Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 08, 2025 ...

Excess Black Smoke Coming from 8.3 Engine

November 17, 2010, 08:06:17 am

This is an FYI;

I have been experiencing more than normal black smoke coming from the exhaust when the engine is under a load.  I had my nephew, who is a diesel mechanic check and he found a break in the exhaust between the turbo and the pacbrake assembly.  This allowed air to escape.  Less air means more fuel, more fuel means more black smoke.
This is on the Cummins 8.3 mechanical engine.

Quote Selected

Re: Excess Black Smoke Coming from 8.3 Engine

Reply #1 – November 17, 2010, 09:16:53 am

A fairly common issue, particularly on the older one piece exhaust manifolds.  The move to the multi- piece manifolds allowed more movement with the heat/cool down cycles without breaking.

Not all exhaust leaks are caused by a broken manifold.  Many times it is a leak between the head and manifold.  If this is the case, many times the manifold can be re-surfaced and will actually be more reliable than a new one piece manifold, as it has been "cured".

Brett Wolfe

Quote Selected

Re: Excess Black Smoke Coming from 8.3 Engine

Reply #2 – November 17, 2010, 09:45:09 am

And means less exhaust braking. . .

Quote Selected

Re: Excess Black Smoke Coming from 8.3 Engine

Reply #3 – November 17, 2010, 09:55:32 am

I'm certainly no diesel expert and usually a little bit thick but I like to learn. I understand the leak can cause less exhaust breaking, but with a leak in the system between the turbo and the exhaust break, the exhaust side of the turbo, how is the fuel to air ratio affected?

Quote Selected

Re: Excess Black Smoke Coming from 8.3 Engine

Reply #4 – November 17, 2010, 10:10:53 am

Kent,

You are on the right track.

An exhaust leak "upstream" of the turbo reduces boost leading to less air in the combustion chambers, lower HP and more smoke.  Potential leaks can be at head to exhaust manifold, manifold itself and manifold to turbo.  Leaks further "downstream" have no effect or very slightly positive (less back pressure).

An exhaust leak "upstream" of an exhaust brake (which is downstream of the turbo) reduces back pressure and therefore braking HP.  An exhaust leak "downstream" of the exhaust brake has no effect on braking HP. Same leak points as above plus turbo to exhaust brake on exhaust brakes mounted on the turbo and at turbo to pipe and pipe to exhaust brake to those mounted further downstream from the turbo.

Brett Wolfe

Quote Selected

Re: Excess Black Smoke Coming from 8.3 Engine

Reply #5 – November 17, 2010, 10:22:07 am

There was a two piece manifold for the 5.9, but not for the 8.3. I had to replace mine on the 280 five years ago. I've not seen or heard of a two piece. Its one big hunk of metal... Problem is how many bolts will break and how good the mech is on extracting them.

Quote Selected

Re: Excess Black Smoke Coming from 8.3 Engine

Reply #6 – November 17, 2010, 02:15:42 pm

In simple terms, a leak between the exhaust ports and the turbo will reduce its ability to make boost, ie, less power and more black smoke.
Also, a leak between the turbo compressor and the intake manifold will also reduce boost to the engine and again more black smoke and less power.
Keep it in simple terms, much easier to understand.
Also a highly restricted air filter will also cause black smoke.

Quote Selected

Re: Excess Black Smoke Coming from 8.3 Engine

Reply #7 – November 17, 2010, 03:51:48 pm

Had a crack in the manifold right next to the turbo on our 99 ISC (2 piece), so the 2 piece design doesn't prevent cracks, but maybe it puts less strain on the bolts (the tech had only a little trouble getting the bolts out, and none needed to be drilled.

Quote Selected