This is a question about the truck in my signature. While sitting still idling I have noticed my truck has a somewhat odd idle. It generally idles around 750 rpms and the needle flutters a little bit. While idling I feel/see a drop in rpms every 10 - 15 seconds. It seems like it drops about 50 rpms and then jumps back real quick. If you blink you would miss it. There arent any codes, FP stays between 12-15 psi all the time. Is this anything to worry about?

 

The way I understand the tach operation is: The computer samples engine rpm at certain intervals and "updates" the tach readout. The fluctuation you see is the needle reflecting the updated engine speed.

 

if its cycling every 8-10seconds it sounds like the AC is cycling, rotate the heat control to OFF as the ac will cycle in the other posns as well. its normal to see a rpm drop when the AC cycles

 

My AC was low on freon. I fixed that problem last night right before I made this post. That was the problem. The AC compressor was cycling on and off making the RPMs flutter.