.
mind you this was a long time ago, i proberly have my wores crossed somewhere.

afaik white is not rich but rather imporoper combustion ie to cold, incorrect timing.

Yep, black smoke is rich mixture, white is imcomplete combustion.

Layto....

lay80n wrote:

tweak'e wrote:proberly right, i wonder if its white smoke due to way to much boost (compression) rather than being lean.
mind you this was a long time ago, i proberly have my wores crossed somewhere.

afaik white is not rich but rather imporoper combustion ie to cold, incorrect timing.

Yep, black smoke is rich mixture, white is imcomplete combustion.

Layto....

x3 Whiite (or whitish/grey) smoke usually means retarded timing.

Andy - sounds like you need to set the timing (may be slightly retarded) - then its time to turn the little screw...