2015 Chevy 2500hd gas, noise when going up moderate hills

[verretcat]

I am hoping this is just a question about how a motor works and nothing is wrong. I recently purchased my first camper (3,800 lbs) and 2500HD, gas, Chevy truck. When towing in tow mode it appears the transmission stays in 5th gear and the motor operates at 2250 rpm. When going up a small hill, nothing really changes. When going up a medium hill and the engine begins to strain, the engine will abruptly make an additional noise and the gas mileage indicator goes from say 8 mpg to 5 mpg. However, it stays in 5th gear and the rpms stay about 2250. As the top of the hill is reached the noise gradually disappears. On steep hills, the noise will begin and a few seconds later the truck will downshift to 4th and rpms go to about 3300. My questions is, is this normal or should I be concerned with the additional noise that occurs without a change in gears? If it is normal, does anyone know what is causing the noise?

[ddphillips52]

What you are hearing is your torque converter unlocking. Completely normal.

[aceinspp]

Unless you are really pulling steep grades with your set up no need to use tow haul mode. I pull my almost 9000 V-lite most of the time in regular mode till I encounter steeper hills. You will find the gas mileage is better also than if you leave in tow haul mode. The noise you maybe hearing may also be the air being drawer into the engine. Typically normal. Later RJD

[DocsDad]

Not to muddy the water and I realize you don't have the Express. I was notified of this because we have the 6.0 on our 4500 chassis sunseeker. We have no issues but your post reminded me of it.

Torque Converter Clutch Wear – 2010-2014 Chevrolet Express GMC Savana

Torque Converter Clutch Wear – 2010-2014 Chevrolet Express GMC SavanaOEMDTC - OEMDTC

[wbdavey]

Verret - I'm going to disagree that the torque converter is unlocking, if that was the cause of the noise increase the engine RPM's should change at the same time. I also disagree with not using "tow/haul", why did the manufacturer put it on the truck if there wasn't some benefit while towing? Now for the cause of the noise, IMO you are hearing the engine cooling fan which is thermostatically controlled. Next time it happens, closely watch your coolant temperature and I'll bet the temp goes up as you start to climb the hill and then comes back down shortly after the new noise starts.

[verretcat]

wbdavey-would the fan coming on also explain the mpg indicator going from 8 mpg to 5 mpg at the exact same time as the noise starting?

[northstar1960]

Originally Posted by verretcat

I am hoping this is just a question about how a motor works and nothing is wrong. I recently purchased my first camper (3,800 lbs) and 2500HD, gas, Chevy truck. When towing in tow mode it appears the transmission stays in 5th gear and the motor operates at 2250 rpm. When going up a small hill, nothing really changes. When going up a medium hill and the engine begins to strain, the engine will abruptly make an additional noise and the gas mileage indicator goes from say 8 mpg to 5 mpg. However, it stays in 5th gear and the rpms stay about 2250. As the top of the hill is reached the noise gradually disappears. On steep hills, the noise will begin and a few seconds later the truck will downshift to 4th and rpms go to about 3300. My questions is, is this normal or should I be concerned with the additional noise that occurs without a change in gears? If it is normal, does anyone know what is causing the noise?

H

Tow / Haul mode just changes the shifts points in the tranny , stays in lower gears longer when starting out before shifting and down shifts at ealry points such as going up a hill or down . turn off your t/h mode and try and shift yourself and see if there's a difference in operation and noise . I up and down shift with the selector better then the t/h mode can do it .same same only better . also when your going up hill with the load expect mpg to drop. normal

[davel1971]

use tow/haul. That's what it's for. I changes the entire operations strategy of the transmission. Not everything you will notice. Better for your vehicle to use it.

[davel1971]

Originally Posted by northstar1960

H

Tow / Haul mode just changes the shifts points in the tranny , stays in lower gears longer when starting out before shifting and down shifts at ealry points such as going up a hill or down . turn off your t/h mode and try and shift yourself and see if there's a difference in operation and noise . I up and down shift with the selector better then the t/h mode can do it .same same only better . also when your going up hill with the load expect mpg to drop. normal

tow/haul also changes the operating pressure holding the clutches. Can you do that manually better than the PCM can?

[davel1971]

Originally Posted by verretcat

wbdavey-would the fan coming on also explain the mpg indicator going from 8 mpg to 5 mpg at the exact same time as the noise starting?

most likely it's the hill decreasing your mileage

[northstar1960]

Originally Posted by davel1971

use tow/haul. That's what it's for. I changes the entire operations strategy of the transmission. Not everything you will notice. Better for your vehicle to use it.

That may be the case with newer trannys . but what changes in tow haul is shift points and that's about it .

[verretcat]

I am no mechanic but it just seems the strain of going up a hill causes something noisy to kick in and send more gas to the engine to keep the vehicle going at 2250 rpms. I guess I was hoping to hear from someone with this engine say that his truck has been doing the same thing for several years without a problem. Maybe it's because I'm a newbe going on my first long trip and wanting everything to go smoothly.

[Tonkatoy77]

Originally Posted by wbdavey

Verret - I'm going to disagree that the torque converter is unlocking, if that was the cause of the noise increase the engine RPM's should change at the same time. I also disagree with not using "tow/haul", why did the manufacturer put it on the truck if there wasn't some benefit while towing? Now for the cause of the noise, IMO you are hearing the engine cooling fan which is thermostatically controlled. Next time it happens, closely watch your coolant temperature and I'll bet the temp goes up as you start to climb the hill and then comes back down shortly after the new noise starts.

X2 on everything said here. The mileage drops because of the hill and because the fan robs a ton of horsepower. That is unless the new trucks come with electric fans.


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[Tonkatoy77]

Originally Posted by northstar1960

That may be the case with newer trannys . but what changes in tow haul is shift points and that's about it .

What are you considering a "newer tranny"? The mapping in the TCM is totally different in T/H. Also, the line pressure to the clutches is different. Has been that way in GM vehicles for 15+ years.


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[northstar1960]

Originally Posted by davel1971

tow/haul also changes the operating pressure holding the clutches. Can you do that manually better than the PCM can?

All it does is change the shift points and nothing else . i was trying to be polite . no extra psi changes , no nothing . and yes i can shift the tranny quicker then the unit because i see the hill coming etc the tranny does not and will only shift when load increases

[jeff64]

I do use the tow haul mode. With all the discussions of substandard tires supplied on the TTs, I've erred on the side of caution an keep as close to 60 mph as possible. I can see in tow/haul one of two things, either it is kept out of converter lockup (more likely) or the 6th gear is locked out. Either way if I don't beat the pee out of it, I'll get about 8.2 mpg with a Leer cap on the bed. RPMs generally less than 2250

[Flybob]

Manual implies that the computer tracks when a trailer is connected and T/H is on or off. Depending on how much data is collected, could impact warranty.

[captainperry]

I have to disagree with previous posts stating " only shift points change in tow mode". On my Ford the forward clutch pack drops out and the O/D band applies. The overdrive band is a weak point, I would rather have the forward clutch applied so I pretty much tow with O/D off.

[youroo]

Originally Posted by verretcat

wbdavey-would the fan coming on also explain the mpg indicator going from 8 mpg to 5 mpg at the exact same time as the noise starting?

Your (6speed) never shifts into (6th Gear) while towing? I have Never had a (Mileage Drop) from (8 to 5) while going up a Hill or going down the Road in the Short period of Time you speak of? How many miles are on this Vehicle? This all seems very odd when your trailer only weights (4500 lbs)! Youroo!!

[Jim Schings]

On the Chevy all the toe/haul switch is to change shift points (higher RPM) and downshift points (to help in braking). On some engines (I am told) it increases alternator output. Never tested that, so it's just a rumor... You can do slightly better by shifting on your own and it won't hurt a thing. (However, normally you will not use OD.) (You can do better than the trans management unit mainly because you have visible access of an upcoming hill.) However, why bother? The tow mode does fairly well on it's own.


Re: Fuel mileage indication. The way the mileage indicator works is this. : It has no real fuel flow sensor. The ECM, knows what the fuel pressure is, the fuel injector fuel rates (pintel size and opening speeds), duration of the injector opening and the RPM of the engine. From that it computes the estimated fuel mileage. On a relatively new vehicle it is pretty accurate. (Due to injector flow it may vary a bit with older vehicles) When you go up a hill the engine demands more air and fuel. The injector pulse duration immediately goes longer. The ECM computes the new MPG... Done deal...


Without hearing the noise you describe it is hard to tell if you have a problem, but I suspect not...