Strange idle only when the engine is warm
 

I have a 2006, 5.9 common rail engine. This thing is making me pull my hair out! It has a random engine miss, only when the coolant is up to operating temperature (200 degrees). When I go to W.O.T the engine comes back down in rpms very slowly. It drives fine, runs good at any rpm other than idle. Has plenty of power, gets 17mpg @ 75mph. It always starts right up. I have no fault codes to help troubleshoot.

Here is what I've done so far:
1. Injector cut out test = no change in the problem
2. Adjust engine valves = no change in the problem
3. Removed and tested all 6 injectors = tested fine (@ room temperature)
4. Changed air & fuel filter = no change in the problem
5. Capped high pressure fuel lines to each injector one at a time. Got engine up to operating temp. (cylinder cut-out test for the fuel supply to the injectors) this made a difference with the idle problem.
6. Replaced 3 injectors = no change in the problem
7. Replaced all of the high pressure jumper tubes = no change in the problem
8. Replaced the fuel rail = no change in the problem
9. Replaced the fuel pressure regulator on the cp3 = no change in the problem
I have $1800.00 into this project so far and have not fixed a thing. Please help if you know something.

really strange-?-?- I pulled the injectors, sent them to a fuels specialty shop they tested them, they said they tested fine. Got the injectors back reinstalled them and the miss went away. I don't know what the problem was.

You probably had a cross over tube that the TQ was off on. When it warmed up it was leaking more than it should have been causing a random miss.

That's a good idea but I don't think so. I had already tried retorqueing the quil tubes, that didn't work. I had replaced the quil tubes with 6 new ones before I sent the injectors in for testing.

injector harness then maybe PRV Call Todd at T&C Diesel Google him!

Maybe, maybe not. The injector to tube interface is a bit picky about how it is set up. By removing the injectors you probably went thru the whole process of install where just replacing the tubes you may not have.

On the other hand, you could just have had a sticky injector that has now been cleaned out. You don't know what the testing consisted of so that could have fixed the issue.

A harness problem will almost always generate codes. If you don't have codes it is good bet it was a mechnaical issue. If the injectors are not new all bets are off, you likely have at least one injector that is out of spec in some part that testing will never find. A slight score on a pintel will take a mocroscope to see and will effect operation as th eclearnaces in the injectors are tight, 2-3 um in a lot of places.

A slow drop of rpms when you let off the throttle is perfectly normal, the ECU will not let the egine de-rev quickly. If it did it would likely blow injectors form the pressure spikes. You cannot just free rev and dump the throttle either or you get wierd reactions as the rail pressure spikes badly. To test that you need to hold rpms for at least 10 seconds to stabilze things then lift off the throttle. If your rpms drop steadily will no jerks or hangs the inejectors are working correctly.