13bt Rough Idle when warm

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Sep 27, 2016
Livingston Montana / Arlington Texas
yall, I'm racking my brain trying to figure out why this truck is idling rough (missing/stumbling), but only when it's nice and warmed up (10 miles on the highway or 30 minutes driving around town, 300+ egt at idle). Before it gets completely warmed up, it runs like a dream. It also runs fine in neutral, and even if it's allowed to roll forward in gear. But warmed up, with your foot on the break in drive, it consistently runs like it's missing a cylinder.

I've had the fuel-system pressurized, there are no leaks between the tank and lift-pump. I've been through the pump banjo-bolts, lines and return lines looking for leaks, and had a mechanic take a look as well. Given the fact that it runs fine cold, coupled with all the unsuccessful searching I've done to find air, I don't think I have air in the lines.

If it was an IP or injector problem I feel like it would also present while cold. The IP was supposedly resealed and injectors were supposedly rebuilt from the mechanic I bought the engine from, but I'm not 100% sure any of that really happened.

I've been reading everything I can get my hands on, and I'm beginning to think I may have a boarder-line timing problem (the engine manual identifies this as a possibility for the problem of rough idle when warm) or possibly something going on with the valves (it's direct-injection and we do a lot of in-town driving)? I've read that you can be just barely in spec, but then when the engine warms and the oil thins out, tolerances can loosen and push the timing over the edge out of spec (though this is more a problem with engines that have timing chains, which can stretch (the 13bt has a timing gear))


If you were me, what tests would you try to run first?
What colour smoke do you get when cold and warm and under acceleration?
And what speed does it idle at?
IMO , timing is often blamed but rarely guilty, unless some eager beaver has been in there playing around with it.
Heat expansion can affect a number of components that are tired or slightly out of spec.
I get the appropriate amount of white smoke on a cold start, and a bit of blue/black smoke while I'm running, In gear, I idle at around 950, and while I'm experience the problem and push the accelerator, I get a fluttering sort of acceleration until the needle is up to about 1300 rpm.

Also, I ran a liqui-moly diesel purge a couple weeks ago (no effect) and changed the oil and oil filter yesterday, 6 liters of mobil-1 synthetic and 1 liter of lucas oil stabilizer (no effect).
My guess is you have problem in the pump somewhere, it maybe something simple like an injector valve or injector spring.

also I read that with direct-inject engines, the oil should stay amber, mine does not.. it goes to black pretty quickly.

Maybe on a new engine with all the whizz bang technology, but this is an ancient design that dates back to the 70's. It probably stays cleaner than a IDI engine, but not for much longer.

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