Looking for some insight on an issue I've been chasing on my Discovery for a bit. I have a 2003 Discovery 4.6 that I have owned for 3 years. Bought it with 135k, and it now has 155k. Throughout my ownership, it has a shutter on cold startup, when RPMs drop, and acceleration that has progressively gotten worse. I have replaced nearly everything on this car throughout the years: spark plugs, wires, coil pack, secondary air injection parts, engine pulleys, MAF, engine mounts. Most recently, the truck died on me over the winter with crank/sometimes no-crank start, and I replaced the Crankshaft Position Sensor (brand new OEM Bosch), and it fired right up. When idling, the car does shake as if it is misfiring. The vibrations are present most notably from 1100-1800 RPM, more so when RPMs drop and on acceleration. The vibration does go down as the car warms up.
This leads me to my next point. When at operating temperature (I have a 180* t-stat), there is excessive heat underneath the driver's-side and passenger-side floorwells. Occasionally, if the engine is hot and I go to restart the truck, it will not turn over or crank. Full power, but nothing. After letting it cool down a bit, it will start. I have no codes. When running, the truck does seem to have a knocking noise near the cats that makes it sound like a diesel engine. The engine shakes in the engine bay, and it hesitates for a quick second on cold starts. It does have a slight rattle underneath near the cats as well when idling. I am at a loss for what to continue to look for. I have zero codes on the dashboard except for the three amigos (P1580) and P0126 for the 180* t-stat that comes and goes. Two mechanics were unable to figure out the issue. I am located on Long Island, NY.

Any insight is appreciated! I love this truck and want to continue driving it forever. It is a very rare Icelandic Blue, making it all the more special to me.

 

Lot to unpack here. Sounds like you have potentially bad/clogged cats, exhaust leak maybe, and a sensor that's contributing to no start when warm (can't remember cam or crank, but it's covered here at length).

This is not a common problem and I may be wrong but the cams on these do kind of wear out especially if not well maintained and this is not going to keep the truck from starting but if/when the time comes to do HG I'd very much look into the condition of the cam.