Thread: Can a diesel engine blow black smoke at idle.

Blocked air intake or exhaust. As a lad I tried to stall an idling diesel by jamming my boot on the exhaust outlet of a mazda t3000 truck. All I got for my pains was a big blast of soot on my overalls.

Yes it can but by rights it shouldn't:

it implies overfuelling at idle which could be difficult to achieve on a healthy motor.

So either the pump timing is completely wrong, the pump is delivery too much fuel, there is an air obstruction in the inlet, there is an exhaust obstruction, or my personal choice, the compression is down and you've got partial combustion.

Last option, and only on a turbo diesel: turbo has siezed or else the hoses have delaminated, but these both count as a restriction.

No, unless as bee utey said there is a restriction or way too much fuel which will probably result in an increase in idle speed where things will once agian balance out the excessive fuel.

Diesels run no throttle butterfly to restrict air input and run excess air compared to what is required for stoichiometric combustion. Black smoke is a sign of excess fuel, at idle the excess air will be able to manage any excess fuel. Untill you start pouring the fuel in it should not generate black smoke.

No idea. But I used to work with some guys who's repertoire of practical jokes included a banana up the exhaust pipe.

Diesels produce measurable soot at idle.

Normally adding fuel with no load would mean more rpm, not more soot (which is I assume the point of your post). However it is conceivable that if the injectors are worn and injecting especially large droplets then you could get sufficient soot at idle.

Originally Posted by isuzurover

Diesels produce measurable soot at idle.

Normally adding fuel with no load would mean more rpm, not more soot (which is I assume the point of your post). However it is conceivable that if the injectors are worn and injecting especially large droplets then you could get sufficient soot at idle.

Or if you have dropped a cylinder from a burned out or not quite sealing valve.

Had me stumped the first time I saw it on a Kubota stationary engine as you would imagine the unburned fuel to be white but it is partially burned fuel which generally goes with revs.

That one was a puff puff puff sort of thing but sounded smooth but was rather low on power.

I'd imagine that it would be less possible on an engine with pre combustion chambers.

Since then I have seen a couple of worn out tractors and possibly 1 or 2 trucks do similar things but I would not be able to tell you what was the cause as I didn't always do the fixing of them.

Cheers Casper

some interesting answers.

heres some clues.

in theory no, a diesel cant produce black smoke at idle (remember Idle for a diesel represents work load not engine RPM)

In reality however yes, a diesel can produce black smoke at Idle RPMS.

so now the question becomes, Why cant a diesel produce black smoke at Idle (perfect world answer)

What are some of the things that can happen to a diesel to make it blow black smoke at Idle? (real world answers)

some of the answers to both parts of this version of the question have already been provided.

Easy. It's hard for them to smoke at idle because there's soo much excess air.
It's easy to make them smoke visibly at idle, leave a rag in the air-box.