Going to see if any of you guys with experience with these engines may have some thoughts. We have a boom lift with one of these engines that I recently adopted. It runs on LP and seems to both idle good and solid at WOT for operating. The issue is the area between off-idle and high idle for operation. It seems to get into a vibration zone that is a bit more than I would consider normal. I realize I4 engines typical can be hard to balance and may have a little buzz to them.

I did note that 2 intake manifold bolts were missing and I have replaced those but entirely possible a small air leak is present. But I have seen something like that cause vibration unless the engine is missing and this ones does not seem to miss a beat anywhere.

It appears the engine front has rubber isolation mounts but the rear bell housing is rigid mounted to the chassis. The vibration does seem more pronounced at the front of the engine around the accessory area.

My basic initial test was to grab the throttle linkage when idling and move it up slightly. With just a little over idle, it really buzzes but as soon as I lower back to idle, it is smooth again.

 

Welcome to the forum

I would wait until engine has been off overnight so stone cold
Then remove fan belt(s) from crank pulley

Start engine and see if you still have the vibration

You can run the engine for about 2 minutes with no water pump, but less time is better, so shut it off if there is still a vibration, if there is no vibration then one of the accessories is causing vibration
You can also HEAR any noise much easier with no cooling fan on

I think it would have a direct drive fan(or electric), but if it has a fan clutch they can cause an rpm specific vibration when failing, that would go away with fan belt off

The crank pulley/harmonic balancer could be rotting out, give it a good look and feel

When was the timing belt last done?
Could be belt or tensioner is on the way out, 6 to 8 years was the change schedule

 

I did a prelim assessment and I will have to study any concerns of disconnecting the belt driven Hoof governor, BUT as I examined the radiator fan, I found obvious damage on the fan. It is a stamped/riveted steel 5 blade design. I first rotated it to a known point so I could roughly check for straightness of the blades or axial runout. There was probably 3/4" variation. Then as I was hand rotating I found at least one blade that struck something hard and curled the tip of the blade a bit. I really cannot get good access here and will probably resort to pulling the radiator so I can examine further.

If the fan is really doing all of this, I would be baffled as it is smooth at full RPM.

I also tried to examine the balancer and looks like the rubber is internal. I really don't want to swap that without some sort of confirmation. The PN on the pulley is E3ZE6312AA. There is at least one on egay. I am not sure if that is a typical pulley for these engines or not? It is pretty typical on industrial applications for them to install more robust or otherwise different parts like manifolds, accessory mounts, etc. But the balancer is probably a common part for something.