Ok i recently purchased a 88 k1500. It HAD the 5.7L but a 305, period correct, donor motor was installed with 75k miles. Im getting a very strange idle. It is idling down just to the point of stalling out but dosent. the aftermarket tach is showing a idle of between 800 and 900. If i unplug the TPS and plug it back in it idles at around 1000 to 1200. but if i pump the gas once it returns to the ****ty idle. I replaced the TPS and im still having the problem...

 

Oh and besides the idel issue when the truck is moving is runs beautifully. Strong, quick response no hesitation or bogging. Oh and when you pop the hood and start the motor it smells very rich, not leaking gas.