My truck has been cutting out and stalling at various speeds. Once the truck comes to a stop after spuddering and backfiring, it starts right up. The check engine light comes on a throws a code for the IAC/stepper motor and coolant thermistor. I have replaced both parts and they appear to be working fine. What else could be causing this problem? I have also replaced cap, wires, checked for cracked vacuum hoses, checked base idle and timing, and replaced coil pack. 95 D1 with 4.6

'95 DI w/ 4.6? Replaced cap and coil pack?

our rrc was throwing codes all the time , stepper motor codes , airleak codes and a maf sensor code, replaced maf sensor and all proplems and left. as mentioned above replace coil pack and ignigtion parts as this can lead to symptoms described, hope this helps ! :smokin:

Paul,
Yes, a D1 with a 4.6 and I replaced ignition coil and distributor cap. I hope I'm using the right vocab. Stalling is still occurring at high speeds. If I let off the gas as it starts to studder, it seems to help some in the prevention of a total stall.

A DI with a 4.6 is curious to begin with. NAS? A 4.6 with a distributor is even moreso.

Coil packs are inconsistent with cap/rotor.

If it's a GEMS engine, check the vacuum lines and valve cover breathers. And reset adaptive values.

If it's a Lucas, set the base idle correctly and check the tee piece.

I pulled the 3.9 and installed the 4.6. No GEMS. What T piece are you referring to?

Often-times, the vacuum line from the flame arrester will become horribly clogged, leading to all manner of problems. The T-piece is the plastic tee between the hoses running from the passenger side valve cover to the plenum.

You have replaced the IAC valve, leading me to believe that you may also have a circuitry problem between the ECU and the devices.

Have you set the base idle? Cleaned the IAC valve casting? You might tear the MAF sensor apart and use some contact cleaner to clean the wires.

When you did the swap, did you remap the ECU? You may be running slightly lean and this may be causing some issues. While the volumes are not vastly different, the 14 CUx ECU only has so much variability it can utilize with respect to A/F ratio control.

Back firing implies a lean condition. Did you change the fuel pressure regulator to handle the increased displacement? Don't burn your pistons. See www.rpiv8.com on 4.6 conversions.

Thanks for all the help. The truck is in at the local shop. I'll report back.

After reading RPI's site again, I think that I don't need a different fuel pressure regulater since I have a reprogrammed ECU. The new ECU should handle the fuel requirements for the engine.