g9volt

2000 2500 6.0 120k miles. On cold mornings it can idle rough and chug so bad it literally rocks the truck, for about 10 minutes, until the temp gauge reads near 200°, then it smooths out completely like I flipped a switch. After that it will if you take off too fast it it stumbles/cuts out momentarily but keeps accelerating, but that also clears up within a few miles. The whole fuel system is original, only change is cylinder heads 5 years ago, because of so many broken exhaust manifold bolts. Also, after initial warm-up, it can set for 3 hours and starts up perfect. There is also a very loud sucking noise from air cleaner (K&N setup) that quits when engine smooths out at full warm up. It does the same thing on warm days, but only takes as little as 3 minutes to smooth out, still full warm-up. It also , at first, threw out code P0336 (crank sensor) but the CEL would go out when engine smoothed out. Now when during rough idle during warm-up, there's no CEL anymore. HELP!

jw11432

So it sounds like when the engine is below a certain temperature threshold, it doesn't run right. Now I know that upon cold starts, the ECU enriches the fuel mixture. My spitball idea is that maybe you have a faulty temp sensor, and as a result, the ECU never gets the data that your engine is cold, so you're always running an ideal operating temperature mixture, as opposed to the required rich mixture when cold.
I could be mistaken in how things operate in these trucks, but if you have a scanner, that could be something to look at, what the coolant temp is when you do a cold start. If it's reading higher than it should, that could likely be the problem. If not that, perhaps some other component causing this issue. But I could be wrong about the cause as well.

Jeffdt1966

There is a fairly common problem of intake gasket failures on older and higher mileage motors . The symptoms act similar to your description but instead of being related to the thermostat opening it's the motor warming up and the heat causing expansion of intake and gasket thus sealing minor leaks until they cool off enough to open again . This also makes them very cold natured in the winter with starting being easier in summer or after they have already warmed up . You can check for this just like where chasing a vacuum leak ( with is exactly what is ) spray whatever you would normally spray to find vacuum leaks . Spray all around the base of the intake with the truck running and listen for any repeatable idle changes . I use starting fluid spray so mine would rev. Up if I hit a leak ..... But I don't recommend it , its highly flammable and I have been told that I shouldn't use it for leak detection .... But I love how well it works and I do it anyway . If you use this method and something goes bad ... Don't blame me .... I don't recommend it . A simple Google search will offer several different options that may work just as well . good luck ... This is just something I would check with those symptoms....