hello all. Over the past few days Ive noticed the truck beginning to run rough. I thought at first mabey I had a bad tire, but then I decided it was actually the motor. Its got kind of a rumble in it.
Its gotten worse yesterday and today. At high speed (55-60) its almost unnoticable, but as I let off the accelerator it starts to rumble and then shake. It isnt bouncing. or shaking side to side.. more like the motor vibrating, loud and hard. The rpms seem a little lower than usual as well. My memory isnt what it used to be, but it SEEMS like the last time it did this, the harmonic balancer was bad. Is there a simple way I can check it? Does it sound like this is probably whats wrong? I think I replaced the last one myself but its been a few years, and I want to be sure thats the problem before I buy a new one. Thanks guys..

 

Start the engine, and have a look at the balancer..... see if it's doing anything it shouldn't.

Getting any engine codes? How old are the O2 sensors?

 

The check engine light comes on about every four or five minutes and flashes six times.
I just started it up and stood off to the side and watched it idle. The balancer does look like its got a wobble to it. Like perhaps its warped?

 

I assume you have the 5.9 liter? If so, the balancer isn't perfectly round.... it also helps balance the rotating assembly, so, there is a bunch cut out. That makes it look like it is wobbling, when it really isn't. If you watch the front edge of the balancer, it it moves back and forth, THAT is a problem.

CEL flashing indicates an engine miss. (and would also make for a rough idle.....) Can you check codes?

 

it doesnt idle rough, its actually a loud hard vibration from the motor. It isnt constant, it happens when Im doing say, fifty to sixty mph and I let off the gas. at first it was like it shuddered, but now its much louder and harder and kinda scary. I cant really check codes. and looking at it as its running, the balancer does "appear" to kind of go in and out. Hard to explain kind of because it spins so fast.. but looking at the front edge while it spins.. it appears as if there are two front edges? like its sliding in and out. I doubt if I can get a vidio of it but I may try.

 

yeh, looking again with it not running, I see what you mean about a section being cut out. That Would give the illusion that its wobbling. Now I dont know.

 

Check your u-joints in the rear driveshaft. Balancer wouldn't induce that kind of vibration.

 

The u joints dont appear worn or loose at all. I grabbed the drive shaft and tried to shake it. Tight as can be. Nothing underneath looks out of place.

 

Try bumping the trans into neutral before you decelerate, see if that changes anything. (careful though, don't wanna hit reverse by accident.)

 

Might want to take the truck to a shop with a lift, get it up in the air with someone behind the wheel and rev the motor a bit while observing the balancer from below - if the balancer is moving in and out, its likely bad. You can also check it while the vehicle is at rest and see if the rubber sandwiched between the hub and the outer ring of the balancer appears deteriorated - if so, get a new balancer.

If not the balancer, also check the flexplate by removing the front lower inspection cover and see if the balance weights are still in the front of the flexplate.