Sounds like it's directly related to idle, where the engine is very susceptible to vacuum leaks, poor spark, and dirty injectors.

You can put a coil on it, but I'd check the vacuum at idle and look for a wandering rpm at idle too. If it wanders at idle, then you have more info. If the vacuum is low or fluctuates, you also will have very good data points to add to the diagnostics.

 

I tempted fate and tried to drive in to work this morning. Fate won and I'm on the roadside waiting for a tow. Too early for parts store and can't stay in this location.

Started right up this morning then just died completely approaching an intersection. Cranks, no start. Code shows only the one P0531. Now gotta find the relays.

Found em, switched relays and no-start condition persists. Don't have multimeter with me so can't check much else. Waiting for stores to open to get a coil. Fingers crossed.

 

Preemptive question, if it's not the coil, what would be next to check, PCM? Do I check first for power going to the coil? How best to do that? If the PCM, any suggestions for diagnosis there?

 

Check the connectors at the PCM, they like to corrode, and cause all sorts of interesting problems.

Problem is, it only fails long enough for the engine to quit, so, no telling what is actually happening...... Given the age of the truck, a relay, or the coil itself *might* be the problem, but, if it persists after changing them...... Well, gonna have to wait until you have a solid failure, and troubleshoot from there.

 

Today I had that "solid failure", truck left me on the roadside this morning with a "no-start" condition. It cranks but no spark.

So tonight I checked the coil. It tested good for primary (1.0 Ohm) and secondary (12,220 Ohm) resistance. Next I checked the coil harness at the coil connector. The - side had a good ground. The + side had no voltage.

I had switched relays around between the ASD and others (horn/trailer tow) and nothing changed.

I checked voltage when cranking the engine and it only came up to 0.2 volts.

I'm not sure what to check next as there should be voltage (12.4v) to the coil harness, right?

The PCM was recently replaced. The original PCM failed suddenly right after a remanufactured Mopar transmission was installed. The shop replaced two PCMs before they got it running again. This one has been in for less than 800 miles. I'm wondering whether there is anything related to the transmission swap the shop could have screwed up causing the truck to eat PCMs????

 

You will only get power to the coil and injectors for about 3 seconds when you turn the key to on..... If the PCM sees the engine turning though, there should be 12 volts there. Coil gets power from the ASD relay, as do the injectors, so, check voltage at one or more injectors as well, see if you are actually getting battery voltage there.

 

Doing a little poking around on the web this morning and found info about the PCM needing to be programmed specific to the VIN? Since a shop (who hasn't done much right so far) installed the last PCM could it be they just plugged in whatever they got from the parts store without programming, could that have screwed something up?

 

that's only an issue on the 98 and up PCM's. Your truck shouldn't care.

Of course, WHERE they got the PCM makes a big difference. Not all vendors are created equal...... I have seen folks go thru four PCM's before they get one that actually works right.

 

I think I found the problem!! (Sorry long winded post here)

Kept digging yesterday and learned some more. I started by checking the OBD-I "key flash code" as I remembered sometimes they were different than the OBD-II Scanner codes. This investigation returned a 43, Ignition coil circuit fault.

Checking this I learned that I had been checking the wrong wire for voltage at the coil input. After checking the code with the key I decided to give it a crank, and IT FIRED RIGHT UP!! ?? It was idling fine and had good throttle response. At the least I hoped to get it back in my driveway for further diagnosis.

Since it was running I decided to see if I could replicate the failure. In initially it was a random sputter/stall I was thinking "short". I tugged and wiggled all the connectors on the PCM and their harnesses, nothing. So I went to the coil input wire and started from where I could reach it on the valve cover and started pushing, bending, wiggling that wire. Doing this I got the idle to hiccup and finally it stalled.

I "straightened" it back out, no kinks, and tried starting and she fired right up. Wiggled/tugged the wire again -- STALLED! Bingo, I hope.

So I unplugged the connector.



The wire had been routed too close to the exhaust manifold and the split conduit cover had been melted away. I pulled this off and the tape from the wires back to the manifold. I could not find any damage to the wires; no pin holes, burnt marks, cuts, bared wire, broken insulation. Nothing felt brittle.

I continued with the failure replications and isolated it to the plug/connectors. Only when movement occurred here did the engine stall/cut out. Dis-assembly of the plug showed that the wires were firm in the connectors and not damaged there either. Now I checked the wires for continuity, resistance and voltage. All tested OK as far as my understanding goes.

So, the only thing I can think of is the continuity between the pins in the coil and the contacts in the harness plug. There has got to be some play here.

I know the last time the "shop" had the truck they "tried fixing" a different issue by swapping in a new coil. The old coil had been on the truck for 15 years! Maybe the "new" coil had slightly larger pins causing the ID of the harness connectors to get enlarged and thus a little loose now, or just removing the plug broke 15yrs of solid electrical contact. They put my old coil back in.

Anyway, my next step it to add some dielectric grease to the plug connector to attempt to improve the continuity between the pins and connectors. If that doesn't solve it I'll look for a new plug pigtail. Hopefully that solves all this.

 

Pull the coil, and check the pins for the connector on the coil. They may just be dirty.....