I used Bosch. Lol. I haven't cleaned the TB yet. The one I'm using is a Fastman that was on my 94. I had to send it in to get the throttle shaft swapped for the 97 style cables. I swapped sensors from the TB that was on the 5.2 engine. I have an air/fuel gauge installed and could use my scanner and compare the readings between the scanner and gauge.

 

Ooops.

and:

That might be enlightening.

 

I hooked up my scanner and drove around. My scanner shows my o2 sensors staying within range. My A/F gauge shows it from 14.8-15.2 at 60 mph, which is about 2200 rpm with overdrive off. When I let off the accelerator, the ratio falls to the 10.8-11.4 range, then it goes up to the 13.8-14.2 range. I'm going from a lean condition at 60 mph to a rich condition when letting off the accelerator, then adjust to a slight rich condition at idle.

 

That's pretty normal.

If you haven't already, clean the throttle body. Pay special attention to the IAC well. They like to crud up, and cause issues. That *might* help, but, I suspect you are going to need a proper tune on there to completely solve it.

 

Judging from the variety of the sensors dodge used in these trucks the ECM uses whats called a "speed density" equation to determine fuel injection pulse widths for each intake stroke on each cylinder.

basically an array of sensors such as your MAP sensor, IAT sensor, and to an extent the TPS all determine the quality of the atmosphere inside the intake manifold, which implies how many molecules of oxygen exist in each cubic foot of air that passes through the manifold, affected by temperature, vacuum, throttle position, etc. which basically implies the "load" of the engine.

then the ecm compares this to a pre-programmed table that compares the rpm of the engine (and how much volume it should be displacing for a 5.2 or 5.9 engine at that rpm) and cross references the load (the amount of oxygen per volume in the manifold) to determine how much oxygen the engine is actually ingesting and at what rate.

as mentioned before by HeyYou, since your engine is of a bigger displacement then the computer thinks, the engine will run incredibly lean.. I'm surprised its even functional considering the differences in a 5.2 and an approximately 6.6 litre engine.

Since gasoline needs to be mixed with intake air in as fine of a mist as possible, and since barometric pressure has a great effect on how well gasoline remains suspended or re-condenses, the TPS is used by the ECM to either enrich or lean the mixture based on your throttle changes.

when you let off of the throttle, the still-decelerating engine pulls a considerable amount of vacuum across the intake manifold behind the closed butterflies, which makes gasoline very succeptible to evaporating and becoming more finely mixed with the intake air. as such, I can see most car manufacturers programming the computer to slightly lean the mixture during closing-throttle transitions in order to save fuel and help emissions. combined with the fact that your base fuel-injection map is already very lean for the displacement you are ACTUALLY running, this could be leaning the engine out enough to stall it.

You said the throttle response is strong when you OPEN the throttle (and the TPS tells the computer to enrich the mixture due to the DROP in vacuum), so I bet you could (VERY CAREFULLY) experiment with disconnecting the TPS and having opposite results: weak throttle response, but solid during throttle closing.

Regardless, considering having a custom tune burned into an ECM can be both costly and complex, some short cuts modern-day-efi-hot-rodders sometimes take involves tricking the ECM.

For instance, you can consider simply buying a set of fuel injectors with a higher rate of flow, or upping the fuel-injection line pressure for example in proportion to the amount of displacement you have gained going from a 318 to a 408. However, these shortcuts are highly dependant on the new engine having a similar torque curve (which is an essence a VE curve) for them to be effective solutions.

Personally when I got done building my 408 I'm just slapping a carb on the bastard.

 

I thought a would try a 5.9 ecm, until I get a tune in the truck. Picked up a used 5.9 ecm from a local salvage yard. After instillation, I started getting a IAC code. I pulled off throttle body to clean it, but everything was clean. The IAC motor appears to be pretty new and is probably a BWD brand from O'Reilly Auto. After installing the throttle body, check engine light was still on. I swapped ecm back to the 5.2 that came with the truck and check engine light does not come on. I will continue to wait for my tune.

 

Have you attempted to clear the code present on the 5.9 ECU? Don't forget that it very well could have been saved in the ECU's memory from whichever truck it was pulled from, hence why swapping the ECU's is the only obvious variable between code and no code. But no matter what until you get that tune, go easy on the darn thing. Running as lean as we suspect, high RPMs and high temps are a sure fire way to have a catostrophic combustion quality that may destroy the engine.

 

I made sure there were no stored codes with the 5.2 ecm. When I 1st put the 5.9 ecm on, I got a code when I started it up. I cleared the code but it popped right back up. After swapping back to the 5.2, I looked for codes, but it passed and had no stored codes. I'm going to return the ecm. I'm out of options, so I will have to sit and wait for Ryan to contact me. After spending 3months on this project, I'm wanting to drive it !! Lol.

 

while i agree about the tune the iac is what keeps the engine runnin when you let off the pedal. lean or not if it's not letting air in the engines gonna die when you let off the pedal. or at least it's going to sound like it wants to die. it's a cheap part, toss in a new one if you wanna play a little till you get the tune.