PCM is on the firewall, passenger side.

Removing the IAC while the engine was running *should* have made it rev uncontrollably... That's a big vacuum leak, perhaps they just unplugged it? (which would just disable it.)

When you first start the truck, does it flare up to 1500 RPM or so, then settle back down?

 

That would have absolutely nothing to do with it...... I suspect the IAC is either bad, or, the PCM simply isn't controlling it. Do you have a volt meter?

 

Start with finding the vacuum leak.

 

When I first start the truck, the RPMs will jump up and down for e few seconds, then dive down and the truck dies. He's had a vacuum leak for a while now. He could not find it and 4 wheel drive will not work (assuming because of the leak). He said when he traced the vacuum lines, he was lost when he got to the cluster of lines around the firewall because he was unsure which was which when it came out the other side. The diagrams only showed general routing information. Yes, I have a voltmeter. Could the fact that he has had the vacuum leak for a while now be the cause of the issues at hand?

 

Vacuum leak will usually cause a high idle, not stalling.... though, fixing it WOULD be a good idea. Given that your 4x4 doesn't work though...... makes the problem easier to find. Should be two lines coming up behind the engine from the transfer case, one will simply have a cap on it, that's the vent line. The other is vacuum supply for the CAD system. (I believe that the line was originally white.... may not be anymore.... ) You can get to the lines down on the transfer case fairly easily..... Make sure you are getting vacuum down TO the t-case switch first. (which I suspect will be ok.) There are two metal lines that run along the t-case crossmember, to the frame rail, and up toward the front axle. They are notorious for rusting out, giving you a nice vacuum leak, and ensuring your front axle won't work the way it is supposed to.

As for the idle..... I would unplug the IAC motor, turn the ignition on, and verify you have 12 volts on two of the pins in the connector.

 

Unhooking the iac motor, is this where the voltmeter comes in to play? If so, could you be so kind as to explain how to use it? I have found myself alone and left with a truck that will not run lol. But I am a quick learner! When it comes to the older vehicles, I had no problem holding my own. These newer ones are quite the different story. I am learning though. Will the metal lines for the vacuum show the rust on the outside or do they need to be removed to determine their condition?

 

IAC: Yep. You want the connector end. There are four pins in there. Set your meter on like 20 volts DC, stick the black lead on the negative battery terminal, and do what you need to so it will stay there. Test each of the four pins in the IAC connector (with the ignition on, engine not running), you should see 12 volts on two of them. Nothing on the other two. (it varies which ones will have voltage, so long as two do, and two don't, that is correct.)

Metal lines: The may or may not be obviously compromised. (and they are a pain to get out.....) To make life easy...... (for certain values thereof......) you could pull the connector off the actuator on the CAD on the front axle. Pay attention to which way it comes off.... that's important. With the t-case in 2wd, you should have vacuum on the outermost port, nothing on the other. Put it in 4x4, and they should be the opposite. (engine has to be running for this test, so, be careful under there.) If you don't have vacuum there, need to work back thru the system, and see where you lose it, move back to the other end of the metal lines, where the soft lines come down from the t-case, pull those off, and check there. (again, which way the go on there is important, so, pay attention. ) If you have vacuum there, the metal lines are likely toast. (either rusted out, or, collapsed, and not passing vacuum.) Just keep working your way back thru the system, till you find the problem. This is a good resource on how it all works.

I like the older vehicles better too... were easier to work on. (aside from carbs... I hate carbs. ) But, having now dealt with the computerized stuff for a few years.... ya kinda get used to it. Once you realize how it all works, troubleshooting gets a LOT easier.

Have you downloaded the service manual for your truck yet? If not, you can get one here.

 

Thank you so much. I will attempt your suggestions tomorrow as it is currently dark and rainy here. I totally agree on the carbs. We always drove Chevys (I had a 1968 Chevy Impala) and the stock carbs were Rochester which I think were the worst of them. If our carb screwed up, we would replace it with a Holley lol. Although finding a vacuum leak on a carbed engine is quite simple.
I have already found the resource you suggested concerning the 4 wheel drive a while back. I have also downloaded the service manual last year and it has been helpful in the past. It does not, however, tell you how to use certain tools and things like voltmeters.
I really appreciate the help from everyone in this forum. I am especially impressed with how knowledgeable and responsive everyone is.

 

Let us know what you find, and we can go from there.

 

I spoke to the mechanic today and he said he checked the pins on the connector. Please be patient with me as I am trying to pick up where they left off. My husband only had so much money for a mechanic so when the limit was reached, we were on our own again.
Anyway, he said the pins on the iac connector were good. As far as the 4 wheel drive not working, the mechanic said that he was able to fix the lines and the 4 wheel light on the dash went out. But when it was shifted to 4 wheel drive, the light would come on as if in 4 wheel only the front axle would not move. My husband had a broken fork a while back and he replaced all parts in the axle. Could it be possible that he may have something in backwards? I know you can put a clutch plate in backward, could this be the case here?