Another question on my "new" 1989 F-150. I bought this truck last
week in Colorado (to tow a crippled dodge home and eventually replace
my '84 Ranger). I ran it around town (6-7000 ft), mainly on the front
tank, but a little on the back while I changed the oil and generally
checked it out prior to the trip. No worries. As I climbed above
8500 ft, (using the full rear tank, front was about half full), I got
a strong gas smell and it started to buck and jump. I pulled over,
and opened both gas caps. The front hissed for a second, and was
fine, the back vented (vapor only) for what seemed like a long time
(minute, more? Seemed longer but I didn't time it). I put both caps
on, and proceeded over the pass using the front tank, w/ no problems.
In the next town, I filled both tanks and put both caps on tight.
Tried the rear tank, ran fine until I got up to altitude (above 8K,
forget exactly), and repeated symptoms. I switched to the front tank,
symptoms went away. I stopped and removed the cap from the rear,
vapor flowed (but not for as long). I then put the back cap on loose
(one full turn out from tight). I switched back to the rear (once I
was moving again) and it worked fine for several more ups and downs.
I filled up again in Gunnison, and left the rear cap loose. It worked
fine to well above 10,000 ft, then started bucking again. I switched
to the front tank,and climbed the pass. The smell of fuel persisted,
and at the top (first chance I took to look at it), there was liquid
fuel spilling out of the (loose) cap I took the cap off and fuel
gushed out for a long time (again, I didn't time it, it seemed long).
I replaced the rear cap, (with a $7 stant, not a motorcraft), and
tightening the cap caused a similar problem on a lower pass later on.
I can suck air in through all three caps, but not blow out through
them w/ lung power. Running it loose was fine, and running it tight
is fine if I don't climb. (All of these symptoms were while towing a
minivan, if that makes any difference).
There seemed to be recent evidence of fuel being pushed out a
connection up by the (square plastic) box that I assume is the
unneeded fuel being sent back to the tank (it's fuel injected, 302).
There seems to be one return line, which T's down by the front tank
(how does it know which tank to send the fuel back to?).
The truck is now at 1600 feet, and runs fine with caps tight. I'm
probably 10 hours away from anywhere I can test any possible fixes,
but I'd like to know what the problem is/was.
I'm pretty sure I'm going to keep this truck, so will be buying a
manual shortly but any help in the meantime is appreciated.