Bucking Under Heavy Load?

[Cruecible]

So I had my mechanic install the timing belt kit back in May and it all went pretty smooth. He said that he likes to install the belts a little loose to allow them time to settle in, then he adjusts the tension as needed. The power steering belt was the first one to act up, so I took it back in and he adjusted it and it worked perfect after that. About a month ago (haven't driven it much in the past month), I noticed that while under heavy acceleration, my truck bucked rather hard while I heard the supercharger spool, but then went away and was fine when I left off the gas. I thought this was pretty strange, so I punched the gas while already going about 40 and it was fine.

Over the past couple days, I noticed it doing the same thing so I immediately thought it was the A/C belt that also runs to the supercharger. I popped the hood and felt the tension on the belt as having more deflection than recommended in the FSM so I tightened it as best as I could. Still same issue, so I took it back to my mechanic where he checked all the belts and got them adjusted properly. I drove off, and again it seems perfect under normal load and driving conditions. This morning, I had to punch it after turning into traffic, and same thing where it bucks really hard. I thought it was maybe traction control kicking in or something, so just for curiosity I turned the VDC off and punched it from a dead start and it still bucked.

Called the mechanic and he won't be able to get me in until Friday to do a ride along so he can try and diagnose it. I'm really confused on what it could be, as all the belts are nice and snug. I'm thinking maybe something got knocked loose during the timing belt install? Just for recap, this only happens from a dead stop or around 5mph when I floor it. Never happens any other time. I haven't done any new performance mods or anything else, aside from rear diff drain and fill along with the transfer case. Is it possible that with all new belts that it's delivering more power than before with the worn out parts and I'm not used to the traction control kicking in?

Any ideas on what to check before I take it in?

 

[granitex]

If anything he should have had the tension on the timing belt a tad tight instead of loose, all that they can do is stretch a bit and get looser, they can't shrink to fit.

 

[Cruecible]

I'm a little nervous that it might be the timing belt. I should've clarified and said that his tension preference applied to the drive belts only, like the steering belt and the aforementioned belt that feeds the supercharger. The timing belt is really the only thing I can immediately think of as being the problem.

I drove it at lunch with the a/c off and it still is happening, and not so much only when I punch it, but probably anything above 70% throttle from about 7mph down. The throttle cutting issue probably still happens at a higher speed, but I don't notice it because it doesn't buck as pronounced as it does at a lower speed.

 

[Cruecible]

Read through the FSM and I didn't come across anything that sticks out as being a possible issue. It has to be something related to when they installed the timing belt kit, and he can't get me in until Friday to check it out which is disappointing. My MPG's are normal, so at least that's one less thing to worry about. I think I'll run by Autozone to see if any codes pull up. I haven't thought to look at the RPM's while the bucking happens, I'll do that too.

 

[Cruecible]

Could just be a coincidence that it happened now, and not related at all.

My first thought in reading all this is that your boost from the SC is cutting out/getting released. To me that sounds like your KS is going out. Might not be dead just yet, but could be on it's way. When they fail, they open a little valve at the back of the SC and let out all but roughly 2 lbs of boost. This "could" be the cause of your "bucking". Have you had the codes pulled yet?

Cheers,
JP

Not yet, I'll be able to have them checked in an hour. The KS was replaced when I bought it a little over two years ago. And when I floor it on the interstate, everything sounds fine and strong including the supercharger. It's really weird man. Like I said, my initial instinct was the supercharger belt was slipping, causing it to buck or jerk as it was trying to get "traction" on a pulley. I mean, that seems to be the most logical and simple conclusion to me. But I went down yesterday to my mechanic and he tightened everything up and I checked the belt deflection both in the morning and at lunch and it was good. Is it possible it's maybe too tight now? I have no clue, but I trust him enough to feel that he'll know what the issue is when I take him for a drive.

 

[TJTJ]

If you floor it from a stop, it SOUNDS LIKE you have an initial acceleration, and then the "buck" where I'm thinking the acceleration stops, so your head, etc, goes forward as if you braked, and then it "catches again", and accelerates again...so your head swings back as you go forward.

(That's what your "buck" seems to appear as?)

I'd run the codes as discussed, but, typically...a situation like that means you either lost spark or fuel at WOT.

So, a ground strap or electrical connection might come loose on a jolt of acceleration, say when the battery rocks back, and then reconnect, etc...but, generally, bumps might trigger it too.

The distributor might be on its way out.

There might be a clog in your fuel filter/line, etc.

An injector clog, etc, could do that too.

The engine is twisting under acceleration, and the air intake from the air box to the engine is being flexed, and a leak can develop, causing the loss of vacuum...and if a motor mount is bad, that would make that twisting worse.




Overall, things that are worse under WOT are fuel flow, airflow, and spark...so, I'd be checking from dizzy to plug, fuel delivery, and intake integrity, in that order.

 

[Cruecible]

I think you're exactly right TJ. I have a bad motor mount that I need to replace (already have a new mount in the garage) but have been putting off. I think something is happening when the engine twists that is causing the power jerks to happen. I'm going outside right now to see how tough this will be to replace

 

[Cruecible]

Well, bad motor mount was the culprit in that it probably accelerated a torn intake boot. FML. I didn't see any feasible way to jack up the engine safely to be able to get the motor mount off, so if someone has a pro-tip for that I'd sure appreciate it.

While on the subject, if someone has a spare intake boot they want to sell, shoot me a message.

 

[Cruecible]

New intake hose is $50 at the local dealership. I don't feel confident in installing the motor mount myself, so my mechanic will cut me a deal on the labor and it'll work out good.

Thanks for the assistance gentleman, I truly appreciate it!

 

[TJTJ]

Glad you got it all sorted out!

I like it when the diagnostic guess turns out to be the correct solution.

It saves SO much time.

 

[Cruecible]

For sure! I just wish I was more confident and had the "proper" tools to do the engine mount myself, but I'd rather it be done correctly then have me tool around on it for a few hours.

 

[Cruecible]

Got it all fixed. There wasn't anyway he could get to the top bolts of the mount easily, so he unbolted the front diff to drop it a couple inches to make life easier. The old one was non-existent and was overdue for replacement. Plus side of this ordeal is that my gas mileage has gone up significantly from babying it around town.