So to recap when the fuel system is in open loop mode it seems to run fine. As soon as it warms up and transitions to closed loop mode, it hesitates and backfires. I think it is running very lean. The question is what sensors would cause this behavior?

I keep throwing parts at this. Not a whole lot left.

New ECM
new MAP
new IAC
new distributor, rotor, plugs and wires
new fuel pump
new Crankshaft position sensor
new camshaft position sensor

no active or pending codes.

 

Is it still doing this? When it acts up what is the temp? The pic you posted is reading 122F, Is that up to temp or still warming up?

 

yes it does that. It will get into a loop where it cycles from like 2700 to 3000 to 2700 and undulates maybe every second or so.

No, it doesn’t do it until it gets hot around 200° usually, but I see what you mean. Ill pay more attention to the temp.

 

That's likely an O2 sensor problem then. Easy to test, just unplug 'em. (will set codes) If it runs/drives better with 'em unplugged, replace 'em. NTK or Denso sensors only though please, these trucks really don't like Bosch.

 

With both oxygen sensors unplugged, all of my issues appear to be gone. I’ll order some sensors!

 

Dodge 1990 fuel injectors coils failed when they got hot and stopped gas delivery--like running out of gas or power, but then when they cooled down they returned to normal function. Rated worst in the car industry. Solution was to replace all Dodge fuel injectors with Bosch!!!

 

A 90 is throttle body injection and has 2 injectors. The op has a 99 witch is sequential (8 injectors). I had a 90 and put 200K on it with not one issue with the injectors.

 

My new oxygen sensors came in today. Installed, cleared codes and BAM! All is well! Thanks for your support!