Big Swings in EGT's

[Jfoster]

Here is my scenario: You climb the pass loaded, working the truck for the last 20 miles, the final 5-7 miles you’re running around 1250 EGT and 20 PSI. Once you get to the top and start down, the EGT’s drop like a rock and can end up well below 300 in a matter of minutes. (I don’t tow a lot so I’m having a tough time justifying an exhaust brake. ) What’s going on inside the engine/turbo with such a drastic temperature change in a short time? Is valve warping a possibility? What about the turbo? When possible I try to feed a little fuel on the downhill side to cool it more slowly, but on some hills its just not possible to do this.

 

[Ranger Dick]

Not totally sure on this, but when you are climbing, you are combusting relatively large amounts of fuel and air, since you need high power to do this. Since the diesel is a heat engine, there is not a constant ratio of fuel to air. Hence under heavy load operation, higer combustions temperatures due to more fuel. once you peak the summit and go to idle condition, the amount of fuel drops dramatically, I heard down to 85:1 air/fuel by weight. Your motor is basically pumping air through and with relativly high engine rpm at idle throttle, Since the probe is measuring Exhaust gas temp, and most of the gas is now air, that's why the low temps.

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01. 5 ETH/DEE QC, 2500 4X4, 3. 54 Everything but leather, added SPA Boost/EGT guage.
68 Jeep CJ5, V6, 225ci, sm420, warn overdrive, full floating detroit locked 44 in back,
2. 5" superlift, power steering, 4. 88gears. Mobiweld welder and other neat stuff.

 

[Jfoster]

I guess my question was unclear. I know what is happening inside to CAUSE the drop, (nice explanation by the way), but what are the EFFECTS on the internal part from the fast drop in temp? IE: rapid cooling of metals, gaskets, bushings, etc... ... sorry if I was unclear.

 

[illflem]

Matt explained it pretty well. Not to worry about the sudden drop going downhill, you are measuring the temperature of the exhaust gas not the engine. The gas will cool rapidly when you aren't adding fuel, but the engine itself does so more slowly. If your coolant temp was hitting bottom rapidly right after cresting the hill then you could worry.

 

[roadranger]

So this is not the root cause of the single piece exhaust manifold cracking??- extreme temp changes?

The exhaust brake would help even out the temp extremes, wouldn't it?

 

[hammong]

Cast iron is used for exhaust manifolds for just that reason -- it's very resistant to the effects of extreme heating/cooling without cracking. The "problem" with our manifolds that leads to cracking is the amount of weight hung on them (turbo, forward portion of exhaust tract, etc, coupled with engine vibration, heat cycling, AND potential casting flaws or mounting inconsistencies.

A solid stainless exhaust manifold might fit the bill, as well as reject more heat, but you're gonna be talking big $$$ for a replacement.

Greg

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Y2K Dodge Cummins QC 2500 SLT Laramie 4x4 Long Bed, 4. 10/Auto, Forest Green/Driftwood, 265x75R16 Michelin LTX, 6 speaker CD, Camper/Tow/Plow Packages, Westin Black Nerfs, Mopar bedliner, Ultra bright diamondplate toolbox, K&N Filtercharger, Autometer Ultralite EGT and Boost gauges in 2-gauge pillar w/tweeter, DD Stage II injectors, boost module, and boost elbow. Yee haw!
http://www.rints.com/hammond for photos!

 

[BillGotthelf]

If you need a rebuild at 300,000 miles someone has abused that cummins. With proper care (even bombed) you can go 500,000+ miles to rebuild. I have used oil analysis for my past three dodges and all my heavy equipment because of the oil filtration system we use. Oil good with over 300,000 miles on it and no abnormal wear metals. Also never had problem towing 400,000+ miles heavy loads with rapid up and down egt's.

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1996-3500 4x4 extended cab tst #5 plate, K&N air filter,Primeloc fuel filter kit,4" exhaust,Purodyne bypass oil fiter system, bigger injectors (215),b&m trans pan,Amsoil all around,temp gauges out the wazzo, much transmission work including TST upgrade kit and Bill Kondalays TC, Transfer Flow 98 gal bed tank,double tow 33' 5th wheel and 12' trailer w/motorcycle(Honda Valkyarie Interstate)Also have 1999-250-stock.
Installing governor spring kit(BD)on dualie.

 

[Ranger Dick]

Sorry, misinterpreted the question. When I hear that these motors go 300,000 miles before rebuild, I would presume that this type of operation, eg going from full throttle to idle is anticipated and is acceptable. As to what actually is happening inside the motor etc. THat is a good question. But it's outta my league. Matt

 

[TowPro]

Just a note, an exhaust brake will slow down the rate of cooling on the down hill side. I did read this quick change in temps, can warp an exhaust valve (in theroy anyway).