I have a 2000 Ranger 2.3L with aluminium intakes (upper and lower halves). The truck started idling rough a couple months ago and threw P0171. It's still drivable but idles rougher than normal at stop signs. When I spray starter fluid around the back of the engine on the driver's side, it speeds up. I checked all the vacuum hoses, EGR gasket, and EGR pipe and all were good. I removed the upper half of the intake to check that gasket (which was good) and verified tight bolts mating the lower half of the intake to the head. There isn't anything else on that corner of the engine to suck in air.

Question, would a cracked intake manifold exhibit a rough idle?

 

A vacuum leak would and cracked intake would be a vacuum leak, but so would an intake gasket leak which would be much more likely than cracked intake.

Get yourself a cheap cigar and a bottle of Scotch(Some say Scotch is optional, it isn't, lol)
remove upper intake again.
duct tape the lower intake hole with a hose added
pour a glass of Scotch and light cigar
puff it up and blow smoke into hose>>into intake
watch for smoke coming out of leak
keep puffing and blowing(and drinking)

 

Update: The gasket between the lower intake manifold and the head was cracked on cylinder #4. I tried the cigar and was able to see smoke coming out of that general area but I couldn't pinpoint it. However, I could hear the air escaping when I blew so I moved a stethoscope around in that area until I found it. While I had the intake off, I installed four injector kits (i.e. o-rings and spray caps). It runs much smoother now. Thanks for your help.

 

Good work!

Thanks for updating your thread, may help another member in the future

How was the Scotch?