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Rawze

Some info about white smoke and valve lash on demandated common rail engines.

This is some info I found recently. It has to do with someone asking why they sometimes see white smoke accompanied with what can be described as a fuel or oil smell after a long idle, or sometimes during warm-up. Also the the change in engine sound after a de-mandate, why the engine sound would change from a smooth tight-sounding engine to more like its older cousins with that all too familiar valve-lash rattle after a de-mandate...

I just found this bit of information on an official research paper on 2250/2350/X-15's and other common rail engines that have DOC/DPF/SCR systems ...

The document discusses the fact that moisture accumulates in all of the exhaust components, especially the larger components (the DOC/DPF/SCR cans) and combines with normally found tiny amounts of un-burnt fuel from pulse injection cycles, oils, or other contaminants like soot/carbon to which any engine can produce even when the engine is considered perfectly healthy.

it then goes on to quote...

Quote: When operating the vehicle after extended idle periods, white smoke has been observed when the exhaust components have not yet come up to full operating temperatures. This is often accompanied with a pungent odour described as an un-burnt fuel or oil smell. To avoid excessive white smoke/odour conditions, a load is placed on the engine during extended idle and during warm-up periods using the electronic VG controlled turbocharger to rapidly increase the temperatures of the exhaust components as a prevention to drive off moisture and bring after-treatment components up to operating temperatures.

I would imagine that if someone de-mandated the engine and removed the extra load at idle, this "white smoke" after idling for long periods, or when engine is cold and there is moisture in the exhaust components due to sudden temperature change after engine starts up, that white smoke out of the exhaust would be a common occurrence on these common rail engines no matter what someone did.

- I know for a fact that the 2250/2350's after they are cranked up will jump into SCR-warmup mode after about 5 - 10 seconds when they have factory stock programming, or a poor quality de-mandate has been done where modes have not been re-mapped properly. This can be heard by listening to the subtle sound of the engine change + the sound of the turbo ramping up too.

The rapid warm-up of the exhaust components to prevent this white smoke/moisture/pungent smell condition has some ill effects though. - That is, when the engine does this, it also consumes about 3.5 - 4x the amount of fuel at idle the whole time it is doing this because the VG turbocharger is placing an internal load on the engine by making it labour against the turbo-induced exhaust restrictions. It can have a fuel consumption of as much as 40-50+ mg/stroke or more on the engine vs. it only needing about 15-18 mg/stroke when this feature is removed through custom tuning/programming.

In the end, the short answer is that if someone de-mandates their truck and wants the engine to not consume excessive amounts of fuel at idle,... a "white smoke" with a pungent fuel or oil smell can almost be expected sometimes, depending on humidity, temp outside, and other factors. Reducing valve guide seal leaks and other events or conditions to minimize any fuel or oil getting into the exhaust as much as possible can help, but may not completely solve it.

It Seems to be a trade-off...

4x more fuel consumption at idle and a nicer looking exhaust when engine is warming up + no valve lash sound because engine is under a constant load at all times?. - This result only has the effect of making the engine sound exactly like it did before the de-mandate the truck owner/driver "feel better" even though the engine consumes more fuel?

-- or --

A bit of white smoke and unburnt fuel (or oil) smell on occasion + some valve lash during warm-up because that is what a diesel enigne does when there is no load on it any more and it is warming up. Some lash sounds can be removed/reduced through programming of injection cycle pulses and timing offsets at idle, but nothing much can stop it completely unless a load is place on the engine (scenario above). All of this varies depending on on conditions outside, but then the benefit of a truck that uses 4x less fuel at idle. The reality is that with the common rail fuel system engines, that pulse injection cycle system is not perfect no mater how well it is programmed.

smorgan87

When ya tune an engine to run and last like they used to ya gotta expect the rattling snds and smoke they used to make.