HEGO stuck on 0.0 volts at idle
 

The new Bsoch HEGO on my '88 F-250 5.8L, manual transmission, is stuck on 0.0 volts at idle and only begins switching above 2000RPM. Propane enrichment makes the HEGO respond at idle so I assume the engine is running lean. There is also the random spitting sound of a lean idle misfire at the tailpipe. Vacuum gauge is steady at 16 in. , fuel pressure is 35 psi at the rail, and I cannot locate any vacuum leaks after testing all lines and reservoirs. All intake manifold gaskets and PCV valve are new. MAP, ECT and IAT sensors test at normal specs. Injectors were cleaned , flow tested and matched for performance. No EGR leaks. Tried a known good replacement ECM with no change. Any ideas as to why the engine is running lean?