Truck recently started dying if you don't go above 1000rpm when going forward or backward in drive or reverse. Truck idles at 400-500 rpm but not idling rough. We just replaced the map sensor today and the iac was replaced a few months ago along with new plugs, and new ignition coil, new distributor cap, new egr. We are at a complete loss now as to what the issue is. No check engine light. Runs super going down the road when above 1500 rpm. any ideas would be awesome...

 

What engine?

 

3.9 v6 auto

 

Quick thought - Have you checked the plenum gasket for leaks? Easiest is to open the throttle body (with the motor off!) and peer down into the plenum, and see if it's oily.

Another thought - see if you can use a hand vacuum pump on the EGR valve and verify it's not stuck open.

RwP

 

You can check the Catalytic Converter too. You can take the temp with a laser thermometer (about $20 or so at Home Despot) and if it is hotter in front its plugged. You can also smack it not too hard, with a rubber mallet and see if it runs better. I was ambiguous about the temp info I got so I whacked it a couple times on both sides and it ran better. Now I know I need to change it. I did have a bad plenum which I fixed as well. An oily plenum will destroy your Cat.

 

Plenum looked good I did bang on cat and no changes

 

Let us start at the start. Don't throw any more money at it until you diagnose it. If it was running good then started this something good probably went bad and that should be detectable.
Go here and follow the instructions for doing a code check; https://dodgeforum.com/forum/1st-gen...-96-codes.html
See if there are any codes.
I waited a long time to do it but I finally bought a MT2500 scan tool off ebay and it helps a lot. The main advantage aside form being able to see info about what the engine is doing is that you can change settings like reset the new IAC you just put in. http://buy1.snapon.com/products/diag...ge_chooser.asp
Make sure you get the right cartridges and an instruction book with it. It should only cost $200-$300 or so. If you can't there are work around's the guys on here will be able to help with.

 

Map sensor voltage reads at 4 with key on and not running and drops to zero when running at idle if that helps with anything.

 

Did the code check got no codes other than 12 and 55

 

Hmmm. OK Check the butterfly to see if it is oily on the bottom side. This is how I diagnosed the plenum. When I looked in it was too dark to see anything, turns out when I had it off it was caked with burnt oil. And see if you can get a laser thermometer to test the temp of your Cat front and back when the motor has been driven like 10 miles. Make sure your wires and plugs are good and well connected. The crank sensor comes to mind as well but I do not know how to diagnose it.