Hi All,

Recently been having some issues with my '98 Ranger, 4.0, 4x4, 5 speed. When at operating temp, sometimes when coming to a stop, the idle drops to 350-400ish and will hang there, not stalling, until it jumps back up to regular idle. This is a new motor with a newer SMP IAC. I've already replaced the IAC with a new Motorcraft one and the issue persists. When doing an idle relearn with the new IAC, it never "bounced" between RPM's while learning. My concern is lack of oil pressure when the truck is idling that low.

Whats the next course of action to look at? I've unplugged the IAC and the truck idles fine around 500 - so I'm assuming no leaks of sorts. The IAC does not sit 'flush' to the intake manifold, but covering any gaps does not change anything. Could it be the alternator?

Thanks yall

 

How many miles are on your truck?

I doubt it's the alternator but go through the motions anyway.
a) Check morning voltage of battery.
b) Check voltage at battery during operation.

I've always read that unplugging the IAC valve caused the running engine to sputter or even die.
Idling fine after unplugging means you have an air leak.....again, I'VE READ.

I've read cleaning the MAF sensor is a must.
Inspecting and cleaning the throttle body is simple and easy enough to do.
Thoroughly inspect the vacuum lines on your nearly 30 year old truck. Are there any aged plastic vacuum trees?
Also, the entire PCV system is a potential vacuum leak source. PCV valve in place? Any hoses or pipes or rubber elbows coming from it to the intake can age.

 

if you unplug your IAC and you have no change in idling most likely you have a vacuum leak. You should notice a difference

 

There is a difference. It idles steady at about 500 instead of the usually 650-700.