Good evening, I have a 1989 Chevy Cheyenne 2WD manual transmission 4.3L that will not idle unless the map sensor vacuum hose is unplugged. I honestly have no idea what could cause this. A little background: the intake gaskets were leaking so I changed them. The truck was running fine before but after it started the current problem. I have changed out the fuel pump, fuel filter, distributor cap, rotor button, ignition module, map sensor, the distributor, coil, spark plugs, and plug wires. It's getting plenty of fuel and spark. Just refuses to idle with the map sensor vacuum plugged in. Now, I have hooked a timing light to it when I have the map sensor vacuum hose unplugged and the timing mark on the harmonic balancer sets at 12 o'clock when it's idling and when I move the distributor, advancing or retarding the timing mark will move down to the timing mark on the block. Resulting in the engine stalling out. I have moved the distributor one tooth over from TDC and still have the same results. Even moved it to 180° out and it will not fire in that position. Any thoughts or ideas are welcomed. Thanks in advance! Have a great day.

 

What happens when you take the (one side, I don't know which) off the map sensor, and put your thumb over it?

 

As long as the vacuum hose, from TBI to map sensor, is not plugged into the map sensor it will idle for 2 to 3 minutes and then stall. Doesn't matter if the vacuum hose is closed off or not. Ive pulled the front vacuum off on the tbi...and it doesn't change. Thank you for your responding.

 

Update: I had a bad injector that was causing this issue. It's now changed and truck is working beautifully. Have a great day.