My 94 Roadmaster engine gets hot and heater blows cold air while in drive gear. If I put in neutral, engine cools down and heater blows hot. Also sometimes while car is idling it will begin running roughly, service engine light will appear, and car will die. When I restart, engine will run smoothly and service engine light will be off.

 

Tough question, for me anyways. Sounds like almost something more mechanical dealing with radiator and engine cooling system. Thermostat comes to mind and sometimes new are installed backwards. Knowing Buick somewhat also wouldn't be surprised if a diverter valve system is there to transfer available heat to inside car before engine is actually up to temp.; possibly for turn key drive aways in winter. Any new hoses installed lately, may be reversed?

Never made a living as a mechanic so hope one has the answer on this one. Always did all my own work though and had 63Whiskey MOS in Army (wheeled vehicle mechanic) for instructional intent only, among a few others (11B, 79D).

 

You need to pull the codes from the computer. The 94 has an odd sort of OBD1 1/2 system. You need a code reader, or someone who can pull the codes. The local GM dealer may be able to do it for you, if they still have the equipment. The codes will help you determine where to look. The coolant temp sensor may be the issue, although the symptom also could have something to do with bleeding the coolant system correctly. I believe this, or a similar issue, has been addressed on the longroof forum, or impalassforum.com. I personally have not worked on the RM LT1s, but getting any codes from the computer is extremely helpful.

You may have another issue as well. There are some really good resources on the above forums.