When my ford 7.3 turbo diesel is cold, it makes a loud hissing sound about every 10 sec and last for 10 sec. Anyone know what could cause it? Thank you. Jeff

I've worked on a 1999 7.3L F350 PSD and it's one awesome truck. Many diesels don't like winter and they let you know this in different ways. Try using your block heater or synthetic oil and see if it helps. From experience, I can tell you diesels work much better in cold weather with synthetics. Personally, if I owned such a work-horse, I would only use synthetic diesel oil. I would also install a by-pass oil filter to filter that 15 qt. oil sump, which would allow me to get my money's worth out of the synthetic oil through extended drain intervals. This can be safely done through regular used oil analysis (UOA). This configuration will allow drain intervals from 60,000 to 200,000 miles. This is common these days. Check out Amsoil's lineup of diesel products.

Poleman; This sound like some pressure being relieved (I have little knowledge of this engine! ). By-pass like a waste-gate (if so equiped). EGR exercising in a stickey fashon (why on a cold engine? ) Fuel Pressure returning to tank. Put that in your MIX. Good luck.