I have a 1991 TBI 366cu in BB all stock on propane. The engine was running great until a few weeks ago.
The problem began with backfiring intermitently while driving. I was getting the following codes: 12, 43 and 44. During this time the engine became very sluggish while starting out. Almost unable to pull itself on a slight incline. After a few seconds on flat ground, it had a significant increase in power, almost like normal. As I start climbing a hill, it looses power quickly and seems to operate better if I gear down and keep the rpm's high. When it began to back fire, I still had good power. Then one day it had lost power. I am wondering if the back fire may have damaged something. I cannot find any visible or audible vacuum leaks.
I had found a bad plug wire shorting to the valve cover. I replaced the following items: Plug wires, rotor, distributor cap, and new knock sensor. The propane vapourizor was rebuilt last year and was working good, I do not think I have a lack of fuel delivery problem? I had tried a used ESC module and made no difference. I measured the voltage from the ESC to the ECM and it was a steady 9.4v. The manual calls for between 8-10v. I was told that the knock sensor was engine specific. I did not know this at the time of purchase, so I removed it and installed the old one. By measureing the voltage from the ESC, I do not think it is a knock sensor problem. After changing out these parts, my last ride only showed a code 12 and code 44. It starts and idles good, the back firing is gone, for now. Are any of the sensors engine specific as with the knock sensor? Could the temp sensor be creating this code 44. The lower power problem is consitent. Any ideas and comments would be great...

 

Code 12 is a Coolant Temperature Sensor code and Code 44 is Lean Exhaust. IIRC, GM ran alot of the big blocks with retarded timing advance when the engine is cold, if the ECM believes the engine is cold, it will retard the timing. That will prevent it from passing the forced knock test (code 43) and possibly pass unburned a/f mixture through the exhaust system and past the O2 sensor (code 44)

 

Fast355, thanks for the info, seems to make sense although it shows in the code manual that code 14 & 15 is coolant temp sensor or circuit fault, voltage low or high respectively and code 12 is "Diagnostic mode; no distributor signal to Electronic Control Module;System Pass". If the sensor were faulty, is there a case where the ECM would not detect it? I also agree that it seems the timing is retarded. I had unplugged the brown/black tracer wire to the distributor for electronic advance and checked the timing with a light. It showed ten degrees advance. I adjusted the distributor to approximately 2 degrees to see if it made a difference. Not really, did not run as smooth at idle. I set the distributor back to 10deg advance.