Join Date: Mar 2019
Thanked 0 Times in 0 Posts
At this point, you probably don't need yet another direction from the peanut gallery, but this sounds similar to behavior I've run into on other vehicles.
Vibration at low RPM is, quite often, due to 1 or more cylinders either not firing or partially firing (slightly off timing, dirty plug, aging plug wire, etc.) At a higher RPM, the mass of the engine smooths out the "bumps", similar to a flywheel. The fact that things improved when you replaced the spark plugs is important. That leaves other potential misfire culprits such as old plug wires or the actual timing. Plug wires are fairly straight-forward. Timing is a bit trickier to sleuth.
If your timing belt is showing its age, you may have some "float" in the timing. Again, this would be less noticeable at higher RPMs but would affect all cylinders, not just one. If one or more cylinders is firing slightly too soon, or too late, you'll lose some power and it feels like a dull thud. You may also have a coil pack that is no longer sending spark to one cylinder, or sending a weakened spark. I once had a van that literally kept eating coil packs.
I tend to start diagnosing things from the simplest, least invasive (expensive) and work my way up from there. If it is a timing belt, you'll want to deal with that sooner rather than later. If it fails, that can be potentially...really bad. lol