07 5.9 cruise control bucking

[G Ogre]

Does anyone here have the definitive answer for the "bucking" that some owners, like me, experience with CC engaged, cresting a hill typically as the cruise reaches a point that it's feathering the throttle? I've had it since I bought my truck used, an 07 5.9 48RE, with 34K in 2011. It now has 95K. I've searched the web until I'm blue in the face, looking at countless theories, but nothing that's an absolute, "this is it, this is what you must do to cure it" response. Seems a lot of owners experienced the same thing, but most just live with it. The most credible (at least to me) theory I read had to do with a slight leak in an injector or two, causing the ECM to read an issue with rail pressure, which in turn caused the ECM to snap the fuel off and on repeatedly. I've been in a number of Ram dealer service departments, and I've yet to find a single person that will admit ever hearing about the condition. Hmmm....

Since the story I read (might have been here) seemed the most credible, and my OCD won't accept this as normal, my plan has been to go after the injectors at some point. In preparation, I added a FASS Titanium pump and filter setup, the thought being that if I'm throwing big money at an injector swap, at least they'd have a clean fuel supply going forward.

Any help would be appreciated. If injectors are the answer, so be it, but I'd really like to KNOW it'll fix it, as I'm 50K - 100K (I'd hope) away from having to change them because they're worn out.

I've been a member for quite a while, and really enjoy the Chrysler CSI stories written by Stan Gotti. This feels like a question for Stan...

 

[Motorhead]

I have a 5.9, 2007 and mine does the same thing. My truck is a stick shift. Do you have a programmer on your truck? Stock setting, the bucking is barely noticable. When I have my Smarty jr set up, the bucking is more noticable.

 

[G Ogre]

No tuner, bone stock here. It varies in intensity sometimes, but seems to be the terrain/conditions that affect it.

 

[Cloudmaster Don]

My 2006 2500HD SRW does the same, as does my 2002 Jeep Liberty, as well as a friend's 2003 3500 DRW. He is a pretty knowledgeable guy who believes "they all do that", and it is inherent in the design of the truck's electronic controls. He recommends just slipping out of cruise control when the bucking becomes annoying. There is apparently no fix. AAARGHHH!!!

 

[Motorhead]

My truck being a stick and clutch out, you automatic guys are in drive or Over drive and your torque convertor is locked. Any speed deviation with a locked drive train to the engine could cause the bucking. Older Automatics without locking torque convertors had the slip in the convertor to absorb any bucking. That's my theory but not necessarilly the truth.

 

[Cloudmaster Don]

Hmmm... I neglected to mention that both of my vehicles have automatics with lockup torque convertors, while my friend's is a stick. Since all three do it, what does that tell us? That automatics will buck with CC engaged and so will sticks? And that to avoid it one must either de-clutch or shut off the CC? Having automatics, my solution is to disable the CC, then re-engage when appropriate.

 

[Motorhead]

Don, I most of the time will apply a tad bit of throttle to overcome the bucking.

 

[Cloudmaster Don]

I've tried that technique, and found it works well enough on some hills, but by the time I've arrived at the bottom of most hills, I'm going much faster than I wanted to. Maybe it's where I live (San Diego), or maybe it's just me.

 

[TConver]

Mine was doing the same thing nothing fixed it until I replaced the injectors.

 

[G Ogre]

Mine was doing the same thing nothing fixed it until I replaced the injectors.

Thanks! You're the second one that has confirmed that an injector swap solved it. Hate to do it this early, but it makes me crazy when thing don't work right.

 

[NAllen]

Thanks! You're the second one that has confirmed that an injector swap solved it. Hate to do it this early, but it makes me crazy when thing don't work right.

I have 2003 5.9 5600 6sp mine did the same thing awhile back when new injectors installed it stopped.

 

[G Ogre]

Thanks! Good to know!

 

[Regcabguy]

I've tried that technique, and found it works well enough on some hills, but by the time I've arrived at the bottom of most hills, I'm going much faster than I wanted to. Maybe it's where I live (San Diego), or maybe it's just me.

I live here too. Never an issue with mine . 1st Smarty and now Anarchy EFI Level 1. I don't tow but do carry 1800# around all the time. G-56.

 

[cerberusiam]

I guess it just depends on the truck. Multiple weird acting injectors, mixed new and reman, tuned, un-tuned, deleted, not deleted, in several different trucks, the CC has always been rock solid, towing or empty. These trucks frequently have a unique personality.

 

[Cloudmaster Don]

Correspondence about this problem with a Washington friend of mine produced the following explanation. This is the same guy I referred to in my initial post (#4) on this topic, who drives a 2003 3500 DRW with a manual transmission. He wishes to remain anonymous with the quip that I should refer to him as a "grumpy old man in Washington." Here is his analysis:
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Not sure why replacing the injectors would matter. The problem is (as I see it) as you approach the point that the programmed speed has been reached on level ground, it STILL takes torque to maintain the speed, so the engine is under some load, which it likes, so the driveline stays under that load so any slack/backlash in it never comes into play.
Downhill, however, it takes close to zero engine torque to maintain speed (Mr Gravity is helping us out). If it's REALLY a downhill, it's easy - idle for sure.
On gentle slopes, however, the cruise tries to go to idle, but almost immediately a bit more than idle is needed, but by then, it's too much, so it goes full idle again, and so forth. That causes the driveline to come up against it's slop in both directions.
Some cruise controls have a sensitivity adjustment (or did back in the 60s/70s/80s) that allowed the speed to vary a mph or two so that "hunting" wouldn't happen. In today's digital world, not so. They are precise and try too hard to maintain speed exactly.
In gasoline cars, the torque converters are not so tight you'd feel the hunting, but in diesels they have a low stall speed, so there is less slippage so you DO feel the hunting, plus truck differentials probably have more backlash (you can rotate the driveshaft more degrees in either direction before the gears touch).
It's much more annoying in my manual transmission Cummins because there is NO SLIPPAGE since it's direct drive, and you feel the backlash in the driveline.
"Backlash" is the amount of rotation of the engine crankshaft in degrees that are required when reversing direction to take up the slack in the gearing (torque converter/transmission gears/differential gears). A soft torque converter and auto trans on a gas engine damp out such backlash much better than a "tight" torque converter with an auto trans, or worse yet a manual trans/clutch.
That backlash added to the cruise control trying to find the perfect throttle setting downhill (no engine load) gives that jerky response you're feeling.
Besides, I put a brand new set of injectors in my engine a couple of years back when my re-manufactured engine had maybe 20k miles on it (I'd bought a complete set still sealed in plastic tubes as spares for $1000 because they were a steal) but I had the old injectors tested and they were said to be perfect, so I reused them. But it seemed dumb to have new ones in a box when they might improve performance and MPG, so I put them in only to find no better performance, and a 15% MPG fuel economy loss!

Anyway, I have exactly the same cruise control bucking with the new injectors as the old ones. EXACTLY the same, so I wouldn't spend the bucks if you don't need them for other reasons. The bucking (oscillation, really) depends on the slope. Most of the time it's mild, so is mildly annoying, but some hills (just the perfect slope to cause the phenomenon) cause really badly annoying bucking (that I'm sure is not great for the driveline, either) so I just disconnect the cruise and re-initiate it on more level terrain. After 9 years of owning the truck, I'm resolved to it I guess.
=====================================================================
So that's the word from the "grumpy old man," whose opinion and explanation I value just as I do the opinions and explanations of others on this forum.

 

[cerberusiam]

Not sure why replacing the injectors would matter.

ECM is constantly monitoring engine rpm from individual and composite injection events to maintain a demanded rpm. If you have injectors that are not consistent in there injection events it introduces changes to fueling that may not apply correctly on the next revolution. While injectors may idle relatively smooth once demanded operations increase there variance can increase leading to minute surging and\or bucking.

Downhill, however, it takes close to zero engine torque to maintain speed (Mr Gravity is helping us out).

Gravity will only negate the need for fueling when it can overcome wind and rolling resistance. On most roads that stabilizes in the 30 mph range depending on truck, tires, etc. Engine will still fuel till make HP and TQ and still be running against a load. Back to bad injectors, if they are not balanced and consistent you will get surging and possibly bucking.

A soft torque converter and auto trans on a gas engine damp out such backlash much better than a "tight" torque converter with an auto trans, or worse yet a manual trans/clutch.
That backlash added to the cruise control trying to find the perfect throttle setting downhill (no engine load) gives that jerky response you're feeling.

Modern automatic transmission function the same way as a manual transmission, the TC locks in most conditions so TC softness or tightness has no bearing. At anything above 50 mph and normal loads TC is locked so it is 1 to 1 thru the trans. An auto trans a built in buffer to the backlash on coast, the sprag clutch in the OD section but only if speed will over run inputs and it usually does not unless an EB is used. The speed pcontrol on these trucks has sensitivity but not so much it can compensate for out of range issues cause by the fuel system, it is looking at speed only and not taking into accoubt what the ECM is going so it reacts badly at times.

I put a brand new set of injectors in my engine a couple of years back when my re-manufactured engine had maybe 20k miles on it (I'd bought a complete set still sealed in plastic tubes as spares for $1000 because they were a steal) but I had the old injectors tested and they were said to be perfect, so I reused them. But it seemed dumb to have new ones in a box when they might improve performance and MPG, so I put them in only to find no better performance, and a 15% MPG fuel economy loss! Anyway, I have exactly the same cruise control bucking with the new injectors as the old ones. EXACTLY the same

I would say your deal was not so much of a deal. If you are losing efficacy by changing injectors there is a difference that could translate to a problem. There are few testing places in the US that has either the equipment or experience to adequately test CR injectors. There testing algorithms are not anywhere near accurate or comprehensive enough to find balance or consistency issues. The specs Bosch publishes are to broad in a lot of cases and will lead to idle issues, rough running in certain rpms, surging and\or other problems at speed.

As I posted I have a mixture of trucks and tunes, none of them have a surging or bucking issue as they sit now. The onyl times I have nticed ANY issues with speed control it has always been a fuel system issue and mostly injectors.

 

[Cloudmaster Don]

cerberusiam,

Your expertise is much appreciated. Thank you. I will forward this to my friend in Washington, and see how he replies. Yes, there really is such a guy as I do not have the technical background to analyze such problems, much less write about it.

I know of a shop with injector testing equipment, but do not know what kind it is, nor how much experience they have. I will make a list of questions to ask before interviewing that shop.

 

[G Ogre]

ECM is constantly monitoring engine rpm from individual and composite injection events to maintain a demanded rpm. If you have injectors that are not consistent in there injection events it introduces changes to fueling that may not apply correctly on the next revolution. While injectors may idle relatively smooth once demanded operations increase there variance can increase leading to minute surging and\or bucking.

It's more than minute...quite funny to see how new passengers react..."what the hell is wrong?" as they grab the armrest!

Gravity will only negate the need for fueling when it can overcome wind and rolling resistance. On most roads that stabilizes in the 30 mph range depending on truck, tires, etc. Engine will still fuel till make HP and TQ and still be running against a load. Back to bad injectors, if they are not balanced and consistent you will get surging and possibly bucking.

Yep, only happens when the CC is sort of feathering throttle...on steeper downhills, zero throttle, it stops.

As I posted I have a mixture of trucks and tunes, none of them have a surging or bucking issue as they sit now. The only times I have noticed ANY issues with speed control it has always been a fuel system issue and mostly injectors.

Thanks very much for weighing in on this...really is leading me to an injector replacement. A couple of questions: who's or what injectors would you use? And I was reading some references to all the weird things that can be attributed to fuel control actuators. While I'm generally not a fan of blindly throwing parts at a perceived problem, this is likely the last truck I'll ever own, so I look at things in the really long term. If it's something that'll likely fail down the road, I don't mind getting in front of it. In your opinion, any chance it could affect the CC condition? Thanks!

 

[cerberusiam]

FCA can cause some weird things also, it is what controls actual rail pressure. If it doesn't work correctly the ECM will try to extend it s duty cycle to create more rail pressure then pull it back if\when it goes too far then it will go back to far and the cycle starts over. It is a PWM solenoid that is moving a little 3 cornered block to control amount of fuel into the high pressure side of the CP-3 to generate demanded rail pressure. With ANY elector-mechanical device little things can cause big issues down stream. If you see more than a 500-600 psi swing in rail pressure it could effect a lot of things.

Top of the line injectors are BBI. Better QC, issues with stock nozzles fixed, etc. New Bosch injectors, OE or the marine injectors are the next best. Anything else tends to be a crap shoot because there is no consistent control on quality and configuration across vendors, they all think the know best and how to do it.

 

[verk]

It's much more annoying in my manual transmission Cummins because there is NO SLIPPAGE since it's direct drive, and you feel the backlash in the driveline.
"Backlash" is the amount of rotation of the engine crankshaft in degrees that are required when reversing direction to take up the slack in the gearing (torque converter/transmission gears/differential gears). A soft torque converter and auto trans on a gas engine damp out such backlash much better than a "tight" torque converter with an auto trans, or worse yet a manual trans/clutch.
That backlash added to the cruise control trying to find the perfect throttle setting downhill (no engine load) gives that jerky response you're feeling.
...
Anyway, I have exactly the same cruise control bucking with the new injectors as the old ones. EXACTLY the same, so I wouldn't spend the bucks if you don't need them for other reasons. The bucking (oscillation, really) depends on the slope. Most of the time it's mild, so is mildly annoying, but some hills (just the perfect slope to cause the phenomenon) cause really badly annoying bucking (that I'm sure is not great for the driveline, either) so I just disconnect the cruise and re-initiate it on more level terrain. After 9 years of owning the truck, I'm resolved to it I guess.

This is the best explanation for a phenomenon that I too have been experiencing for a while. When truck was new this didn't happen at all, but as backlash and overall play in drivetrain have increased over the years so has this phenomenon. Thanks to anonymous for writing about it so descriptively.