Fella's,
I have a question that I'm sure someone can answer right away in reference to egt's while going downhill. Why do they drop so low (about 200* pre turbo on my gauge), while traveling at the same speed and throttle position? Just wondering, cause I don't know everything about diesel operation. Just by posting here, I seem to learn something new about diesels everyday.

 

You probably were not going downhill at the same speed and throttle position as when going up the hill or on flat land. Unless you were going some god-awful excessive rate of speed that made gravity a minor factor, you would have picked up speed unless you let off the throttle. But that seems obvious so I'm guessing I misunderstood your question. If you are, instead, saying that you are holding a steady speed and throttle position while going downhill and, while doing so, the EGT's keep dropping until they get to below 200 degrees, the reason for that is it takes a little while for all that metal to cool off. The engine is pumping a lot of air without creating much heat from combustion and that cools everything down. You can get pretty much all the way down to ambient (outside) temperature coasting down a hill with no throttle.

 

Badunit,
I guess what may have sounded ok to me, may have been confusing to others. You got the second part of your reply right. I believe you misunderstood my question or I did't ask the question very well, but you followed up with the answer I was looking for. I recall my old Suburban gasser would stay the same temp, I guess it because it was not pulling as much air. As I stated, I dont know everything. Thanks for the reply.

 

Diesels are a strange breed compared to gassers...

First, though it may seem strange, on an uphill grade it's much better to shift down and rev the motor... Not because you can't pull the higher gear, but because you'll be dumping excess fuel into the cylinders and causing EGT's to go out of sight. Not nearly the same thing as a gasser which needs to be downshifted in order to make the grade.

Then there's the throttle thing. A diesel is always wide open. No throttle (little fuel) and lots of air don't make for much heat. You can expect the block to cool rather quickly. A gasser (which runs hotter anyway) doesn't suffer from the same circumstances as the throttle plate all but stops airflow and reduces heat loss at low power settings.

The best advice I can give is that you really can't apply gasser theory to a diesel. It's a breed apart. Then, when you start to deal with a I-6 and not a V-8, the differences become that much greater.

Handle the I-6 like you might handle a V-8 gasser and you'll be mistreating the engine. The saving grace is that the Cummins will suffer the indignity without damage, but you'll pay the bill in economy lost.

 

Even if you don't move your foot on the accelerator, when load decreases (you start going downhill) the governor (ECM in your case) will cut back on the fuel to prevent RPM from rising. So even though your foot hasn't moved, the engine is getting less fuel - so the EGT drops.

And like badunit said, there is a lot of air flowing through these engines at all times. When there is almost no fuel (coasting downhill) things cool off very quickly.

 

All good explainations that have answered my question. Thanks fella's.

NORD
I completely agree with being abusive to your engine and paying for it in the end. Unfortunately, diesel 101 is'nt offered at my job (I work with electroincs) so I ask here all those things I have questions about. This is my 1st diesel, so excuse me if a question may sound stupid or something I should know.

 

if you get right off the throttle and downshift on a downhill, the engine can go to 0% fueling, so the engine just turns into an airpump, and no fuel gets burned...

 

Yeah, and if you reset your mpg computer, it says 99.9.

 

or 0 L/100km for the metric canadians...