6.4 towing hold back trouble

[Coltws]

I went out and tried this today with my 5.7 8 speed and exactly like he said when going downhill in 1 gear when you hit a certain rpm point it stops engine braking and its like the pcm opened the throttle a little bit and it accelerates then you have to get on the brakes to maintain your slow decent.

If you don't hit your brakes it will probably start holding back again between 3 and 4k and slow itself down to 2000 something rpms and then take off again. At least mine does unless I am loaded it can't slow it back down after getting the rpms that high.

 

[retired]

We drive hwy 12 from Missoula to Boise all the time and even without a load it suxs having to brake all the time.

 

[Coltws]

Here is a link to a video I uploaded sorry it didn't turn out very good but I think you will get the idea. you can't see in the cell phone camera how much the throttle plate is opening but I felt like sound and watching the rpms would help more. when the rpms get down to just under 2k the computer takes over and gives it gas. towing in the mountains is pretty awful. https://youtu.be/SmDwAn8DeQU

 

[Coltws]

Without experiencing the situation first hand it's hard to know exactly what is going on, or how unusual or non-typical the situation is compared to other vehicles. But in general the engine itself can only provide so much "hold bacK" force. If you are expecting the truck to always be able to keep the speed low regardless of the downward grade or load it's not going to happen unless either yourself or the ECM applies the brakes.

In any case I doubt the engine is truly accelerating as in the throttle is being applied, but rather just the weight of the truck and load is pushing it down the hill and the engine doesn't have enough compression braking to completely counteract the downward force, and hence it increases speed. The perceived change in speed, or hold back force, is probably related to the transmission and torque converter unlocking. If you are driving down the road and let off the gas pedal you just start coasting and don't get any major engine braking at all, even if you let it coast all the way to a stop. The ECM at some point will also unlock the trans and converter to keep the engine from over-revving. Yes, you can blow up an engine by over-revving it while going downhill even at no throttle....I've personally seen it on a manual transmission pickup where the person meant to downshift from 5th to 4th going down a steep grade, but went from 5th to 2nd instead. At 70 mph that caused the engine to hit over 7,000 rpm and spit a rod out of the block.....again this happened with their foot completely off the gas pedal.

Sorry for taking so long to respond on this thread I started I got my confirmation of what was going on. The truck accelerates on its own going down hill. The dealer told me if I found a fix I would be a millionaire. I kinda wish they would find a fix and fix everyone's truck. It's not much of a problem on the highway but on steep windy hills in the mountains that you want first or second gear it is awful. Not to seem rude but I am extremely mechanical and if you haven't driven one of these trucks and tried low gear hold back to experience it then maybe commenting and basically saying I don't understand what I'm talking about isn't what you should be doing. I put a camera in the intake and watched as the truck opens and closes the throttle plate. It will hold back until the rpms get down to about 2200 and then the throttle opens up untill it hits about 4000 and then the throttle closes. Again not trying to be rude but I fix , equipment, trucks, cars, I work on electrical, hydraulics, motors, jet boats, excavators and much more so I'm not in need of someone to explain to me how hold back on a gas engine works thanks for attempting to help though.

 

[Choupique]

That's stupid. I wonder why it does that? My 2018 2500 diesel will try to maintain whatever speed the cruise is set at, or whatever speed you were at the last time you touched the throttle or brake pedal. It works great, but it does swing up and down about 10 mph. What's your trucks speed doing while this is going on? With the exhaust brake in auto, the diesels will brake until they slow down to target speed and then open back up until speed is target + 10 mph, and then engage the brake again.

 

[hemihustlin]

its the pcm trying to maintain proper catalytic converter oxygen levels

during a long WOT event you can feel and hear the throttle opening and closing back to where it was before to give a slight bump to the A/F ratio. when looking at the o2 sensor readings on a scan tool this is easily identifiable by the big blip it gives during the o2 signal flatline of a WOT event
I am sure this is related to the same emissions systems protection programming and that it can be seen with a scan tool.
if you have some budget for custom tuning thats where i would look to solve this problem that Chrysler will say is "normal operation"

 

[09SilverRam]

Did you compare cruise + tow haul to without cruise (I’m assuming you are running without cruise) and see if the computer behaves differently?

All of the 4th gens I’ve towed with are early 6 speeds but cruise+tow haul and manually setting highest gear has given the most predictable downhill speed.

 

[2003F350]

Okay so...I am not going to claim to be an expert on this, but I am pretty sure that hemihustlin has it basically right.

You are trying to hold a speed under a speed that Cruise Control can be active (I believe 25mph is the cutoff), so the computer has no idea what speed you want to maintain. Gravity is pulling you down the hill, and gravity is going to cause you to accelerate if there's no real force opposing it (brakes, engine compression, friction, etc). Since you're under 25 mph you can't tell the computer 'hey I want to be at THIS speed' so it can't do calculations to keep you slow.

Couple this with the engine sensors trying to keep it running in a range of emissions factors, and you're going to end up in this scenario. The engine needs a certain air/fuel ratio to keep the O2 sensors happy, and with no other input coming in (i.e. a speed setpoint from the Cruise Control module) it is going to do whatever is necessary to keep those O2 sensors happy. Thus at low speed, as RPMs increase the A/F ratio changes, so the throttle plate needs to adjust slightly. Which, at such low speeds, will allow you to speed up because it isn't actively engine braking like it will with the cruise set.

Likely, if you were traveling at a higher speed, you wouldn't notice it as much. But at those speeds, you're going to need to work the brakes.