All kinds of bucking fun

S

StainlessRRA

Advocatus diaboli
Joined
Oct 23, 2008
Messages
6,316
My truck wants to "Buck" under very specific circumstances. It's bad enough to chirp the tires or spill drinks and absolutely driving me up the wall. Let's play guess which sensor is bad...

The buck happens consistently in the 2100 RPM range. Nowhere else. This happened once previously, but after a few changes were made in the tune it seems to accentuate the problem. The truck needs to be at least halfway to temp and the air needs to be cooler, in the 60s. It has happened with warmer weather but it is far more common with denser air. And I also need to be light on the throttle. Around 20% on the tps. If I get on the throttle harder the issue is nonexistent. I also have more smoke than I think is reasonable. While I have a set of II 88% overs that aren't ideal, they shouldn't smoke this much. The truck definitely doesn't spool or respond the way I feel it should.

I've done the obvious stuff. When I checked for boost leaks, I found a small one. Fixing it made the problem worse. I also notice on the logs the exact moment I have this buck the truck is at about - 3.4 degrees of timing. I'm inclined to believe a map sensor could be the culprit and was tempted to order a 10 bar sensor. The really annoying part here is trying to be on the highway, I have to avoid that entire RPM range. The sudden jolts and little puff of white smoke are making me nervous and getting really frustrating. Any ideas?

03 5.9 with two pumps, II 88%, and a little 66mm.
 
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So what did the sensors look like in the data log?
 
JWhite said:
So what did the sensors look like in the data log?
Click to expand...

Any sensor in particular?

I dug out an old boost gauge and hooked it up. Boost reference seems to be correct. But yet again this morning it feels like a completely cuts the power for half a second around 2100 RPM and puffs white smoke.
 
I'm going to guess FCA controller, or rail pressure sensor.
What is the rail pressure looking like when it bucks ?
 
Yea...I'd bet it's a pump controller issue. Ditch the dual pumps and just put a ten or a twelve mil on it.
 
StainlessRRA said:
Any sensor in particular?

I dug out an old boost gauge and hooked it up. Boost reference seems to be correct. But yet again this morning it feels like a completely cuts the power for half a second around 2100 RPM and puffs white smoke.
Click to expand...

Uh, all of them? You had a data log on it, should have looked at every pressure and temperature available to see what is flaking out. At this point it's throwing darts at a board
 
Snedge said:
I'm going to guess FCA controller, or rail pressure sensor.
What is the rail pressure looking like when it bucks ?
Click to expand...

Roughly 18k PSI and very close command vs actual. There is a new 6.7 rail sensor in it, which the tune is supposed to be scaled for. I don't know what they scaling is for a 6.7 vs 5.9 sensor is however. I had wondered if this issue was possible if the tune had accidentally been scaled for a 5.9 sensor.
 
Phooker said:
Yea...I'd bet it's a pump controller issue. Ditch the dual pumps and just put a ten or a twelve mil on it.
Click to expand...

I would be more apt to believe this if it wasn't happening at the exact same RPM in the exact same circumstances.

JWhite said:
Uh, all of them? You had a data log on it, should have looked at every pressure and temperature available to see what is flaking out. At this point it's throwing darts at a board
Click to expand...

Two things stand out to me. The timing being negative and what the final timing correction factor is. Nothing else looks strange.
 
What does your tuner say the problem is? When did this issue start happening? can you take a video of the 'bucking'? Can you floor it and skip through the 2100 RPM bucking nonsense?
 
StainlessRRA said:
I would be more apt to believe this if it wasn't happening at the exact same RPM in the exact same circumstances.



Two things stand out to me. The timing being negative and what the final timing correction factor is. Nothing else looks strange.
Click to expand...

What does your fuel quantity do? You suspected the map sensor, but does it go haywire in the log?
 
WUnderwood said:
What does your tuner say the problem is? When did this issue start happening? can you take a video of the 'bucking'? Can you floor it and skip through the 2100 RPM bucking nonsense?
Click to expand...

No idea, they're a bit busy. It was right after a tune update that incorporated negative timing to cut the smoke. It had happened once before but not as pronounced. I have video but it's rather pointless as it's hard to see. You really feel and hear it in the truck but not as much on video. Yes.. If I give it any decent amount of throttle there is no issue. But I really don't want to drive that a jackass all the time.

I don't want to blame the tune. I think I have a deeper problem that this is bringing out.
 
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Snedge said:
TPS ?
Click to expand...
I meant to ask about related gearing and road speed.
With the added torque for my fummins, it is doing the exact same thing. If I hit a certain speed in overdrive, I've either got a wrap problem or a driveline that's causing the issue. It will buck as long as I drive there. Throttle up or down kills it, additional torque application kills it.

I also question the cam position sensor on the other Kyle's issue. I'm not sure how heavily the cummins engines base timing events on the cps

Sent from my motorola one 5G using Tapatalk
 
do you think it is over retarding the timing....and if ya mash it it brings more timing back in and it goes away

I have no idea ..this is over my head mostly...just keyboard mechanicing
 
dieselbeef said:
do you think it is over retarding the timing....and if ya mash it it brings more timing back in and it goes away



I have no idea ..this is over my head mostly...just keyboard mechanicing
Click to expand...
I've had that thought, but haven't gotten a log.

Sent from my motorola one 5G using Tapatalk
 
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