Weird Nailing Symptom

[R Leo]

Ok, rewind to just before Thanksgiving. I loaded Marlene with wife, dogs and daggone near straight WVO and drove 200+ miles to Dickinson for Turkey with Dad and Stepmom. Temps were mild (not cold) but, I did run the fuel heater all the way...no plugging or solidification issues with the WVO. For the return, I topped off with diesel (7-8 gallons) and returned home.

Pretty soon after the return, I began to notice a slight nailing at drifting throttle settings and med-high rpm (2800 rpm) if the car wasn't fully warmed up or hot. The nailing gradually became worse as the ambient temperature dropped approacing the Xmas holidays. By the week before Xmas, it was bad enough that I was beginning to think something was loose under the valve cover.

Also, because of the suddenly emerging fungus problem with the 300D, I'd been running straight diesel since the return from Dickinson. Consequently, I haven't been heating the fuel when driving (it had also blown a fuse).

Anyway, the nailing seemed to be getting more and more prevalent until last weekend when, I replaced the fuel heater fuse and got the heater working. I ran around for a couple of days with the heater operating to see if it was going to pop this fuse and suddenly realized that the nailing had ceased.

No nailing at all, with or without the heater now. Granted, the ambient temps have been ultra mild (50°F at night, mid 60s day). Today, the weather has turned to crap again and we're in for some low temps for the next few days. I'll see if the nail returns.

Any observations? Comments?