944 engine vibration

[GTR34]

Hi guys,

I have an 87 944 with some odd sounds coming out of it. When the rpms reach 3,500 - 4,000, I get this raspy, very bad sounding vibration. It sounds like the engine is chewing something up or there's some sort of worn out part creating the vibration, but it only happens in that 500 rpm range, it's pretty smooth everywhere else. What could this possibly be?

[Gawernator]

Weird... could the balance shafts be out of time or the sprocket installed 180 degrees off?

[mytrplseven]

First, check the heat shields near the exhaust headers, then the one over the muffler. If that's not it, I'd suspect motor mounts. Following that, I'd pull the belt cover and see what the status is of the balance belt. If it's broken, damaged or real sloppy, you're there. If it breaks, it'll likely take the timing belt with it and then your really screwed. Do you see a drop or fluctuation of oil pressure when this is happening? That could mean broken oil pickup tube.
Does the clutch feel ok? Any vibrations coming through the shifter? Does this go away when the AC is switched off? You could have a loose or failing serpentine belt.
That's the stuff I'd look into that's common areas others have run into with vibration issues.

[Gawernator]

You would think if it was bad motor mounts that he would notice it at idle

[GTR34]

I do notice vibration through the shifter, the belts have apparently been replaced 2 years ago. The clutch feels fine. So you're saying it could be the belt? Funny thing is, the guy who owned it previously balanced GE turbine engines for a living for 20 years, so if that's it, it may be a huge fail on his part :b

[mytrplseven]

Thing is, there's no balance belts on the GE engines. I flew them for 30 years and have yet to see one .

[GTR34]

Right, but being an engine balancer, you'd think he would take care to make sure the balance shafts and belt are correctly installed, but who knows. Should I be concerned about this vibration?

[Gawernator]

I feel some vibration through the shifter, I'd say it's "mild" mostly on the lower RPM range. So depends on what your vibration is. It's not enough to shake my hand or anything.

[tamathumper]

At least on the 968, that's exactly the RPM range where you'd expect a harsh vibration if one of the balance shafts was 180 degrees out of phase, and looking at the PET the balance shaft setups are effectively the same.

It can also cause shifter "walking" from front to back, sometimes severe. I had a shop screw up my balance shafts during a routine maintenance, then deny the shafts were out of alignment on several follow-up visits to address the vibration. I even changed out the clutch and dual mass flywheel on their "advice". I ended up taking the belt off and fixing it myself, and the problem was instantly solved. Guess who I haven't been back to since?

Even with a good balance shaft setup, there's intermittent vibration or "buzz" in the shifter and/or it's boot, especially if the top knob is worn.

I used to work for GE Steam Turbines, with Engineering, and there's no correlation between the two.