I'm trying to help a co-worker with his Dakota. He's been complaining of a long crank time for several months. The fuel pressure is good, battery voltage is good, and vacuum is a steady 19 in Hg at idle when the A/C clutch is not engaged. For a long time there were no diagnostic codes. He also said the fuel economy had dropped. Shortly after his initial complaints we replaced the spark plugs, wires, distributor cap, and rotor, all of which showed significant wear. About a week later he did a cylinder decarb with SeaFoam since the problem really was no better. He got a lot of dark exhaust, but it didn't fix the crank time or fuel economy issue.
Recently he began to complain that randomly the truck would stumble when letting off the throttle and rarely it might even stall. I had him run with my CarChip for a bit so I could datalog a few trips. The intake air temp is right on, coolant temperature seems reasonable, manifold pressure seems good. The Sensor 1 O2 sensors toggle frequently and engine and vehicle speeds are right on and track nicely to manifold pressure. Oddly though, the Carchip logged a P0320 diagnostic code at startup for the lase several trips that we logged. That code suggests an issue with the Ignition/Distributor engine speed input circuit. After reading on the Internet it seemed several folks suggest the Crankshaft Position (CKP) sensor is the typical culprit. So last night I backprobed the CKP and CMP sensors and and monitored them with a scope to see if anything stands out. Both look good (see image) and are present as soon as the starter begins to crank the engine.
One odd think I did notice with the mechanical vacuum gauge installed on one of the otherwise capped ports on the side of the intake manifold is that when the A/C compressor clutch would engage, the vacuum would drop to 15 in Hg which is borderline good/bad. With the clutch unplugged, this didn't seem to be an issue. I thought maybe the IAC was not keeping up so I removed it, inspected and cleaned it (not too dirty). I reinstalled and left the IAC unplugged. With the A/C clutch connected, the warm engine started and idled okay, but the idle speed seemed to fluctuate a bit. When the A/C clutch kicked in, the vacuum did not drop as much, but the idle fluctuation got more noticeable. Plugging the IAC back in put everything back to smooth idle and low vacuum with A/C clutch engaged.
So based on the scope plot, I don't see the CMP or CKP being an issue. I also can't understand how an IAC, if slow, would trigger a P0320 code. Additionally, my scan tool can tell the PCM what to use for desired idle and the IAC is able to accommodate idle speeds up to at least 1800 rpm, so it seems to be functioning.
If you're still reading this, I'd like to thank you for your interest. I'd also like to know if you have any suggestions on additional items to check, or what your experience tells you is probably the issue. Admittedly I'm not a Chrysler guy so I don't know if there are common issues that may cause this type of code.
-Rod