So the other day I was driving back home and my car starting acing really weird--bucking, almost-stalling, idle loping up about 200rpm and back down. I've been having a problem keeping my CPS plugged in and getting misfire codes, so I figured it had something to do with that...

Well, I was wrong. I got a code p0123 and managed to get my car home to check it out. Tried to turn off the code but it wouldn't go away. Replaced the TPS and the CEL went off and the car started up and ran fine until it heated up. Then it started with the same old symptoms.

It idles fine, but as soon as I put my foot on the accelerator from a stop light the rpms drop to about 50 and the car almost/sometimes stalls out. I'm thinking it might be an alternator issue (since engine speed seems to affect it) but I'm really not sure where to go from here. Any advice would be greatly appreciated!!!

 

i dont think its an alternator issue. if it was you would have to constantly jump it and it wouldnt stay running for long.

to me it sounds like its just missing bad. check all the ignitions. then check intake, especially the air flow sensor.

 

There is no Air Flow Sensor on the 2.5 v6. There is a MAP sensor but that looks OK...

Someone on another forum suggested Coolant Temp Sensor, signing that if the car thinks it's cold all the time it'll act really funky...

 

I was chasing an irradic idle and the car would actually lunge while coming to a stop.
Turned out to be the Crank Position Sensor. I know you said that wasn't unplugged, but I would not completely rule it out. I ohmed mine with a meter and compared it to a new one and found the old one to have higher resistance. So I assumed it was not feeding a good signal 100% of the time. That maybe the PCM thought the engine was running slower than it actually was, so it would attempt to increase RPM. Replacement fixed irradic/harsh shifts as well.

You should probably check for a vacuum leak as well. It may be leaning out and not getting enough fuel fast enough to compensate. The Idle Air Valve could be gunked up from carbon buildup and is slow to react so a look at that couldn't hurt.

 

Thanks to everyone's help.

After running through the TPS checks, I realized the new one I installed was indeed working, and my rough starts were coming from somewhere else:

Coolant Temp Sensor

Installed the new one, reset the battery, car coughed once, started, and has run like a dream since. Think I'm finally done chasing issues. Thanks to everyone!!!

 

Does anyone know where there's a table of resistances vs. temperature for coolant sensors? I've used a resistor in place of an intake air temp sensor to fool the ECM into adding extra fuel, but that was something in the 80's that had only used basic operating principles.
I would like it for troubleshooting purposes now though.

 

Honestly everything I've seen says that if you suspect it's the CTS, just replace it. For $15, $20 it's totally worth it since testing the resistance on those suckers can be pretty tough if you don't have a control over the temperature of the fluid.

Anyway, this is what my sources say:
At 200F (operating temp) resistance should be between 700 and 1,000 ohms
At 70F (room temp) resistance should be between 7,000 and 13,000 ohms

 

i was going to say coolant tempature sensor but i see you got it figured out... great job