No check engine light when it happens. It's the old EEC-IV system. It seems codes vanish when you turn it off. Of course, the two times it's happened, I was nowhere near a place to scan it. I've even had it checked at work, but, as I said, no codes.

 

I would check the timing.

 

So, this advice applied to the '90 bricknose that I had, but you may want to spend some time looking at the ignition switch on the top of the column, under the dash. My truck was doing a similar thing due to the contacts wearing our in the "run" position. If you don't want to replace the switch right off the bat (and it was a massive pain in the rear) simply bypass the OEM switch leg for the "Run" position with a simple toggle that you can get at any auto parts or home improvement store. If that solves your problem, then simply replace the switch and remove the bypass. Hope this helps!

 

Okay so I need to bump this thread with a new post. I have a 92 F250 300 2WD ZF5. It has 270,000 miles on it and was running great! No issues at all. All of a sudden It has started dropping power while i'm driving down the road. I don't think it's the trans as it will do it in every gear. I disconnected the green vacuum line from the EGR valve and it didn't seem to make a difference. I've checked all the spark plugs and wires and I have a lot of white residue on the all the plugs. It also seems to not do it all the time, I drove it to and from work yesterday and it never did it. I drove it this morning and it bucked like a bull on the interstate. Any suggestions?

 

Pull codes and tell us what they are.
Dave ----

 

If I'm reading the flashes right i'm getting 33, 79, 212, 332 all with the key on engine off.

 

If you're reading them right, couple for the egr, one the ac is on or the pin for it is shorted to ground. Got one for the spout connector circuit/tach. Visually inspect the harness to see if there are any wires rubbed through or melted. Also, you're egr is probably stuck. With it running, tap it lightly(on the casting) with a hammer or wooden handle and see if it affects your idle. If it does I'd replace it. You can try cleaning it, but I don't usually have much luck with that. I have one stuck in my '92 Ranger that I can tap it and make it bog almost all the way down, or I can get the idle to go through the roof.

 

​​​​@Kramercd I tried tapping the EGR while it was idling and it made no change in how it ran. I unplugged the green vacuum line to the EGR and ran the truck up the interstate and didn't have any issues with it. It's really hard to start after it gets warm with it unplugged though.
As far as the cables. I did find two cables tied together right at the AC compressor. I have no idea what they are but that might be the AC code? This truck is all kinds of "pieced together"

 

Put a vacuum pump on the egr and see if it moves. If it actuates like it should, then your sensor is bad. It may be completely seized though. The original one on my ranger was completely encased in carbon.

 

Hey @Kramercd It moves with the vacuum pump. I can hear it making a clank noise when I release it. When I do it running it stutters like it's about to die and as soon as I release the vacuum it idles back to normal. Is it the sensor on the end of the EGR?

 

Does it hold vacuum? The fact that it moves is good in itself, if it doesn't fight the pump and close back, egr is good. Sensor is bad. You can test that with a multimeter set to read ohms. I want to say they're three pin and it's the outside pins. You'll need small alligator clips, or a wire piercing probe. You can open up a safety pin, twist it around the test probe and hit it with electrical tape so it doesn't move. Do one for each and stab them into the back of the connector. You'll want to measure the resistance with the battery unhooked so there's no power on it. Or you could find which one has 12v on it and probe the other one and test its voltage to ground. In either method you will want to watch the meter while you put the vacuum pump on the egr. Should see a smooth increase either in ohms or volts. But I'm going to tell you its the sensor. Clear the codes and read them again. Unhook the battery positive and turn your headlights on and wait for thirty seconds(two minutes is even better). Hook the battery back up and see if it throws any codes.

 

It does hold a vacuum. I left it for maybe three minutes and the needle never dropped. I'll test with the multimeter and report back but I have a feeling you're right that it's the sensor.

 

@Kramercd It will drop voltage across two of the pins when I pull a vacuum on it. Is it possible it's the vacuum pump under the EGR that's not working?

 

Is it a smooth change in voltage? Or is it erratic?

 

Seems pretty smooth. shows 4.9v then drops voltage as i pump it.