Thread: Another IAC question

The method for setting the idle on the 3.9's (presuming 4.0 is similar) involves getting it warm, blocking off the IAC feed pipe and manually adjusting the stop in the throttle body.

So if you've done this, it should be idling around ~600rpm as it was set when you block the IAC feed hose.

There are a host of other things on the 3.9 that affect idle. From engine coolant and fuel temp sensors to calibration on the MAF. If these aren't right then no amount of screwing with the IAC will help.

It is BS about this setting the IAC on a 1996 14CUX 3.9 V8. It is not 4.0. that is D2.

The IAC winds right out every time the ignition is turned off and in as the car is started.

The other thing that can cause fast idle is that the centrifugal advance is stuck on part or full advance, so check by trying gently to move the rotor clockwise a little . It should move about 3mm then click back.

The IAC is a stepper motor which relies on the ECU to provice the signal. The fact that if you left it a while and it started fast may mean a dirty housing. Did you also clean the housing the cone goes into?
Another thing to look for is that the housing is loose on the back of the inlet manifold, so check the screws. Another thing is that the small pipe to the FPR is off or leaking so check that.
Finally you may have bought a dud IAC as the cheap ones are hit and miss.

Originally Posted by PhilipA

It is BS about this setting the IAC on a 1996 14CUX 3.9 V8. It is not 4.0. that is D2.

The IAC winds right out every time the ignition is turned off and in as the car is started.

The other thing that can cause fast idle is that the centrifugal advance is stuck on part or full advance, so check by trying gently to move the rotor clockwise a little . It should move about 3mm then click back.

The IAC is a stepper motor which relies on the ECU to provice the signal. The fact that if you left it a while and it started fast may mean a dirty housing. Did you also clean the housing the cone goes into?
Another thing to look for is that the housing is loose on the back of the inlet manifold, so check the screws. Another thing is that the small pipe to the FPR is off or leaking so check that.
Finally you may have bought a dud IAC as the cheap ones are hit and miss.

Oh did I mention its a USA machine marked Discovery 1 SD 4.o stamped on the Plenum

Does it have a distributor?

If so it is officially known everywhere except the USA as a 3.9.

They made it 4.0 in the USA to match Jeep in engine size for marketing.

Everywhere else the 4.0 marketing designation started with D2 in 1999? with the Thor manifold and Bosch ECU with no distributor.

So if it has a dizzy its a 14CUX 3.9

Regards Philip A

As far as I am aware the 4.0 which first appeared in Aust in the P38 was also put into the North American D1 at the same time - so the OP may very well be a 4.0 Gems engine - definitely prior to the Thor. Same as the 4.6 went into the NA D2 but not ours.

The main differences between the 3.9 and 4.0 blocks noting the actual capacity of both engines is the same - the 4.0 has cross bolted mains, extra ribbing inside the block and as the crankshaft main journals are larger than the 3.9 the casing for the bearings are of a larger diameter. There are other changes bit these are relatively minor. Without major work a 4.0 crank will not fit in a 3.9 block.

Garry

Well the poster should let us know if it has a distributor soon.
The IAC changed a bit about then from a screw in to a bolt on ( common to lots of Chevs) then but there are many cheap IACs sold in the USA that don't work. I know I bought some.
Regards Philip A

Didn't the very early 4.0s have a distributor, I though it was only lost maybe on later Gems engines but definitely once the Thor induction system was introduced.

I do admit I am on shakey ground on what the very early 4.0s and 4.6s had.