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Does slowing down up a steep hill keep engine temp down?

Yahoo Message Number: 74215
I have a 2002 Affinity 42' with C12. Going up the Grapevine two weeks ago with 100 ambient temp I got my CAT up to about 216 on the Silverleaf. I usually get a buzzer if I get to 217F. My question is this, I was peddle to the metal and passing all the trucks. I was doing 50 or so and they 30-35. If I slowed down and went down some gears would my temperature go down? I kept thinking that if I went faster more air would go into the radiator, but maybe that is the wrong thinking. Comments

Bob - 2002 Affinity

Re: Does slowing down up a steep hill keep engine temp down?

Reply #1 –
Yahoo Message Number: 74217
The reason for the trucks slow speed is because they have dropped enough gears to keep the engine RPM's up to keep engine temps low by reducing the work load. I would say you did great pulling that hard grade if you only saw 216 on a 100 deg day. It is fan speed you want not road speed. Another reason for dropping gears. High engine RPM keeps the fan speed up. Next time drop one gear and see what happens.

Leonard

97' Magna 5418

Re: Does slowing down up a steep hill keep engine temp down?

Reply #2 –
Yahoo Message Number: 74231
Hey Leonard,

I agree with you. I always downshift climbing steep long grades to keep the power at the middle of the torque curve and to keep the engine from lugging with an overload.
However, I thought the hydraulic fan speed was controlled by engine coolant temp. Is it controlled by both RPM and temp? I am guessing that the higher RPM also increases the circulation of coolant through the radiator and therefore, helps to maintain a lower temp.
.

Now I am a little confused. Please clarify.

Daron Hairabedian, 98 Allure, 30226

Re: Does slowing down up a steep hill keep engine temp down?

Reply #3 –
Yahoo Message Number: 74236
The fan is driven by a hydraulic pump which is driven by engine RPM. More engine RPM more oil flow, more fan speed. This becomes critical in hard pulls to keep fan speed at max speed. If we want to nit pick, at some point it may be the fan reaches max speed below max engine rpm's, but for simplicity go with the above. Engine temps opens the hydraulic control valve to allow max oil flow, after that, it is all engine RPM.

Leonard

Re: Does slowing down up a steep hill keep engine temp down?

Reply #4 –
Yahoo Message Number: 74239
Leonard,

That is a great, clear explanation of the inner workings of all control mechanisms. I guess I was doing the right things but did not completely understand the "whys". I knew I could count on you to educate me on this as you have on many other issues in the past.
Thanks for your response. I am no longer confused.

Daron

Re: Does slowing down up a steep hill keep engine temp down?

Reply #5 –
Yahoo Message Number: 74242
According to a sheet I have about the fan on older cat engines, if the fan is in the fast mode at idle the fan turns 700 rpm and 1,700 rpm when the engine is around 2000 rpm. So engine speed does make a difference. As Gary use to say at the Allison seminars, shift down to get the rpms up for better cooling.. TWI 2004 intrigue 11731

Re: Does slowing down up a steep hill keep engine temp down?

Reply #6 –
Yahoo Message Number: 74293
Interesting Thread.

Up until two weeks ago, I never saw my temp go above 200 and never slowed down up any hills except grocers grade (HWY 166). The end of July we came from Junction City to Bakersfield down I5 and went up every hill in Cruise Control. Had to accelerate back to 55 MPH from 30 after getting stuck behind some trucks but never got hot. Outside temperature was in the high 90's. (wished my dash air put out a little more)
Two weeks ago I went to Mammoth Lakes and the grade outside of Bishop near the top I hit 220 and then at 221 on the Silverleaf, the temp warning light came on for about 10 seconds as the temp increased to 222 then went down to 210. I was down to 35 MPH 1800 RPM C13 Cat( It think it was in 3rd gear). Outside temp was about 97 degrees. There was a Fleetwood Discovery, Monaco and another CC Intrigue all with Cummins travelling with. The Fleetwood and the Monaco both got temperature warning lights. They were down to 30 MPH even the one not towing. I never downshifted but just left it in cruise control at 55.
1800 RPM is max for my engine. 1350 is max Torgue. Should I have slowed down and manually shifted to get to 1350 RPM to avoid overheating???

07 Intrigue 530 12162
C13 Allison
06 Rubicon LJ

Re: Does slowing down up a steep hill keep engine temp down?

Reply #7 –
Yahoo Message Number: 74314
Yes, you want to manually shift down to keep torque at maxium, and we also want the RPM's to stay around 1350 RPM, so the fan is turning at high speed.
This will help keep the engine at a cooler temperture and still have power pulling the grade.

Gary Obermire

Premier RV Services

Re: Does slowing down up a steep hill keep engine temp down?

Reply #8 –
Yahoo Message Number: 74316
Interesting, indeed.

My Intrigue 530 normally stays at 192 degrees. I carry a lot of "stuff" and tow a heavy Jeep Cherokee.
Traveling in South Dakota and then into Idaho and Washington I found that if I kept it on cruise control up steep hills the temp would go up over 210 occasionally which made me very nervous.
Consistently, if I slow down to +/- 45mph and down shift to 4th or 3rd gear the temp drops right down to the mid 190's. On really steep grades dropping to 30mph (like the truckers do) and down shifting to 3 always works.

Tom Oelsner

2006 Intrigue 12060

drtomo@...

Re: Does slowing down up a steep hill keep engine temp down?

Reply #9 –
Yahoo Message Number: 74321
Thanks Gary. The run from Junction City Oregon to Bakersfield was from your shop to home after you guys did a great repair job on my Slide hydraulics.