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Old 09-10-2019, 10:13 PM   #1
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Cummins ISC 360 HP,when engine RPM lugs down during climbing, engine runs cooler.

This is my first post on this forum. My main language is German, so please excuse "weird" sounding sentences.

Our DP: Damon Tuscany 42 RQ with tag axle, Cummins ISC 360 HP on Freightliner Chassis.

Main Problem was, as experienced by other members, is the constantly repeating temperature fluctuation during climbing hills, which is almost 30 deg. Celsius. This is ca. 37% temperature fluctuation, based on the normal operating temperature of 80 deg. Celsius (180 Fahrenheit). During climbing, engine temperature rises, fan kicks in, engine cools down rapidly and fan kicks out, engine temperature rises again and this cycle continues until reaching the top of the hill.

However, at one time, just by accident, I was very easy on the accelerator, transmission on Economy Mode, while climbing a steep hill, the rpm dropped to 1150. At this point the engine temperature started falling and stayed at around 90 deg. Celsius with out the fan kicking in. At the same time, the indicator for "instant fuel economy" was reading much better values than when operating within 1400-2000 rpm range.

I did duplicate this "phenomena" intentionally in the next climbing, and sure enough, it repeated the same behavior. When engine lugs down to about 1150 rpm, mostly in the 3 rd or 4 th. gear, the engine temperature will not increase that much and with out the fan kicking in. I can't figure out, why the engine is acting this way. It seams to be against all odds.

I usually try to exploit this phenomena while driving through the Canadian Rockies.

I am sure, this is not harming the engine or the power train, as either the engine ECM or the Transmission ECM will not allow operating in damaging rpm range and would force a shifting in to a lower gear.

Would be interesting to know, if other members have experienced this situation as well?
Monika and Robert,
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Old 09-12-2019, 07:24 AM   #2
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I think it has to do with being in Economy mode. That changes the way the tranny shifts, delaying the shifting and sometimes making it hard to force a downshift with it on. Economy mode is made for flatlands.
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Old 09-12-2019, 02:45 PM   #3
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I would not allow engine rpm to "lug" down that low. Engine coolant temp is low because the engine is not developing power. Not going to get into specifics on HP and Torque ratings for your ISC, except to say the ranges are higher then what your allowing rpm to drop too.
Diesel engines don't run cooler when developing increased power to climb a grade or pull a load. Not gonna happen.
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Old 09-13-2019, 01:32 AM   #4
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Recently I had a conversation with Cummins Shop foreman in BC, Canada, regarding this matter. He thinks, it is attributed to the ecm maping. The ecm will compensate by altering parameters to a given operating range. He suspected, the ECM is disabling the EGR at this load and rpm level. The EGR-Cooler being suspended is probably the reason why the coolant temperature is not rising to a point where the second stage fan will kick in. I did verify this with Insite program. While the wife was driving, I was monitoring the operating conditions. Indeed, some where bellow 1200 rpm, the EGR was "commanded 0 open %" and actual EGR position was 0 % open.
I do agree, luging the rpm could damage the engine. But in this case the rpm is not manually manipulated. I am relaying on the Transmission operating in automatic mode, controlled by electronic sensors and default Maximum/Minimum values. However, I do share your concern. regarding luging rpm.
I will contact Cummins Help line and verify, if there is a "gray zone" where the automatic up/down shifting is not always within optimum range. I will post update after contacting Cummins.
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