RV Repair: Presidential furnace problem, electric solenoid, wall thermostat

Question
I have a 1978 motor home with a Coleman Presidential furnace in it and a wall thermostat. .  I have replaced the pilot light assembly.  The burner tube is in great shape as is the burn chamber.  Here is my problem.  I turn the valve to ‘pilot’ and the pilot lights and burns perfect on the first strike.  However, when I turn the valve to ‘on’, the burner lights up and immediately snuffs out.  I thought when I turned the valve to ‘on’ it was supposed to be in the ready state waiting for the thermostat to turn the fan on and then open the gas valve with the electric solenoid valve. (If I turn the fan on manually before I turn the valve to ‘on’ it lights up and burns fine).   Can someone explain what might be wrong with the furnace or if I am doing something wrong.  I hate to buy a brand new furnace when this one has hardly been used.
Also, if I turn the fan on manually and the furnace is running after about 3 minutes the high temp limit shuts the fuel off until it cools a little and then it burns for another few minutes until the limit switch pops again.

Thanks, Tom

Answer
Tom, It really sounds like you have actually 2 problems. The first is your L.P. pressure or it sounds that way. As long as all your ducts are open and none of them are blocked off with something. If you might have mice this is where they seem to like to nest. It also sounds like for some reason the T-Stat is calling for heat whether it is on or not. You can check at the gas valve and see if there is voltage when you just have the pilot lit and then when you turn on the valve. If so then trace the trouble down with the T-stat being the first thing you check next. Usually when the limit switch is working they are still good. The valve can get liquid to it if the tank has been overfilled, as can the regulator. If this has not been apart the orifice should still be in and relatively good. You can check the anticipator at the T-Stat and make sure it is not over .15 as that can cause this sort of trouble as well. Also does the furnace seem to be putting out extremely hot air when it is on, and or sound like a blast furnace? If so then check the L.P. pressure and make sure it is not over 11 1/2 water column inches. Thanks, Bill