Rough idle or stall during cold and warm startups

[Snake3yez]

All - My 2006 Paxton Viper had been running tip-top but the other day I went to cold start it, when I noticed the car made a different noise and I could tell right away something went wrong. I had my motor rebuilt in 2018 with forged internals. During that time I also did a bunch of maintenance on the car. Including the TPS, AIT, IAC, cleaned the TB, filters among other things. I also added a new battery about 2 months ago.

During this idle start issue, It did throw a code = P0123 which I believe indicates low voltage or max voltage coming from the TPS. I replaced it again because I had an extra one and that made no difference. I also did the 02 sensors because mine was still the 12-year-old OEM ones. I unplugged the battery for a day to reset everything, recalibrated the IAC and did multiple drives for the re-learning cycles to complete. Check engine light has not come back on as of this writing, but the car still won’t idle correctly cold or warm. It drives fine if I just control the revs myself.

Typically on a cold start revs would be higher until it warmed up a bit, then it goes to its normal warm idle around 700 or so. When cold I have to give it some gas for it to stay on, and when it warms up it idles at like 500 and not smooth. If I'm driving it at speed the car drives smooth as it did before, and if I stay on the gas at the light it idles fine at 700 or 800.

I’m not sure what else to try at this point so I’m hoping you guys can help me. I’ve read several threads on older and newer gen Vipers with this issue and most said it was a battery issue or the TPS. I’ve replaced those to no avail. Thanks!

[Damiean]

So I don't have my Viper yet, but in the past when any of my cars have given me rough-idle and "feather the gas at a stop light to keep it running" problem, it's been the EGR valve. Might be something to check, they're usually pretty affordable.

[Zubik]

Have you checked for vacuum leaks?

[Snake3yez]

I’ve checked most, if not all the hoses and fittings and didn’t see anything out of place or felt air leaking.

[Snake3yez]

So I don't have my Viper yet, but in the past when any of my cars have given me rough-idle and "feather the gas at a stop light to keep it running" problem, it's been the EGR valve. Might be something to check, they're usually pretty affordable. Interesting. I’ll have a look. I actually have a brand new Purge valve module I haven’t replaced yet. Thanks!

[Snake3yez]

Update: I checked all the vacuum hoses, literally replaced anything related to emissions, like the EVAP purge solenoid, and other idle related components as stated in the original post. What I have found is that Idle Air Control Valve (IAC) is getting stuck all the way out. This is my second one I've replaced in a few months. If I carefully push it back in, re-install it, and re-program it by resetting the battery, then doing the five slow gas pedal pushes to reset it, it runs fine for a few days with no idle issues, then it gets stuck again and won't idle. The IAC is not super dirty or anything when I take it out, and they're basically new.

I don't know what else to try at this point, or can't figure out what's causing the IAC to get stuck open like that after a few days or running fine.

[Gen3CoupeTX]

Your description made me think IAC. Mine has done that too ... Also a Paxton car, 2006 coupe. I replaced my IAC and all was fine. But before I did, I drove it with the IAC disconnected. It throws a code if you do, but that's less of a problem than a wonky idle sitting at a red light. I'm not sure the climate of where you live, but IAC is fairly pointless in warm places like Houston. BTW, I ordered two IAC valves before I got the correct one. The one I got from JonB was correct. My money says you are on the right track! Cheers, Mike M

[Snake3yez]

Your description made me think IAC. Mine has done that too ... Also a Paxton car, 2006 coupe. I replaced my IAC and all was fine. But before I did, I drove it with the IAC disconnected. It throws a code if you do, but that's less of a problem than a wonky idle sitting at a red light. I'm not sure the climate of where you live, but IAC is fairly pointless in warm places like Houston. BTW, I ordered two IAC valves before I got the correct one. The one I got from JonB was correct. My money says you are on the right track! Cheers, Mike M

I'm in Atlanta so hot temps all year round. Interesting that you can drive without it lol. How does it idle when its not plugged in? I ordered the original replacement from Dodge and the part number matches up to the one that was on the car (Assuming OEM). I did order a 2nd one from Parts Geek online. We'll see how that one turns out, but I don't want to be replacing a $50-$140 part (depending on who you buy it from) all the time. Thanks for the advice.

[Gen3CoupeTX]

You can drive without it connected. It will idle at whatever speed your engine normally idles at (which you can adjust) without the 'massaging' an IAC valve would add. While it's disconnected, mine fires right up, drives perfectly fine, and the idle never hunts or bounces around. So if the seemingly-good IAC that's in mine now ever acts up even a bit, I'll disconnect it again. Even talking about this reminds me of the old 'Carb' days where one had to know the nuances of their particular car in any given situation. So much easier now!

[Whiskey]

I had that happen once but it wasn’t the IAC, it was the vac line UNDER the intake mani had dry rotted and was sucking in air. Just nipped the tip. I reconnected it using multiple pairs of long needle nose plyers. Not as easy as it sounds but I really didn’t want to pull off the intake mani. I figured it out by teving the motor with the throttle body opened and spraying starting fluid (lightly) under there. If the RPMS raise...you got a leak.