Hello, I’ve had a couple weird issues come up in the last few weeks and can’t find the help I’m looking for. At first I would start the truck and cold or hot after starting in park the RPM’s would rev from 800-1500 constantly until put into drive. So I put a new IAC in and while it solved the fluctuating rpm in park now when I start the truck cold the rpm’s are 1500+ then drop to around 1000 then to 6-800. If the truck starts hot it’ll rev up to 1500+ then backfire and drop down to where they should be. I was also having a weird issue come up after replacing the IAC where my rpm’s and shifting is off while driving. After going 60+ then coming to a stop the truck won’t drop rpm below 1000 and I can tell it’s shifting is off. I also replaced the TPS thinking it was the issue for shifting and rpm issue while driving. I also pulled a 121 code so I thought replacing the TPS would help. Yet the issues persist. Looking for any help. I’m thinking that maybe the IAC is the wrong one and potentially vacuum line issues. The truck has a rebuilt E40D and 134k miles

 

Also wanted to mention that I did replace the throttle body a few years back with nice one and when I replaced the tps noticed that the throttle body didn’t snap shut when letting go of the mechanism. For some reason I can’t remember if the throttle body should shut on its own without being tugged by the accelerator line. If the throttle body isn’t shutting all the way would that cause this issue?

 

This might work,
If the throttle was depressed any amount when first starting the engine,after replacing the IAC, the PCM will adopt that high idle speed when started.

If you disconnect the negative cable from the battery, turn the headlamp sw on then off,it will remove some memory from the PCM. Mirrors, radio stations etc.
When you reconnect that negative cable, start the engine, BUT do not touch the throttle pedal. The PCM will set the idle at the programmed speed. Wait for it to idle down then take the truck for a normal drive (mile or two) with accelerate and coast The PCM will set the shift points and after a mile or two and you can reset any radio stations etc.

 

Okay meaning disconnect the terminal then try to turn my head lights on and then off. Are throttle bodies supposed to close on their own? Like if you have a throttle body by itself not connected to anything you can turn the top to open the valves but if you let go would it snap shut by itself or would you manually have to turn the top to close the valves or doors within the throttle body

 

If you have a mechanical throttle body, there should be a spring that returns the throttle blade to the closed position. IF,, there is a cable from the throttle pedal to the throttle body , the cable could be sticking. . With the pedal all the way up, the throttle blade should be closed and idle speed is controlled by the IAC valve..
The sudden opening of the throttle blade causes intake vacuum to drop and the manifold absolute pressure sensor ,MAP, signals the PCM to increase fuel.

If that MAP sensor is malfunctioning, it could be the cause of the lean misfire. Sometimes passages get plugged with carbon and the MAP can't do it's job.
96 should have a TPS to report the throttle opening. TPS =Throttle Position Sensor. The TPS is located on the throttle body.

 

Tried this out just now, still running into the backfire. It feels like when accelerating it shifts up fine it’s when coming to a stop where I run into the weird issue. If driving at 45mph + then coming to a stop it feels like the truck doesn’t know how to down shift. The Rpms will sit at 1000-1500 while if driving slow and coming to a stop the Rpms will go down to 6-700. Also noticed that when driving at faster speeds if I let off the gas the rpms will either jump up or stay the same. I’m gonna try switching out the IAC maybe I got the wrong one. I know sometimes I need 94 parts for my 96.

 

When you say backfire, does that mean you are getting a blast out the tailpipe or is it poping back through the intake?

What is the current mileage on the vehicle ?
Is there any way you can have your vehicle checked for any codes at this time?