My Magnum 5.9L started jerking randomly at idle and up to 1500RPM. It gets worse on hot (actually, regular - 180-190 degrees) engine. Spark plugs and wires have been replaced already - no change.
Run with another PCM/ECU - still jerks.
Engine jerk followed by a more intense exhaust. No stalling so far

What's going on?

 

Check for vacuum leaks.

 

Don't have a manometer to measure an exact pressure. But
a) I moved/shook all vacuum hoses on hot engine to replicate those jerks - no success.
b) Disconnected a vacuum hose to check the air flow - it sucks
c) also the engine idle RPM increases from 600 to 1000 and jerks became more visible.

I would say that jerks become more frequent and occur at lower engine temperatures as it was before (but the weather gets warmer as well).

 

The next free and easy thing I'd do is pull and clean the throttle body, and make sure the idle-air stepping motor passageway is clean.

 

Cleaned the throttle body (is wasn't bad, actually). Found 2 gaskets cracked: between the throttle body and engine and the throttle body and air intake. Are they critical?
Also found that a hose fitting adapter, (the hose from an air intake to the left cylinder head cover) is a little bit loose, but wiggling of this adapter does not change anything.

Overall - the problem still exists, no changes at all.

 

Did you look at the plenum for oil pooling when you had the tb off?
http://www.hughesengines.com/Index/p...p?partid=27091

 

The gasket between the throttle body and manifold is indeed critical. The other one, not so much.

 

Good to know - I'll replace it and have a look into the mainfold as Moparite suggested.

I was in mountains this weekend and noticed very frequent missfire when the engine temperature hits 210 degrees. Even on idle. Says once per 1-2 seconds.
Also noticed, if I turn a steering wheel to the very end position, the idle RPM drops from ~600 to 450-500 (on the hot engine) and from ~600 to 550 (on warm).

 

RPM drop at the steering end is normal.

 

Brought the truck to mechanic. He has found 3 problems, but I'm not sure how 2nd and 3rd could be related to the engine temperature.

1. He has found a manifold intake leak. He sprayed propane(!) in the engine compartment and observed a huge change in the engine behavior. - I would agree, that this leak could depend on temperature.
2. He also found that distributor cap should be replaced. - no doubts, it could cause missfire but the engine temperature should not affect on it. I guess.....
3. He also told me that the distributor shaft is loose and suggested to replace a bushing.

How bad is a first problem, could it be fixed "externally" without a removing of manifold?
For sure, I'm going to replace the distributor cap and rotor, which is easy fix and see how it helps...