I have an 86 K5 with the original motor(259K miles) that still runs good but is hard to crank . I don't know if the glow olugs are bad or if the electrical system is bad. It has power windows but are real slow going up or down and all the lights dim if you use the heater fan.. It has a new alternator and two new batteries. So my dilema is figuring out if the low voltage prob is affecting the cranking prob...Im sure you have disscused this before but It may take me a few days to read everything..