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Off the wall question #74480 01/23/07 05:56 PM
Joined: Jan 2006
Posts: 558
Rewired Offline OP
First off I want to apologize for not logging on the forum in a while, BIGTIME computer problems.
I do have a question regarding term
"wet stacking" when it comes to diesel generators, or I assume diesel engines in general?
Am I wrong but does it mean that if you have a diesel that "wet stacks", it is where there is a buildup of unburnt fuel in the engine and exhaust system, caused by the engine not being run long enough and hard enough to get up to proper operating temperature and therefore not completely burning all fuel injected into the engine?
I am just wondering the exact meaning, I heard the term come up again and was not sure ~exactly~ what it meant.
Thx again!

Work Gear for Electricians and the Trades
Re: Off the wall question #74481 01/23/07 07:24 PM
Joined: Feb 2003
Posts: 231
RobbieD Offline
Wet stacking occurs in a diesel engine when the engine is run at too light of a load during the run-in (also known as load banking). This causes unburnt fuel and oil to collect in the exhaust stack. The exhaust stack will be coated with a black, sticky, tar-like substance. If you run a new unit at light loads and/or idle for too long, the rings will not seat properly. If it is a diesel, wet stacking may occur. Both of these conditions will cause poor engine life and performance.

This info is from

Re: Off the wall question #74482 01/23/07 07:51 PM
Joined: May 2002
Posts: 1,716
Roger Offline
Wet stacking in diesel generators is a perfect example of "bigger is not always better".

I am involved with a hospital that sized it's Generators for future growth and have a wet stacking problems at present due to light loading.


Re: Off the wall question #74483 01/23/07 09:26 PM
Joined: Jul 2004
Posts: 783
LarryC Offline
It is very similar to creosote build up in a chimmey of wood burning stoves. By it self, it is not too bad, BUT if you then heat up the chimmney by overfiring the stove or heavily loading the diesel engine, the fuel coating can ignite, resulting in a chimmney / exhaust system fire.

That is one reason when running the generator for annual testing, it should be put under full load.

Re: Off the wall question #74484 01/23/07 10:53 PM
Joined: Sep 2005
Posts: 202
WFO Offline
OK, this piques my curiosity. Our company recently purchased a standby diesel generator. When it gets set up to test run on a weekly basis, we are not planning to actually switch load. Will this running without load cause build up?

[This message has been edited by WFO (edited 01-23-2007).]

Re: Off the wall question #74485 01/23/07 11:26 PM
Joined: Mar 2004
Posts: 251
WFO answer YES All standby gensets should always be run under load during exercizing or testing. Here on the Gulf Coast many businesses and public safety people discovered their standby systems failed after a few hrs of running during the Hurricane due to not exercizing them under load. Some systems have load banks they use during exercizing.

Re: Off the wall question #74486 01/24/07 01:00 AM
Joined: Jan 2006
Posts: 558
Rewired Offline OP
Ok so the way I understand it, IF it happens on a new engine it can cause poor seating of the rings and short engine life and poor performance, BUT it is possible for this to happen on an older unit if it were run a lot while not under load?

I am asking as I am also curious if this could apply to a truck engine, as my boss starts up his one diesel truck that never leaves the yard and shuts it down while still ice cold. The other truck that leaves the yard once in a blue moon he does almost the same thing to.. I guess what he should be doing is not bother to be starting them once a week if they are sitting there months on end, BUT if he does want to fire them up, he should be taking them out and getting them good and hot? Just curious as I don't want him to screw up the "new used" trucks he has bought.
Thanks again!

Re: Off the wall question #74487 01/24/07 08:47 AM
Joined: Mar 2004
Posts: 778
BigB Offline
Some diesel engines are more succeptable to wet stacking than others. For example, the Mercedes Benz diesels can idle for days without wet stacking problems.

[This message has been edited by BigB (edited 01-24-2007).]

Re: Off the wall question #74488 01/24/07 09:02 AM
Joined: Mar 2002
Posts: 360
Trainwire Offline
Our GE 44 Ton locomotive has two 40's vintage Cat d17000 v-8 deseasels in it.

Most of it's time is spent idling around the yard, one or two cars, a quick jolt of power to get things moving.

Had to bring a two heavyweight passenger cars up the hill from the other end of the line.

Wide open, down to about 10 mph, the stacks starting blowing like the steam engines, black smoke, red embers, even some flames out the top of the pipes. Took a good 3 or 4 minutes of running before it cleaned up. Fortunatly it had just rained and everything was sopping wet.

In that case though, the wet stacking is because they are worn out [Linked Image]

Another issue with the standby gennies, would be the length of the exhaust system. A short pipe would be less prone than a long one. The raw fuel condenses in the cool portions of a long pipe and drains back.

Our gensets here at the railroad use oil cooled engines, and they seem to warm up quicker and are less prone to piddling all over themselves.



Re: Off the wall question #74489 01/24/07 11:16 AM
Joined: Feb 2003
Posts: 939
frenchelectrican Offline
Hello guys and ladies :

anyway for wet stacking on diesel engines it is pretty common with just about any size diesel engines running with no or very light load due very low exhaust tempture.

Like example one diesel generator i been working on { Detorit Diesel old two stroker } if running on very load the exhaust temp will show about 400 to 550 degress but by time i put a full load the temp climb up to range of 850~950 degress it will burn off any soot or wet oil off the exhaust manfoid.

the same thing with diesel truck as well with my DT466 diesel truck at idle [ fully warm up ] it will idle with 350-450 degress but full load it will spike up near 1000 degress but on crusing mode useally 750 to 850 degress range.

Natural Gaz and Propane units [ spark ingited units ] wil expect much higher exhuast temp for sure useally about 800 and above

if more question please do post it i will try to get more details on it

Merci , Marc

Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

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