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Old 06-28-2021, 09:04 AM   #1
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Engine Temps while climbing hills

2001 Intrigue, 370 HP 8.9 ISL.
I've read a bunch of posts about fellow owners concerned about engine temps. I wanted to share a few things after our recent trip from Phoenix to San Francisco and back. This is our 2nd Diesel and I've learned allot over the previous gassers we've owned. With the gasser, I always tried to keep my revs low to maximize fuel economy. However, after reading enough I realize the diesel is a different animal and does not do well when "lugged" up a hill (Lugging builds heat). The 6 speed Allison has 5th and 6th gear as overdrive, so any real power required, the trans is working harder with a higher gear ratio thus leading to higher trans temps. I climbed the Grapevine Northbound at 50 -55 mph in 4th gear at2,000 to 2,200 RPM (approx) and never saw higher than 205 on the Silverleaf. It was about 100 outside. Same results on the grapevine going southbound as well as all the other major climbs between AZ and CA. Highest temps I saw the entire trip were 210 climbing the never-ending grade eastbound from Quartzite towards Phoenix at 75 MPH, 5th Gear and 110 temp outside. What I learned and wanted to share is high RPM and proper gear selection is your friend for keeping the temps where they need to be. I hope this helps someone, it made my trip more enjoyable and less stressful so I wanted to share.
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Old 06-28-2021, 09:17 AM   #2
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I agree for the most part, but, your transmission is smarter than you. We normally drive 65-67 mph so once I set the cruise control I leave it alone. If I’m going to hit a big incline I usually speed up to 70 with the cruise set on 65. I just let that transmission do the the work it knows better when to down shift than I do. Just my thoughts.
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Old 06-28-2021, 09:41 AM   #3
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Was pretty much with you until:


"...towards Phoenix at 75 MPH, 5th Gear..."


Hate to even ask about your fuel consumption.


I would suggest if you can't maintain reasonable coolant temperature on non-steep, long grades in high gear, either slow down or find the root cause of overheating.


Totally agree on 2,000 RPM or so and just under WOT for climbing steep, long grades.
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Old 06-28-2021, 09:52 AM   #4
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Great info sharing, and very similar to our results with the ISL370(400) in our previous 40' allure. I found that our temps also dropped, once I changed over to OAT based coolant (Peak Final Charge.), and I feel our Source Engineering Wax Valve for fan speed control, worked well also.

As far as the ECM's between the Cummins and Allison's being smart and working well together - they do. That being said, as a drivers we have the advantage if being able to see road ahead of us. Many coaches have abundant power to weight ratios. I found our Allure did a pretty dang good job, but, once it lost momentum on a hill climb, it was very difficult to recover. So I'd judge the grade ahead, and manually down shift to keep the RPM's in higher RPM ranges. My practice was to downshift again, when the RPM's got to about the 1700 or so range.

Our temps, would even in extreme heat conditions, would run from 195F and towards the end of a long extended climb, in temps say above 100F (114F climbing I17 North going into Flagstaff!) - up to 105-107F.

And of course, I'd turn off the OTR AC, to allow as much power to the drive wheels as possible, under extreme heat conditions.

With the Magna and the Pre DPF ISX600. OEM Fan Control Unit, and a clean radiator and less then 18 months old coolant change to Fleetguard Complete OAT. We ran from Tucson to Yuma repellently in 112F temps. On a few grades (With full fuel tank and water tank and due to some preventative 'slosh cleaning' of the Grey/Black, they were also both about 1/2 full. So, heavier then normal for sure.), we reached 205F, and I did down shift to 5th gear at 68MPH for the grade, suspect it would have pulled it in 6th, but it was HOT outside!

Best to all,
Smitty
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Old 06-28-2021, 12:01 PM   #5
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Originally Posted by wolfe10 View Post
Totally agree on 2,000 RPM or so and just under WOT for climbing steep, long grades.
100% agree. On long sustained uphill grades in my coach, manually downshifting so my RPMs are at or just under 2,000 keeps my speed up and and temps down much better than the Allison does on its own.
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Old 06-28-2021, 01:15 PM   #6
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Quote:
Originally Posted by wolfe10 View Post
Was pretty much with you until:


"...towards Phoenix at 75 MPH, 5th Gear..."


Hate to even ask about your fuel consumption.


I would suggest if you can't maintain reasonable coolant temperature on non-steep, long grades in high gear, either slow down or find the root cause of overheating.


Totally agree on 2,000 RPM or so and just under WOT for climbing steep, long grades.
I have no problems with temps whatsoever, normal temps read 180 to 185. My first trip out I did experience higher temps because I was not driving the coach correctly. I shared this as a help for other new diesel owners.

As far as fuel mileage, I knew someone would chime in on the 75 MPH! Silverleaf showed 8.1 rolling MPG on the level at 75. As far as while climbing, I never show anything better than 4 MPG on the Silverleaf regardless of speed.
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Old 06-28-2021, 01:30 PM   #7
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Silverleaf showed 8.1 rolling MPG on the level at 75.

All I can say is WOW. 8.1 MPG at 75 MPH in 5th gear is amazing!!!
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Old 06-28-2021, 01:39 PM   #8
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If you were on I-10 the speed limit is 75. That's what most semi's are running and its best to stay with the flow of traffic anyway.
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Old 06-28-2021, 01:47 PM   #9
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All I can say is WOW. 8.1 MPG at 75 MPH in 5th gear is amazing!!!
Yeah, I recently got home from a 6500 mile trip and my traveling partner said 65mph was his max (I prefer the posted speed limits) At 65mph Iím running right at 1150 rpm and I was flat towing my Jeep Grand Cherokee. My overall average for the trip was 7.5 mpg
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Old 06-28-2021, 02:42 PM   #10
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All I can say is WOW. 8.1 MPG at 75 MPH in 5th gear is amazing!!!
Sorry for the confusion, 8.1 at 75mph in 6th gear on the flats. No idea what the mileage was climbing the hill at 75 in 5th gear. No point in checking. The posted limit in AZ is 75 in most areas.
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Old 07-31-2021, 09:45 PM   #11
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Yeah, I recently got home from a 6500 mile trip and my traveling partner said 65mph was his max (I prefer the posted speed limits) At 65mph I’m running right at 1150 rpm and I was flat towing my Jeep Grand Cherokee. My overall average for the trip was 7.5 mpg
Most Diesel pushers I've rode in including my 36' Inspire C9 are running between 1600-1725 @ 65 MPH so you might want to check your tach; towing my Jeep Wrangler Unlimited with 35" tires I'm usually running @ 6-6.5 MPG overall but I have hills to climb anywhere I want to go. My last trip between the Grapevine to Fresno a really flat drive I saw 7.5 but the moment I hit the rolling hills on I-5 it went right back down.
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Old 08-01-2021, 08:55 AM   #12
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My tires are rated for 75 MPH but I run at 62. Never in the 2 diesel pushers I have owned seen 1150 rpm running in 6th at hiway speeds.
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Old 08-01-2021, 11:09 AM   #13
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My tires are rated for 75 MPH but I run at 62. Never in the 2 diesel pushers I have owned seen 1150 rpm running in 6th at hiway speeds.
Ok, Iíll assume my tach is off, but my Silverleaf is telling me my MPG and it seems to be fairly close!
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Old 08-01-2021, 10:58 PM   #14
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So in the last 2 days I have gone over all 5 passes on I 90 at or near 100 degrees.
For a diesel, do not set the cruise, you want all the RPMs you can get
The difference between 1500 and 2000 RPMs is 25% more water movement, 25% faster fan speed, and the engine is working way less per revolution. think of riding your bike up a hill in to big of a gear where you have to stand on the pedal to get it to turn.
We run normally about 190 degree but climb to 210 when it is that hot on a grade. When it is really hot, the on-road temps are 120 or more above the asphalt. It does not give a great differential. Generally speaking, I pick a slightly slower speed and try to keep the RPMs about 2000 all the way up. Remeber these same motrs pull trucks over these hills all day but hey have 13 gears, we are totally gearbound in RV's
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