Chevrolet Repair: Engine dies, coolant temp, bypass mode

Question
Have a 1994 c3500 truck with 7.4L. It runns just fine with the map sensor unpluged but as soon as you plug it in. It then runns real ruff then dies. Its almost like it takes away too much fuel. I checked for vaccume leaks with carb clean but did not find any. timing is ok. New gm fuel pump and fuel filter and new cap and rotor have aready been tryed. I don't understand . It runs fine with plenty of power with the map sensor unplugged, but as soon as you plug it back in it dies. Help?  

Answer
Hi Ed, Not sure where to go on this but the first thing I want you to check is the coolant temp sensor for the ECM. Maybe plug the map sensor in and disconnect the temp sensor. Now see if it runs OK. I'm thinking that the coolant sensor is reading too hot and making the engine run Lean!! If you had a scan tool you could check this reading from the sensor. You could also check the 02 sensor to make sure it is reading correctly too. By disconnecting sensor they go into a bypass mode and if this corrects the problem the sensor you disconnected could be the problem. I have only seen 1 map sensor in all my years go bad on a GM.

Sorry I can't help more

Have a Great Day            Mark