1985 745t

A week and a half back my brick decided to become possessed. Random stalls, wierd and rapid tachometer fluctuations, died while driving, etc. I pinned it down via tests of the ICU et al to (I think) either the coil or the hall sensor. Cash being tight and a trip being imminent I decided to swap in a new coil for $40 and hit the road.

Fast forward 2700 miles. The car made it 2/3 across the US to my destination without a SINGLE problem. Started and ran stronger with the new coil, started every time in the morning when I was driving out, ran strong and idled well with only a slight (and acceptable) variation at idle.

However, I go to start it yesterday morning for the second or third time since I've been here and the problem is back in full force. Car sometimes won't start at all (just cranks), car will stumble at slower speeds especially, tach flips and flops, etc.

So can I be sure this is the hall sensor? Its a pricey replacement (even if you decide to just drill out the sensor) and the car is old, so I'd like to be sure before I go spending more cash on it. Just to check, if you get a slight blip on the tach when you turn the key to start cranking BUT you don't get any blips on the tach WHILE its cranking does that mean the Hall Sensor is dead? Mine always gives me one little hop when I first turn the key but then just pins to the bottom of the tachometer.

FYI, jumping the fuel pump whirs it right to life, so its most definitely not a supply issue (yet ANOTHER of my developing problems...).

Thanks in advance for any help.

-rt

p.s. Is heavy duty pressure escaping from the fuel cap on really hot days normal? I'm in UT now, and through SD and out here I've been getting really big wooshes whenever I unscrew the cap as pressure releases from inside the tank (I've smelled teh gas vapors as it equalizes). Is it a sign of a faulty cap?