93 smoke with 99% biodiesel

[Sri Ram]

My buddy's 93 (club cab/chassis, 4x4, 4. 10, 80+gal fuel tank, stock engine/inj pump) smokes on soy biodiesel, he says especially at 2000+rpms, even at opterating temp. Runs "normally" on #2…Air filter looks clean…Why the smoke? Any thoughts appreciated. (Also posting in 1st gen. forum)

 

[JoeBioDiesel]

What color smoke?

Joe

 

[Sri Ram]

He says black (=unburned fuel?)

 

[JoeBioDiesel]

Black smoke on Biodiesel, with no black smoke on #2 doesn't make sense. B99 normally makes about 70% less smoke with the engine running at it's smoke limit. I've seen numbers close to that regardless of the combustion chamber type in every diesel engine I ever fooled with. I've seen Bio make white smoke on older diesels with lower compression ratios(even seen an old Oliver tractor that wouldn't start on it), but I've never seen black smoke simply from switching fuels.

Is he seeing visible smoke or is he just loooking at the tailpipe and seeing the soft black soot building up at the tailpipe tip, and assuming it's smoking more?



The only other thing I can think of, is if the injection pump or injectors are a little worn, switching to Bio improves injection efficiency on some engines. Bio has a high bulk modulus(it's less elastic than #2 at injection pressures). It is pumped more efficiently by the injection pump, and is delivered to the combustion chamber sooner. This results in advanced timing, one of the reasons for Bio's higher NOx emissions. If you think this might be it, lower the smoke limit a little and see if it clears up without a large power loss. If he has his pump timing bumped up already, try backing it off to stock settings and the smoke should go away.

Good Luck,

Joe

 

[Sri Ram]

Thanks, Joe. I'll try to get more information and get back to you. As I stated at the top, his pump appears to be stock (all caps in place, no evidence of tampering). I run the same fuel (same producer and pump) in my truck and have been for a couple of years. I've seen very little smoke except on cold starts, never any black smoke. Just in case: is smoke necessarily a bad thing when running biodiesel?

 

[JoeBioDiesel]

If it's smoking under load at higher RPMs with a stock pump/injectors then something isn't right, and there just might be something to worry about. If it were mine I'd check the pump timing, pull the injectors and pop test them, and check the compression while I had the injectors out... but then I've got all the stuff to do that. I think he should try to figure out the problem with the truck, because most likely that's where the problem is, not with the fuel.



Joe