I've got a fun story. Well not so much fun as interesting
Truck ran like crap for about a year, with no engine codes, just random hesitation and some vibration at idle. I decided to do alot of maintainence to hopefully track it down. replaced valve cover gaskets, cleaned and resealed the injectors, cleaned the TB, upper and lower plenums and decided to do timing belt after 100,000 miles clocked. Found out that my crankshaft was off two teeth from the cams due to stuck idler pulley when changing my timing belt. When I opened up the lower timing cover under the crankshaft pulley, I found a large clump of black sludge and thought maybe an oil leak from the crankshaft seal. I then found a large clump under the tensioner pulley for the belt and as I'm scraping some out, a tail falls out
Somehow a small rodent had gotten under the timing cover. I still can't figure out how it got there, as there are no holes in the plastic covers, no chewed wiring or any damage at all. Truck runs like a dream again, I had driven around with it performing so badly, its like driving a new vehicle!
Judging by how all that was left of it was black sludge and pink stained clumps from a leaky water pump, the only thing that gave it away was the tail. How on earth could it get up in there? I'm starting to think someone would have had to unbolt a couple of fasteners from the top timing cover, pry it back, and drop it in.
Bizarre, right?