Will the ECU hunt for correct idle RPM?

My '87 XJ 4.0 AW4 has had an erratic idle since I got it.. it will basically idle down really low.. to the point where the motor is misfiring, then idle back up to 'normal' RPM. (I dont' have a tac) So far.. from my parts jeep, I've swaped, IAC, Air temp, MAP, and throttle body (with different TPS). The only thing that made a difference was the throttle body. When I cranked it up, it idled evenly, but too high.. I'm guessing about 1200 RPM. I looked at the # of threads showing on the throttle stop screw, and compared it to the old throttle body. The throttle body I had just installed had the stop cranked out quite a bit more. I slowing cranked the stop back in, and left the idle set a bit higher than it had been with the old throttle body.
Now that I have the idle set a bit higher.. I'd guess 800rpm.. you can't tell as much that the idle is erratic.. because when it idles down "low" it isn't low enough to cause it to misfire. I was wondering if.. the ECU is sensitive to the idle RPM and will try to adjust fuel and spark to achieve it? I'll try to buy an aftermarket tac and install it..

One other thing.. before I had started driving the jeep regularly, when the jeep would sit.. for a month or two, the first 20 miles or so it would misfire.. then the misfire would disappear.. That behavior is making me thinking that maybe I have a sticky injector? (I've tried cleaner). I have the whole fuel rail, from my parts jeep, with the injectors and regulator still on it..I was thinking of swapping it over to see if it would fix the problem....

thoughts?

Did you adjust the TPS?

The "base idle screw" on the throttle body is just there to keep the steel throttle plate from slamming fully closed and damaging the bore of the throttle body. The correct setting is to first remove the air tube, then back the 3/32 allen screw out until it no longer effects the throttle body, then screw it back in until just the slightest movement of the throttle plate can be detected.

The IAC stepper motor will handle the Idle, based on commands from the ECU. The ECU receives data from all the sensors except the gauge/idiot light senders.

Your idle needs to be 700-750 rpm in DRIVE with both the engine and trans fully warmed up, after about 20 minutes of operation.

Poor idle could be the result of: bad or out of spec sensors, grounds, wiring, sparkplugs, plug wires, cap, rotor, injectors, vacuum leaks in the intake and exhaust leaks upstream of the o2 sensor, and possibly poor fuel delivery/pressure.

What injectors do you have in there?

You changed a lot of sensors, did you test any of them first?

Any intake leaks? If you have a command-trac do you have a vacuum disconnect D30 axle? Condition of the vacuum reservoir? Where the vacuum line runs under/behind the battery is a common breaking area.

Have you run a compression test lately?

Have you tried a vacuum gauge on the engine? Could be a sticky valve.

All of my sensor came from my parts jeep.. I did not test any of them. I did install them one at at time and noticed no change. I realize there is a chance I could be replacing one bad sensor with another.. I'm just hoping my luck isn't that bad.
The spark plugs wires look new, I've replace the spark plugs and the dist cap. The motor NEVER misfires above idle, and it is also very responsive, it never hesitates, even if it's idle down low, it responds instantly.

As far as the motor goes, I've replaced the head with a reconditioned one, I've replaced the bottom end (crank, and all bearings) I've replaced the timing set, and oil pump. The engine ran the same after the motor work as it did before.
I have not done a compression test..

I'm am running the stock injectors... according to wikipedia the '89 injectors are good for a couple extra HP.. so they may be worth swapping in anyways..

Tomorrow I'll try spraying the vacuum lines with carb cleaner and listen for changes in the idle..

I have not tried a vacuum gauge on the motor.

I haven't tested my TPS.. but I left the TPS on the throttle body that I transfered from my parts jeep.. and because the idle problem is pretty much the same I doubt the TPS adjustment is an issue.. but I'll check it.

I do have a disco 30 front axle. But I have the NP231 transfer case. I have replace all the vacuum lines that from from the transfer case to the vacuum motor on the axle.

I've repaired the vacuum line that runs to the vacuum canister in the bumper, and that fixed my HVAC blend door problem.

It may also be worth noting that I'm getting 20mpg hwy.. so nothing is that far out of wack..

If you have the original Bosch injectors, the two-piece bodies that are pinched together, you may want to get rid of them.

A cheap upgrade is to go to the salvage yard and snag a set of Ford (Bosch) orange-top injectors. They are 19 lb (originals are 18.6), they have 4 nozzles instead of the single of the originals. A SET of them can be had at the salvage yards for about $30, about the same on eBay but then there is shipping. You can use the Ford o-rings and they are cheaper.

The four-nozzle design really helps to atomize the fuel and smooth out the idle. These orange-top injectors were used in 3.8L Fords up through the E350 vans with V8s.

The original two-piece Bosch injectors will eventually start to leak where they are crimped together. That fuel will ignite on the hot exhaust and then BBQ XJ.