Hi, I live where the nights get very cold in the Winter.
I have this '88 Trooper II 4cylinder that is almost bulletproof. It has 188000 miles and runs very good after it warms up.
I have had this problem with a very low idle for 4-5 mins in the morning when the temp is below around 40f. It is getting worse.
When I first start it, it idles fine then after 30-40 seconds, the idle speed will drop to 300-400 rpm. It only happens in the Winter when the engine is very cold. The colder it is, the worse it is. Sometimes it will stall. If I just idle it for a few seconds and drive, the engine stumbles. Sometimes I have to warm it to melt ice and warm it a little inside. After it warms up it does idle and run good. I get a little hesitation once in a while even when it is warm.
A few years ago I blew a head gasket and I had the head milled and ALL the valves and most valve guides replaced. I replaced all the vacuum hoses and I used a teflon coated head gasket. I don't think it is a vacuum hose problem.
It actually sounds like the timing is being retarded.
I have a Haynes repair manual, but it isn't much help.
I am a good mechanic on older cars without all the emission controls.
Any help will be appreciated.

Check the ECM temperature sensor threaded into the underside of the intake manifold. It regulates the amount of fuel injected on a cold start.

Here's how: There are two connectors on a bracket just behind the battery. The larger of these two has the 2 wires from the temp sensor in it. They are BLACK and GREEN w)YELLOW stripe on my 88 truck. The way the connectors are mounted on my truck, these two wires are right on the top of the connector. Simply unplug the connector and put your ohmmeter across those two pins.

Readings should be:

degrees F / K ohms
14 / 7-12
50 / 3-5
68 / 2-3
122 / 0.7-1
176 / 0.2-0.4

Second thing to check is the air regulator mounted on the block wall under the intake manifold (easier to get at if you pull the wheel and mud skirt). It acts in conjunction with the temp sensor to bypass the throttle and get more air to the engine on a cold start. Pull off the big tube
on the side of the throttle body. Cork the port and the tube and start the engine. Pull the cork on the tube and put your thumb over it quickly. On a cold start you should feel a strong vacuum on the tube that diminishes in a couple of minutes. If there's none or not much vacuum the air regulator could be stuck closed. It's a real simple device.
Basically just a disk with a hole in it, on a bimetalic spring. On a cold start the hole lines up with the tube.
As the 12v from the plug heats up the spring, the disk turns
until the hole closes off. The two devices work in tandem
even though they aren't connected. I'd lean toward the temperature sensor as the problem. Make sure there is a good ground to the intake manifold as that could affect the sensor signal.

I posted the message about the low idle problem. After pulling and reseating both ends of all vacuum hoses, I found several hose ends that were cracked and probably leaking. The hose is only 5 years old, and I used the good braided hose. There is a lot of vacuum hose and I guess I will be replacing some of it this summer. I cut the cracked hose back and I no longer have the low idle problem. while I was checking, I also noticed the ignition timing was wrong, it was at 8 degrees instead of the 12 degrees it should be. I corrected that also. Now it idles fine and no more hesitation.
I'll bet a large number of engine problems are caused by vacuum leaks.
I printed the reply from RT1. There is good info there and I will check those sensors as soon as it warms up again.

I had an identical problem for a long time.



found a bad hose end to the "backpressure transducer", cut off the cracked end and reattached - solved my problem. it's a little flying saucer looking thing towards the back of the intake plenum. there is a pic of it in the Haynes in the emissions chapter, it's part of the EGR system.

the air regulator RT1 mentions can often be a source of 2.6 idle woes and because of its location can be a real pain to deal with. you can relocate it above the intake manifold for easy access in the future.