Over the last couple of weeks my '89 F250 with 7.3L IDI has been getting more and more cantankerous. Starts hard; I have to crank and crank sometimes even when the engine is warmed up. When I come up to a stop sign and it idles down, it is running rough. Sometimes it kills when it idles down; sometimes it kills when I am trying to take off from a stop, unless I really rev up the engine first.

Until recently it would idle down to a low idle and just sit there running slow and easy, no tendency to kill.

Fuel delivery problem? I suppose I could try replacing fuel filter first...

 

Fuel filter is a good first try. Could also be a failing lift pump (the mechanical pump mounted low on the engine).
Also possible that some gunk is building up in the injector pump (pump on top of the engine with lines attached). I had a similar problem with my truck shortly after I bought it, most likely because it had been sitting for a while. When I took my foot off the accelerator pedal and hit the clutch, the RPM's would sharply drop and frequently stall before it could settle at idle. If I was slower pulling my foot off it would come to an idle ok.
To fix that, pull the filter off, drain it, then refill with clean ATF. Reinstall the filter. Start the truck and let it run for about 30 seconds, then shut it down. Let sit for a few hours, overnight is best. May have to repeat if it doesn't clear out the first time. The ATF cleans up the deposits that cause parts of the pump to hang up.

 

I've brought up a number of these IDIs that had been sitting for awhile and they are all the same thing. If you let off the throttle sharply they'll stall out. Drive it for a day or two and it's all back to normal.
To the BalerTwineGuy my first gut reaction was an air leak, tecgod has you in a good direction with the pump. Try filling the tanks up all the way in case you have a cracked pickup tube and check at the filter head for air.

EDIT thinking about it some more generally sucking air makes it run real rough but also take off and run like a raped ape intermittently so less likely.

 

Thanks. How does doing this ATF in the filter thing compare with just running some DieselKleen through the tank?

 

By adding it to the fuel filter you give a concentrated dose to the injector pump/injectors, by letting it sit for hours/overnight it has time to loosen things up,

the diesel kleen is a more diluted method although if I knew I was driving a long distance and going to burn through a tank that day I would try it,

Do you know when you IP was last serviced/rebuilt ?
I would definitely try to the ATF/filter first, then I may hook up a can to the front bump and run your fuel from it (search, there's a good instruction to do this) for a day to see if the problem still exists, at least you know what direction of the fuel supply you need to look at that way,


.

 

The ATF in the filter thing is questionable. "Run it for 30 seconds then let it sit." After 30 seconds it's been almost entirely replaced by diesel fuel so there's almost no ATF left to 'soak' into stuff. Many have stated that it takes several minutes to purge fuel through the system, I watched my return line change color when switching from used ATF/WMO to diesel. By 20 seconds it was obvious that it was changing, by 40 there wasn't a noticeable amount of color left. This is going from black 'fuel' to diesel and being watched at the return line disconnected at the valve draining into a pan meaning that the filter and pump had purged even sooner.

As for the cleaning abilities of ATF, many have also touted this for cleaning the engine by adding it to the oil. I've seen tests that show ATF to have less cleaning ability than motor oil. I'm not saying that it can't work, but there's a fair amount of evidence against it. Almost everyone I've asked about it cites the cleanliness of transmissions compared to the dirtiness of engines to 'prove' how well ATF cleans. Transmissions aren't exposed to the stuff that engines are, like combustion by-products, that alone crushes the 'proof'.

Some report that ATF helps, some say it did nothing. There have been countless numbers of things that people swore did what they were supposed to do that have been thoroughly debunked. The placebo effect is strong, I've tested it a few times with different products. The vehicle owner always says that the product did exactly as claimed, 'till I show them that I never installed it.