Recently my truck has started behaving strangely when I put in the clutch and let the engine return to idle speed. Until recently the engine RPMs would smoothly drop back to idle speed and stay there, but for the past couple of days the RPMs will drop to idle, then dip slightly below idle and then go back to idle speed. It doesn't happen every time, nor does it seem to be related to what gear I was in.

Otherwise the truck is running fine. I don't know if it's any cause for concern. Does anyone know what could cause this? The only thing I can think of is a vacuum leak somewhere.

 

No vac on a diesel, because there are on throttle plates. Try running some diesel kleen through it, if that doesn't work start looking into fuel supply, i.e. weak lift pump.

 

Got no lift pump -- I recently deleted the mechanical lift pump and added an electric.

I wonder, though, why I would see an issue at idle but not at full throttle. Wouldn't fuel supply issues be more evident under load?

 

Long story short I saw this happen after a bad experiment with wvo and gas gunked up my pump making it hard to start after. I also had to stab the pedal to rev past the initial idle just to get it to run, if I let off the pedal it would die.

Truck in sig with all original parts seemed to work by the books before this.

I think I caught mine early, changed the fuel filter, put startron in the tanks and filled the fuel filter with atf, ran for a couple minutes, shut the truck off for atleast a day. A little better but still wierd so I did it again, came out the next day and my pumps operation seems to have been normal since.

The same darn thing happened to my vw jetta at the same time, on that pump I got it back working, but was amazed on breakdown to see some major hinderance where the rollers plate was and this nasty chicken skin type stuff making things bind. No other buildup like that was found oddly.

Anyways try the atf thing.

 

Well, my symptoms are a bit different. It starts up fine, idles fine, and runs under load fine. The only thing that is even slightly off is how it returns to idle after it's been running under load, and as I said earlier, it doesn't happen every time. Just kind of weird, and not affecting driveability at all.

I'm leery of running ATF because of the red dye. Don't want to get dipped. But I do have Diesel Kleen and Startron, which I use regularly.

 

It sounds like your pump internals are a little gummed up.
Lots of guys have had good luck filling the filter with a good cleaner additive and cranking it for around 30 seconds, just long enough to fill the pump and injectors with straight cleaner, then kill it and let it set for a few hours or even better overnight.

 

I have limited experience with these things but I have found the atf is a better cleaner than diesel kleen. I would try that stuff first with varying problems first, and it was always atf that would clean things up.

If you don't want it in your tanks, take the pickup(before filter) and return line and run it from a bucket and just purge it all from the pump with a vacuum bleeder before hooking it back up.

They also make a cleaner that you run in real time to wash up the system good called diesel purge. I use that once in a year just for the hell of it. Its not something you just add to the tank/filter as an additive.

And your idle issue was what was happening after my first soak, and that is why I went for a second. It wasn't every time and I was able to cold start my vehicles after the wvo, but the truck wasn't acting quite right when I was driving it around. I was able to start it with high idle and slam the accelerator to the floor but if I let off it just didn't sound right and might stumble more.

I have yet to have that issue come back even once on either car since I did what I described.

If your truck is used and abused with old parts you might be surprised at what is inside one of those pumps even from normal use. Water, perhaps chunks from eroding olive gaskets in the line(just found this on my truck pulling off old stuff for tune up).

I think its worth a try before it gets worse and it may go away without having to spend $$$ on parts.