1989 Ford F-150 Stalling out at idle

Tell me if I understand this correctly. The Engine Computer takes in information from a number of sensors, then figures out when to fire spark plugs, when to fire injectors, and where to set the idle speed.

"all of the EFI stuff has been removed or bypassed but I don't think this has anything to do with the problem."

You removed something that is part of this system, and you expect the computer to keep on doing what is was designed to do? You'll have to explain why something was modified and what was done, otherwise there is no way to know what is working and what isn't. Did someone try to install a carburetor to replace the fuel injection system? If so, you can't expect to find an answer related to the fuel injection system when it isn't even there. I hope a mechanic didn't perform any of these modifications because that shows he doesn't understand how to diagnose or repair the system.

A vacuum leak results in a high idle speed without a corresponding increase in power. The Engine Computer runs a controlled and adjustable vacuum leak while adjusting fuel volume at the same time to adjust idle speed. It sounds like the computer isn't doing that. That is done with the idle speed solenoid, not all the other parts you mentioned.

Hold on. I don't like replacing anything until the diagnosis points to a defective part.

I suspect the fuel injection system is just fine and it's emissions-related stuff that was left off. You won't get away with that on '96 and newer vehicles, but on yours, if the engine has been running okay for two years, whatever modifications were done aren't to blame for how it's running now.

What it sounds like you have is a "simple" case of low idle speed. Where I would start is by removing the idle speed solenoid on the side of the throttle body, then clean out the passages and check if they're blocked with carbon.

Yup, the photo didn't come through. I copy mine into an MS Word document so I can add pretty arrows and stuff, then I copy in into MS Paint and save it as a jpeg file. Those post to this site real easily.

Look on an auto parts store's web site for the idle speed solenoid to see if you're looking at the right part. I use the Rock Auto site for reference quite a bit.

Same part; different terminology.

If you loosen the bolts for that unit while the engine is running, idle speed should go up when the air sneaks in. That will prove the passage isn't plugged with carbon.

That shows that at least half of the air passage is not blocked with carbon. That passage bypasses the throttle blade and the solenoid is in the middle. We know now the passage from the middle to the intake manifold is not blocked.

The next step is to run that solenoid with a scanner to see how it responds. Most scanners allow you to run idle speed up and down, although Ford uses a system that is significantly different than what all other manufacturers use. Some older scanners may not be able to allow you to test the solenoid's response.

You can also watch the percent of air on the Live Data screen to see if the Engine Computer is commanding it to open more but is not getting the desired results, or if it's commanding it to stay closed in response to other sensor readings. If it's requesting low idle speed, you'll usually need a mechanic to interpret those other sensor readings to see why that is.

You need to see a mechanic for the scanner unless you plan on using it often. For a vehicle of the age you and I are driving you can find a pile of decent scanners on eBay for less than a few hundred dollars. I have a Monitor 4000 for my older cars. I also have a Chrysler DRB3 with extra plug-in cards that let it work on cars back to '83 models. A lot of independent shops bought them because with one of those cards it will work on all brands of cars sold in the U.S. '96 and newer up to around 2004 to 2008, depending on model.

For something a little more sophisticated than the earliest scanners, look for a used Snapon MT2500. We call that scanner "the red brick" because it looks like one. Snapon is very proud of their stuff, and they charge accordingly for the very expensive updates every year, but this scanner is old enough that most shops consider them to be obsolete. Newer scanners do everything this one does, plus a whole lot more. This one is updated by replacing the plug-in cartridges. A lot of the kits will have a bunch of cartridges. You need two that work with the vehicles you want to use it on. I never used these scanners very much, but I'm familiar enough with them to know one cartridge needs to be for "Chrysler, GM, and Ford domestic", and the second cartridge is a troubleshooter for the same vehicles. By reading some of the listings on eBay, it won't take long for you to be more familiar with them than I am.

You can also check at auto parts store that rent or borrow tools to see if they have a scanner to loan out, but you have to be sure it will work on your truck. Up through 1995 Ford had a real bizarre way of reading fault codes and viewing data, so you have to be sure the scanner will work with your year and model. If you can save one trip to the shop, consider the scanner a good investment.