91 Blazer map sensor

I've got a '90 Chevy Blazer with a 4.3L in it, 180K miles. Ive been having hard start ups in it and it will not idle for nothin. I normally have to sit out there for a 1/2 hour w/foot on gas trying to keep the engine runnin so it will warm up enough so I can drive it. A friend told me that he thought it was a bad map sensor. How he can to this conclusion I don't know. Can anyone tell me what the symptoms of a bad map sensor are? Would it cause hard starting and no idle?

yes it could, but so can so many other things. when was your last tune-up? check timming, vacuum leaks, and fuel pressure.

I tried a new map sensor and still experienced the same problem so let me elaborate on how he came to the conclusion that it was a bad map sensor. What he did was unplug the wires from the map sensor and the engine would idle good and would have good response when he stepped on the gas. This would work for just a little bit and then the engine would run rough again then die. In order to get the engine started again, he would plug in the map sensor and it would start right up and run crappy again. The minute he unplugged the map it would come right out of it and have good response w/accelerator. I would also like to add that when he had the map sensor unplugged and it seemed to be running good he would put it in gear then the engine would load up and die. So it's obviously something else, what could it be? any ideas? Could it be the timing, if so, how do I check this exactly? could it be the IAC-can I clean this and how? could it be the EGR valve and can this be cleaned and how? sorry for the long post but if anyone had anything to add please do.thanx

I forgot to add that the SES is not on (but it does work) and therefore there are no codes. My engine code is a "Z" and it is a 4.3L twin injected throttle body (TBI)

ive got same year and motor. when was the last tune-up, fuel pressure check? check for vacuumleaks. for a timming check you need a timming light.