Truck Repair: Map Sensor, speedometer cable, gmc suburban

Question
I have a 1988 GMC Suburban 1500 with a rebuilt jasper 350 with about 40000 miles on it.  It seems to be eating map sensors for breakfast lunch and dinner.  The fourth sensor i've put on it since about two months ago lasted one day and the fifth is sitting in the truck.  Most of the parts have been replaced on this thing.  It does need a speedometer cable which cause it to throw a code for VSS I believe.  The Map sensors seem to just stop working after awhile it runs great after putting a new one in and then after time it throws a code (34) and starts running badly smelling of gas and backfiring under load or when letting off.  I've purchased two kinds of map sensors the better one lasting the least ammount of time.  Any help with this would be appreciated.  Thanks

Answer
Hello Theron,

Actuall, I don't think the map sensor is causin the backfire, but rather, the backfire is what is destroying the map sensors.
Map sensors have a diaphragm intended to measure vacuum, and are not meant to absorb a large burst of pressure, which is what the backfire does.

Timing and fuel pressure would be the most common cause of the backfire, followed by valve problems.

The missing VSS signal will contribute to the timing possibility, since that signal is heavily relied upon for mixture and spark timing.

Van