I did a quick search and found a few similar issues, but not something quite identical.

1990 7.3 manual transmission

My truck is having issues with idle. When I first start it up it will idle fine. And 50+% of the time it will idle fine. But seems to be getting progressively worse at coming down to an idle, i.e. approaching a stop light, or coasting around a curve. It will simply stall.

I have not noticed any problems with the way it's running when I'm driving, have not had it stall any time other than when trying to idle. And when it does idle, it is fine, no smoke etc.

It doesn't seem to be related to engine speed. i.e. when I hold the throttle at ~800rpm (idle is usually around 800, maybe a tad lower) it runs fine, but if I take my foot off the pedal it will die.

If I were to describe this like an old gas engine, it would be like the throttle "set screw" had gone missing.. except that it doesn't do that ALL the time, but it is getting worse. Happened basically every stop light today on the way to work.

When this issue first started it happened occasionally when the engine was cold and that was it.. minor annoyance. Now it is still doing it when the engine is hot, and often enough to be a real problem.

Any ideas?

 

Sounds like the metering valve in the IP is sticking. Get a large jug of Diesel Clean, Howes or some other diesel additive, and a new fuel filter. Fill the new filter with the additive and spin it on. Next put double or tripple the amount of additive called for in the fuel tank. Start the motor and let run for 30 seconds and shut it off, and let it sit overnight if possible. This lets the additive soak in the pump, disolving the deposits that is causing the sticking of the metering valve. The next morning take it for a hard drive. This procedure usually fixes your problem.

 

I was thinking of just running some SeaFoam, but sounds like you're suggesting a much stronger mix than a bottle of SeaFoam will get me. Will have to fiddle with this when I get back from travelling for the holiday, but that definitely sounds like a reasonable solution.

Why would this be getting progressively worse though? This is a brand new IP installed in Oct/Nov (don't remember exactly), at the time with new filter and injection hoses and fitting.

Thanks,

 

These old pumps were designed back when ther was a lot of sulfer in the fuel. The sulfer acted a a lubricator for the pump. The fuel we have now has hardly any. Lubrisity packages are suspost to be added at the refinerys. Are you running a good quality of fuel? I have run 2 stroke oil in the fuel tanks when I fill. 1 qt. split between the 2 tanks. Any cheap 2 stroke oil designed to be injected works. It even gave me a small boost in fuel milage.

 

I have heard of the 2 stroke oil trick, might look into that.

Unfortunately our choice for diesel here is pretty much Sheetz or no-name.

I think I'm gonna get some seafoam and 2-stroke and just splash enough fuel in one tank to mix it up and bring it back to work and see if it's any better when I head home... if that doesn't help it then I'll try your more aggressive suggestion after the holidays.