I recently bought myself into this truck, and now I'm discovering some issues that I didn't notice before.

When sitting at red lights for ant extended period of time, our will idle rough, and sometimes stall out. Another case that it stalls out is when at low speeds (i.e. parking lot) when pulling into the parking space, it will stall out as well. The engine idles in gear less than 500 RPM, and dips down to around 200-350 RPM at times. I feel the low idle is the root cause of the issue because of the power steering pump and brake boster placing load on the engine at less than design idle could result in the stall. It idles fine in park/neutral, but reverse/drive is rough.

I have heard everything from alternator issues, bad battery cell, ecm issues, dirty tbi, and such, but I'm unsure where to start.

At idle the alternator outputs less than 14.5V, but at cruising speed, its right on the nose. Temperature stable less than 210F, oil pressure stable at slightly greater than 40 psig (less than 40 psig at idle, but stable)

Ant direction would be very much appreciated. Thanks!

-Mike

Vehicle specs

Welcome to the forum Michael.
How many miles are on the truck?
Have you scanned for stored codes?
Just going to throw out some ideas..
The IAC controls idle and often just cleaning them may restore idle.
Lots of carbon can cause sticking issues to the IAC pintle movement so getting some throttle body cleaner & cleaning may resolve the idle drop issue.
Vacuum leaks..time to look for cracked hoses!
If it has lots of miles a solid tune up may help from fuel filter, to spark plugs, checking throttle plate movement, cleaning throttle body/intake/fresh air filter, fuel pressure at rail & FPR =fuel pressure regulator check and...battery load test...
Often when you buy used if you go though it and do some checks/ cleaning /replacements, you will familiarize yourself with the engine & know whats good & what is not.
Hope this helps....

135KMi, no check for stored codes yet.
I am unfamiliar as to what an IAC is, as this is my first modern gas truck to date.
Plugs are brand new, and im running a tank of super unleaded with ~10oz of Lucas oil fuel treatment.
Everywhere is closed today being Memorial Day, and rightfully so.
I figure tomorrow I'll go out and survey the computer at a local auto shop.
The battery tested bad, but still does not struggle at all during startup, although it could pose a problem when for bearing load when RPM dips down to <350RPM.
I hope its nothing more than a dirty fuel system.

Thanks!
-Mike

Hello Michael, the IAC = Intake Air Control. Possibly someone with a 2003 6.0 engine can show the IAC location for that motor.
Can you tell me how old that battery is and if it has removable top caps to add distilled water? If it is older & tested bad you want to consider replacing it. Generally what I like to do is to clean the terminals & trickle charge a battery overnight (4amp) range and use a battery tester or hydrometer which cost like $5 at the local parts stores. You basically remove the caps after charging, and test each cell using the battery tester to view the state of charge or specific gravity reading of each cell. Of course a voltage reading is important as well.
I also use a 100 amp load tester to ensure it is in a passing area, and if not based on the sensitive electronics on modern vehicles, it is a good idea to have a powerful battery 800CCA+ in good condition.
Strange things can happen & go awonk as a battery or alternator begins to fail...
If you had an electronic battery test completed did it give you the voltage & current rating of remaining Cold cranking amps (CCA) and what it was new (lets say 800CCA) ?

If I remember what I saw correctly, I was unable to observe a remaining CCA value. However, I do believe I observed a 2.71VDC Cell, or multiple cells under load after it displayed BAD BATTERY.

I will most likely be replacing the battery tomorrow, and may consider cleaning the Throttle Body, as well as possibly replace the TPS.

I have observed no codes being thrown, but I haven't had the chance to see anything with any type of engine scanner. I will be doing that first thing in the morning.

If it has the fly by wire throttle body (no throttle cable) it has no IAC motor, the computer adjust idle by adjusting the throttle plate. If it has a bunch of carbon build up on the back side if the throttle plate it can cause idle issues because the computer has a set value thinking the engine is getting so much air. So take off the TB clean it and reinstall it and see what happens.

I intend on replacing the battery first thing in the morning, and removing and cleaning the TB. If that doesn't do the trick, I'm going to find an auto shop around here and have them run an engine scanner on it to read all of the specs and operating conditions.

Thanks for all of the prompt replies everyone!
-Mike

75 racer, are you associated with the U.*. Submarine Force by chance?

the carbon buildup on the back of the throttle blade on the ls series is extremely frequent issue, i believe there is a tsb on it. there is no iac, the opening is so small when it runs right so it has to be perfectly clean