Went on a "no heat" call to find system tripping on high limit. I had a 70 deg. RA and a 238 deg. SA!!! The blower motor is working at 120v on high speed. I found no obstruction in the RA duct, filters, registers open, etc. I then opened up SA plenum and also found no obstruction. Nothing touching limit switch. Shined flashlight into heat xchanger and couldn't see any light penetration indicating a crack. Pump pressure rated at 140 psi and that is what I got. I bumped it down 15 psi but didn't help. Nozzle clean and clear but changed to factory nozzle. Found a little crud between electrodes and cleaned off. No flue blockage either. System has a history of bad control boards and this current one is also malfunctioning. I don't feel the board is related to the high delta. This was my first time at this residence and noticed last tech added a time delay relay between blower heat tap on board and blower. Why would this have been done? Also, the system (according to the customer) had been working fine until now. Any ideas?