Proper Shifting to Reduce Engine Temperature

[JCat]

I have had my RV for 12 years and I think I learned something new.

From the very first month I had the RV I experienced overheating when traveling in temps over 100 degrees outside.
I have a CAT C7 and the temp alarm trips at 220 degrees.

I took it in to FL and they showed me how both radiators were caked with oil, dirt and road debris, and they cleaned it all out.

I then found out my oil sump was not plumed to the end of my RV, so the oil that comes off this hose was going directly into the fan and onto the radiator. I fixed this by having it plumed to the end past the radiator.

Then once a year I break out the simple green and clean both the radiators.

All has been good for a while.

This month I decided to go to Northern AZ.
I'm heading north on 17 going up, up, up.
Then I hit this really steep grade and the temp starts going up, up up.

I manually downshift to 3rd gear to keep up the momentum.
The temp goes to 222 degrees.

I try something I never tried before, I downshift to 2nd gear and let off the gas a little so the transmission is not trying to go to 3rd gear.
When all of a sudden the engine temp start to drop, and keeps dropping all the way to 213, well below the overheat alarm.

So now I'm thinking I should downshift to 2nd gear when I hit these really steep grades.

Did I finally figure out how to drive my RV correctly in this situation ?

Let me know, and please refrain from saying "yes stupid" or "it only took you 12 years to figure this out, DUMMY ?"

[hipower]

J-Rooster wrote:

hipower wrote:
Those of us who have some trucking experience in our backgrounds drove when pyrometers were prevalent. Measuring your exhaust temperature was almost a neccesity to keep from cracking a piston or exhaust manifold in the early days of turbocharging OTR diesels.

I don't recall the exact temps suggested but they were different depending on the probe placement, pre or post turbo. Somewhere in the dark recesses of my mind I think it was 1200 degrees pre turbo and 800 degrees post turbo, but I can't swear that is correct anymore.

The key here is that if your EGT was too high you downshifted a gear and backed off on the throttle to keep the EGT in an acceptable range. Using our coolant temps to guide us is just a less accurate way of doing the same thing.

Bottom line, if you experience temps that make you uncomfortable while climbing a grade downshift and reduce throttle input to bring the temps back down.

J-Cat, glad to hear that your really learning about your CAT Motor and what it takes to get it running good. Highpower, those were the good old days! Keeping those diesel motors right on 2150 RPM's and if you shifted into neutral with the Jake Brake on it would kill the motor! My boss perfered us to take our foot off the throttle when the pyro got to 1000.

Those were interesting days indeed Rooster. Naturally aspirated diesels making 250 HP or less in most cases. Gas engines that were lucky to make 150 HP. Five speed main transmissions with three and four speed auxiliary boxes or two speed rears. Putting the diesel fuel to them to where there was a black cloud of exhaust that nearly blocked out the sun when you pulled away from a stop and at every shift and a foot of fire coming out the stack lighting up the night sky. Cabs so hot in the summer you could hardly stand it and so cold in the winter you couldn't take off your coat and gloves and an interior ice scraper was almost a neccesity. Big loads and brakes that were barely passable on the level and scared the **** out of you in the mountains.

Makes me understand that the good old days weren't really all that good, but we had fun and made a little money along the way. Today you have a hard time finding drivers or 'wanna be' drivers that can even shift a manual transmission. Nostalgia is a wonderful thing and our memories only seem to keep the good and forget the bad.

[SoCalDesertRid1]

Gears are a torque multiplier. When you shift to a lower ratio gear, you have greater torque multiplication. Therefore, the engine doesn't have to work as hard when operating in lower gears. This means less heat made by the engine.

[Snomas]

I've been down shifting for 14 years to lower temp. When you live in Colorado you learn to do that very quickly !

[wolfe10]

Just to clarify terminology, on a rear radiator diesel:

Behind the fan is the CAC (Charge Air Cooler). It is an air to air cooler for clean air coming from the turbo and going to the intake manifold.

Behind that is the Radiator. This is the one with coolant in it.

Air flows from the front (actually from under the coach) through the CAC and then the radiator and then exhausts to the back of the coach.

[JCat]

Triker, did you even read my original post, it talks to each one of your items.

The only thing I did differently this time was go to 2nd gear and slow down.
I always downshifted to 3rd when going uphills.

As far as cleaning the radiator from other posts, I spray and clean from the front and back of both radiators.

[J-Rooster]

hipower wrote:
Those of us who have some trucking experience in our backgrounds drove when pyrometers were prevalent. Measuring your exhaust temperature was almost a neccesity to keep from cracking a piston or exhaust manifold in the early days of turbocharging OTR diesels.

I don't recall the exact temps suggested but they were different depending on the probe placement, pre or post turbo. Somewhere in the dark recesses of my mind I think it was 1200 degrees pre turbo and 800 degrees post turbo, but I can't swear that is correct anymore.

The key here is that if your EGT was too high you downshifted a gear and backed off on the throttle to keep the EGT in an acceptable range. Using our coolant temps to guide us is just a less accurate way of doing the same thing.

Bottom line, if you experience temps that make you uncomfortable while climbing a grade downshift and reduce throttle input to bring the temps back down.

J-Cat, glad to hear that your really learning about your CAT Motor and what it takes to get it running good. Highpower, those were the good old days! Keeping those diesel motors right on 2150 RPM's and if you shifted into neutral with the Jake Brake on it would kill the motor! My boss perfered us to take our foot off the throttle when the pyro got to 1000.

[wolfe10]

OK, a little more detail.

Wally is correct-- lower RPM= better MPG.

But (yes, LARGE BUT) if coolant temperatures rise, MPG is a LOT less important than keeping the engine from overheating.

So, indeed if coolant temperatures are not rising, low RPM is the ticket (as long as it is above peak torque RPM-- for Cat C7 that is 1,440 RPM).

If temperature starts to rise, downshift to raise RPM's and back out of throttle. Yes, lower MPG, HAPPIER ENGINE!

[wallynm]

We run ours a ~ 1600 RPM which is much closer to peak torque which is 1450 RPM.

tinkerer wrote:
That is exactly the same thing I do when climbing that same grade. I like to keep the rpms at 2000 or above. This also a C7 Cat.

[hipower]

Those of us who have some trucking experience in our backgrounds drove when pyrometers were prevalent. Measuring your exhaust temperature was almost a neccesity to keep from cracking a piston or exhaust manifold in the early days of turbocharging OTR diesels.

I don't recall the exact temps suggested but they were different depending on the probe placement, pre or post turbo. Somewhere in the dark recesses of my mind I think it was 1200 degrees pre turbo and 800 degrees post turbo, but I can't swear that is correct anymore.

The key here is that if your EGT was too high you downshifted a gear and backed off on the throttle to keep the EGT in an acceptable range. Using our coolant temps to guide us is just a less accurate way of doing the same thing.

Bottom line, if you experience temps that make you uncomfortable while climbing a grade downshift and reduce throttle input to bring the temps back down.

[4x4van]

ricelake922 wrote:
This article is very interesting but if you have automatic how do you downshift to help the engine overheating issue?

Seriously?

[Triker33]

Since you joined these forums in 2004. How did you ever miss any of the many post on how to go up hills with a Diesel engine? And to downshift manual to keep RPM's up to help cool.

Glad you got it figured out even though it took 12 years.
You may need to put a slobber tube extension on and clean the CAC every 2-3 years now.

[BigRabbitMan]

DrewE wrote:

wolfe10 wrote:
Bottom line: It takes XX HP to climb a grade. If you generate that HP at 2000 RPM instead of 1600 RPM, the FAN and WATER PUMP are turning 400 TIMES A MINUTE FASTER.

It takes xx hp to go up a hill at a certain speed. At a lower (vehicle) speed, it of course requires less power. If by downshifting the vehicle speed also slows down, you not only have possibly more water and air flow, but less heat generated by the engine as well--a double improvement.

These two posts are spot on. Another way to describe it is that it takes XX amount of "work" to get your coach to the top of the grade. By using a lower gear, your engine gains leverage against its connection to the ground reducing its work load, and since you go slower, the work needed to get to the top is spread over a longer time period so the cooling system has more time to get rid of the heat that is generated getting to the top. On one long, steep grade I found myself all the way down to first gear to lighten the load on the engine and to spread the remaining load over a long period of time.

[DSDP_Don]

"JCat".....You say that you break out the Simple Green to clean your radiator. Are you talking about spraying the one side of the radiator and the one side of the Charge Air Cooler you can access? If so, I think you might be missing what's causing your base heating issues.

On your coach, there should be a large metal plate covering your engine, probably under the bed. When you lift this cover, you'll see the radiator and Charge Air Cooler (CAC) are sandwiched about 3" apart. Take a light and look down between the two. I'll bet you'll see what looks like hair growing in there. This cooling system configuration is VERY difficult to clean because there is no access.

My sister had the same chassis and her coach would run fine until we hit a grade in the heat. We spent quite a while using a garden sprayer and soaking the two inside faces of the radiator and CAC with Simple Green or Dawn. Then we used a homemade flusher we made with pvc, that looked like one of those toilet wands.

Using hot water directly from the house water heater, we rinsed the surfaces until the water finally ran clean. No more overheating. If you hit it once a year or so, it won't get nasty.

[CharlesinGA]

ricelake922 wrote:
This article is very interesting but if you have automatic how do you downshift to help the engine overheating issue?

On my Sprinter based C class, you bump the shift lever to the left to downshift, and to the right allows the transmission to go up a gear, (or more depending on how low you were and how many times you move the lever). My first reaction after putting the shifter in Drive is to bump it once to the left to limit it to 4th.

Charles