Bucking

[iamdub]

Oh well, it was an easy thing to check out and now I have another thing that we ruled out.

Right, which is the point- rule out the easy/free stuff first.

So, if it were the distributor, wouldn’t I experience this issue at all speeds?

Not necessarily. A shaft spinning inside a set of bushings with slack can intermittently get off balance and "buzz" as it vibrates. The bushings in the dizzy would have to be really worn for this to happen, though. Speaking of- the drive gear on the end of the dizzy is another common failure point.

I keep going back to how I only feel this violent bucking in fourth OD, when relatively low in the RPM range. It’s like I am asking for power, and something is preventing it from giving it to me unless I open the throttle enough for it to decide that it needs to drop a gear. In lower gears, and at higher rpm’s in any gear, it seems to be ready for whatever I ask it to do.

Detailing it like this sounds like it's load-based. Say you're cruising at a steady speed on flat road and you come to an incline, such as a steep overpass or bridge approach. The truck slows down because of the incline but you hold the throttle right where you had it. The truck's speed and RPM drop but the engine makes a deeper sound since it's under more load ("lugging"). Is this a scenario where the bucking would happen? What if you maintained that throttle position and lightly touched the brake pedal, just enough to trigger the switch but not apply the brakes?

Do you have access to a scan tool that can display live data? Seeing what the sensors are reporting and TCC status when it bucks would really help. Do you have access to a large trailer that you could tow to induce a load for testing purposes?

 

[Danneman]

Not necessarily. A shaft spinning inside a set of bushings with slack can intermittently get off balance and "buzz" as it vibrates. The bushings in the dizzy would have to be really worn for this to happen, though. Speaking of- the drive gear on the end of the dizzy is another common failure point.

First of all, I really appreciate you help me think this through. Thanks a ton. This site is so helpful.

If I pull the cap and rotor, is that enough for me to be able to check for shaft play? And drive gear wear? Or would the whole distributor have to come out? Not entirely familiar with this part, I've not worked on one before. But pulling cap and rotor I can definitely handle.

Detailing it like this sounds like it's load-based. Say you're cruising at a steady speed on flat road and you come to an incline, such as a steep overpass or bridge approach. The truck slows down because of the incline but you hold the throttle right where you had it. The truck's speed and RPM drop but the engine makes a deeper sound since it's under more load ("lugging"). Is this a scenario where the bucking would happen?

Absolutely, yes, lugging it is. Deeper, more muffled sound. When I hear that, in the scenario you describe it's only a second before the bucking starts. Now, in the hill situation, there will be no pushing through it (no speeding up past it) until gear drop / rev increase. But in other, non-hill situations (just regular resistance from things like wind), as I open throttle it will continue to buck but I will make headway with my speed eventually, just pretty slowly is all, until my revs are closer to 1800-2000.

What if you maintained that throttle position and lightly touched the brake pedal, just enough to trigger the switch but not apply the brakes?

Rev increase immediately, no bucking. I believe this is the TCC lockup test, eh?

Do you have access to a scan tool that can display live data? Seeing what the sensors are reporting and TCC status when it bucks would really help. Do you have access to a large trailer that you could tow to induce a load for testing purposes?

I don't have access to a scan tool, but I'm not opposed to purchasing (even if expensive) as I plan to keep this truck for as long as it's rolling. Is there a recommended brand/model?

I did pull a reasonably heavy trailer recently (rented...), Chicago to St. Louis and back, and it was painful. Bucking more pronounced.

 

[iamdub]

First of all, I really appreciate you help me think this through. Thanks a ton. This site is so helpful.


This site IS a great resource for help, info and just plain BSing about all subject matter with cronies of all walks. I'm happy to continue trying to fulfill my obligation to give as much as I've received here and elsewhere.


If I pull the cap and rotor, is that enough for me to be able to check for shaft play? And drive gear wear? Or would the whole distributor have to come out? Not entirely familiar with this part, I've not worked on one before. But pulling cap and rotor I can definitely handle.


It would be a simple and preliminary check, yes. Pop off the cap and inspect the terminals and the tip of the rotor for corrosion (although I'm certain this isn't your root problem). See if the rotor has any lateral play in it. You can kinda check the drive gear for wear without pulling the dizzy if you knew what you were feeling for. You should be able to twist the rotor a few degrees in one direction and it should rise up a little and snap back to it's original position if you let go. This is due to the drive gears being helical and it rides up the teeth when you twist it backward. It's difficult to explain as it's something you really have to develop a feel for with experience. But, the movement should feel fluid, smooth and "guided" with not a lot of slack. Again, I don't think ignition timing is your problem, it's just another simple factor to rule out. If anything with the cap and rotor does look sketchy, they're cheap and easy enough to replace. I highly recommend ACDelco or Borg Warner parts for ignition components on these engines. Avoid the parts store's house brands, especially Autozone's.


Absolutely, yes, lugging it is. Deeper, more muffled sound. When I hear that, in the scenario you describe it's only a second before the bucking starts. Now, in the hill situation, there will be no pushing through it (no speeding up past it) until gear drop / rev increase. But in other, non-hill situations (just regular resistance from things like wind), as I open throttle it will continue to buck but I will make headway with my speed eventually, just pretty slowly is all, until my revs are closer to 1800-2000.


Ah. Now we're on a trail. Definitely sounds like the ECM is confused about how the fueling and/or timing curves should be adjusted. This is likely due to it receiving misinformation of some sort. This is where being able to see a live data stream would really help. You could see what the MAF and MAP is reporting up until and during the bucking. Another free test: Unplug the MAF and test drive in a situtation where it would normally act up. Oh- you HAVE made sure your intake ducting and all is tight, right? I know that bonnet can easily be on top of the lip on the throttle body rather than hooked underneath it. Any idea what your fuel mileage is or if you've noticed a drop since these issues began?


Rev increase immediately, no bucking. I believe this is the TCC lockup test, eh?


Yes. I was curious if the bucking stops immediately when the TCC unlocks and the RPM jumps up that few hundred- still trying to isolate the problem to the engine or trans.


I don't have access to a scan tool, but I'm not opposed to purchasing (even if expensive) as I plan to keep this truck for as long as it's rolling. Is there a recommended brand/model?


A Tech2 is the ultimate for a GM vehicle. If you want one that you can use on other vehicles, you can get a lesser-capable "universal" model that can still be very powerful in helping you diagnose things. There are devices you plug into your ALDL port and they sync wirelessly to your phone and an app reads and displays the info. These seem to be the cheapest option and I believe most would give you the live data you could really use to narrow down the cause of your issue. I've never really researched them, but any amount of Googling would get you started. I have a $200+ scan tool that does almost everything a one-way tool can do, including ABS, SRS, transmission info, etc. I wish I had known about the Tech2 (or the "clones") being just ~$100 more before I bought it. But it's still good to have for the non-GM vehicles I work on.


I did pull a reasonably heavy trailer recently (rented...), Chicago to St. Louis and back, and it was painful. Bucking more pronounced.


The more pronounced bucking under increased load coincides with where we're headed thus far. Bringing up another rudimentary check- have you checked your trans fluid lately?

 

[Danneman]

All right. Great stuff. I think I'll hold on checking distro then for now, and come back to that if other things that seem more likely to be culprit don't pan out.

Your wondering about MAF and MAP info got me thinking...I always thought of this, early on, as some air/fuel trim thing, but I never considered MAP because I somehow got the impression that this truck didn't even have one! I always thought most vehicles had one or the other, and I knew I had a MAF sensor and have cleaned it a few times (and eventually replaced, for the heck of it).

So now, my question is whether I buy the scan tool now (I will eventually...) or try the MAP sensor first. I think I'm going to try option #2 and see where that gets me. And, if nowhere, that will again be something ruled out.

 

[Danneman]

Ok, so new MAP sensor and wow did it make a difference in overall performance but did not address the issue - although something interesting did happen for the first time today, that might be a clue.

I seem to stay in the higher end of 3 for longer and I have more power in the middle of 4. Noticeable at "city" speeds (like 45 mph zones) and very noticeable merging onto highway. At the low end of 4, light throttle, I still lug and then I get my bucking. BUT, three times right in a row today, around 55ish mph and still accelerating to match highway speed, the truck couldn't decide whether to stay in 4 or drop. Back and forth...2 seconds in 4, then 2 seconds in 3.

 

[iamdub]

Ok, so new MAP sensor and wow did it make a difference in overall performance but did not address the issue - although something interesting did happen for the first time today, that might be a clue.

I seem to stay in the higher end of 3 for longer and I have more power in the middle of 4. Noticeable at "city" speeds (like 45 mph zones) and very noticeable merging onto highway. At the low end of 4, light throttle, I still lug and then I get my bucking. BUT, three times right in a row today, around 55ish mph and still accelerating to match highway speed, the truck couldn't decide whether to stay in 4 or drop. Back and forth...2 seconds in 4, then 2 seconds in 3.

Ehh... This sounds like a trans issue, but still doesn't rule out an ECM and/or sensor issue, either. Have you tried manually shifting it through all gears from a stop to highway speed? Start in first and accelerate until you're around half-throttle. This should put you around redline. You don't have to bounce it off the rev limiter. Keep your throttle pedal steady and shift to second. Again, keep your right foot right where it's at. Wind out second then shift to third, then to D for fourth (overdrive). Take note if it does anything and at what RPM or gear.

Checked the trans fluid level and color yet? Try unplugging the MAF and driving?

 

[RET423]

I had similar symptoms once when my 02 sensor was sending a signal to the ecm that the fuel mixture was too lean, the signal was within range so no check engine light but power was poor and the bucking at lower RPM's was prominent. I disconnected the 02 sensor and everything smoothed right out although the check engine light came on, I surmised that ecm was using general settings without the 02 sensor input that were not great but better than the 02 sensor was providing so I replaced the 02 sensor and everything was great again.

When codes are not thrown these can be tricky to diagnose but that was what mine turned out to be.

 

[Danneman]

Ehh... This sounds like a trans issue, but still doesn't rule out an ECM and/or sensor issue, either. Have you tried manually shifting it through all gears from a stop to highway speed? Start in first and accelerate until you're around half-throttle. This should put you around redline. You don't have to bounce it off the rev limiter. Keep your throttle pedal steady and shift to second. Again, keep your right foot right where it's at. Wind out second then shift to third, then to D for fourth (overdrive). Take note if it does anything and at what RPM or gear.

Checked the trans fluid level and color yet? Try unplugging the MAF and driving?

I have not tried the manual shifting test, that sounds a little scary (or, it will for others around me, with that flowmaster!) but I'm game and I'll try to find a place around here where I can do it. Tough to find good test roads around the Chicago suburbs (I miss my country roads and soybean fields).

Trans fluid is definitely good.

it's ok to drive without the MAF? Highway speed, b/w 65-75?

 

[Danneman]

I had similar symptoms once when my 02 sensor was sending a signal to the ecm that the fuel mixture was too lean, the signal was within range so no check engine light but power was poor and the bucking at lower RPM's was prominent. I disconnected the 02 sensor and everything smoothed right out although the check engine light came on, I surmised that ecm was using general settings without the 02 sensor input that were not great but better than the 02 sensor was providing so I replaced the 02 sensor and everything was great again.

When codes are not thrown these can be tricky to diagnose but that was what mine turned out to be.

That sounds pretty promising...having experienced the symptoms...which sensor(s)? You're talking about the ones in the exhaust, I assume...before the cats or after? Or are there are other O2 sensors I'm maybe not aware of?

 

[iamdub]

I have not tried the manual shifting test, that sounds a little scary (or, it will for others around me, with that flowmaster!) but I'm game and I'll try to find a place around here where I can do it. Tough to find good test roads around the Chicago suburbs (I miss my country roads and soybean fields).

Trans fluid is definitely good.

it's ok to drive without the MAF? Highway speed, b/w 65-75?

Ugh. Yeah, you're gonna have to wait until later in the evening or drive out to the woods.

With the MAF unplugged, the ECM defaults to a basic speed density fueling table. Totally safe and a great way to test for a faulty MAF. It'll act up when you first unplug it, but it should smooth out and run plenty well enough to test drive.