1998 5.2l p0320

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.
http://i538.photobucket.com/albums/ff342/raschmidt/Automotive/jay_sen2.jpg

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

Is this worse when the engine is warm/hot?

The crankshaft position sensor failing is very common for all Chrysler vehicles, especially when the engine is warm/hot. We have changed many CKP sensors in our shop for customers with this exact complaint.

I would start by replacing the CKP sensor. Try it and let us know.

Thank you for the reply. The long crank seems worse when the engine is cold. The stumble/stall is hard to say as he lives so close to work the engine isn't really getting up to temp before he gets to work.

I did fail to mention that while the the fuel pressure is good when the key is first turned to run and when the engine is running, it drops to nearly zero within probably 10 seconds of turning the key off. This seems very rapid to me. I made sure that it wasn't a loose connection of the fuel pressure gauge on the fuel rail. Since the pressure regulator is in the tank with the fuel pump we haven't been able to check/replace that yet. I understand we may be fighting more than one problem here.

-Rod

Re: fuel pressure quick drop after ignition off.

I've seen this dilemma quite a few times in Dodge trucks & Jeeps. The flexible clear (whiteish) tube in the fuel pump assy will develop a crack and allow the fuel pressure to leak back. Also can cause a long crank time for starting.

Ahhh, okay, thanks for the info. We were hoping to not have to get to the pump assembly. So in your mind is it easier to remove the bed or drop the tank to access the pump?

-Rod

I think jwalker in post #4 above is probably on the right track now. Last week we finally replaced the CKP sensor and it did not make any change in the way the truck starts. We didn't recheck the fuel pressure to confirm that it still drops quickly, but it certainly still cranks a long time before starting.

The CKP sensor wasn't too bad to replace once we removed the passenger side splash shield/wheel well liner. We also tied a string on to the old connector so we could easily route the new connector back along the same path. My POM riveter got quite a work out reinstalling the splash shield. There were something like 11 plastic rivets holding that in place.

-Rod

Im having the same problem with a 1997 3.9l V6, i just replaced the ckp tonight and im still not able to start it. I got a p0320 before i replaced the sensor and cleared the code. But i haven't been able to get the truck to start the past few days. Replacing the ckp did nothing....