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My 2002 Intrepid w/ 2.7 engine has been sitting since last November while I try to diagnose an ignition problem. Experts, please help!
Car starts and idles normally. At approx 2k RPM, get a rough idle and the check engine light comes on: code p0351 (problem w/ ignition circuit to #1 cylinder).
I've tried swapping the coil with the one for cylinder #4, but no change -- same result on cylinder #1.
Using a VOM meter at the connector, I've confirmed that the green/brown supply wire to the #1 coil is showing approx 13.7 volts with engine running -- normal charging voltage. Using a test light, I get a nice, bright light for both #1 and #4, indicating that there's no significant difference in voltage drop between circuits.
With the connectors unplugged, I get approx 4 volts on the coil control circuit (brown/tan wires) for both #1 and #4. (I'm assuming my meter is showing average voltage, b/c it isn't fast enough to show the on/off pulses.)
However, there is one significant difference between #1 and #4 (my reference circuit). When I start the car with coil #1 disconnected, there's no CE code or stumble at idle -- only when revved to approx 2k. However, with the engine idling, if I pull the connector for #4 coil, the engine immediately stumbles and the CE light illuminates.
I tried hooking up a noid light to the control circuit for #1 and #4 and neither one triggers any flashes -- I'm guessing the approx 4 volt supply isn't high enough to light up the bulb.
I've removed the plenum and unwrapped ALL the wiring, looking for a break or short. Didn't find anything. Also unwrapped wiring from engine to the PCM (engine computer) and didn't find anything.
The connector for the ignition circuits to the PCM looks good -- all receptacles for male pins in proper position, no evidence of corrosion. Locking latch also works.
So my tentative conclusion:
--the #1 ignition circuit isn't firing at all (which is why the engine doesn't care whether the coil is connected or not).
--the likeliest cause is a bad circuit in the PCM, not the wiring.
Therefore, my plan is to buy a used PCM, have it flashed to show my car's correct mileage, and install it. I know there are Youtube videos on how to resolder the internal PCM connections, which I haven't tried. But if I do that, and the #1 circuit still doesn't work, I'm (almost) back to square 1 -- and don't know if my repair failed to work, or if it's still some kind of wiring problem, not the PCM.
This car, the newest in my fleet, is supposed to be my daily driver, and has otherwise been very reliable. A few years ago, I drove from Washington DC to Seattle round trip, and it performed flawlessly. It still has only around 100k miles (verified by Experian when I bought the car from a private party about five years ago).
All feedback appreciated -- in particular re my plan to bite the bullet and buy a replacement used PCM. Thanks in advance!
 

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i used to have an electronic test light that would just light up if the pcm ground was working.
being a circuit code and not a misfire, it sounds about right for a diagnosis.
these things batch fire at low rpm, then single cylinder fire at higher rpm.
sounds like #1 is having troubles at speed.
wire's the same colour at the pcm plug if you want to chase continuity and resistance for sure, but probablity is good it's the pcm from what you state above.
switching coils rules out coil over circuit.
 

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First, check the little capacitors in the ignition coil supply wire. They can short to ground and cause this code. Especially the 2.7's. Simple way, unplug and see if the code and issues return. Also swap the ASD relay with a known good relay to rule that out.

Coil resistance should be 0.6-0.9 Ohms; on old cars, all of them can be out;

Silly part, replace the camshaft sensor; to rule it out. It takes over at 2,000 RPM, and I can remember some of the more difficult issues I ran into in my mechanic days were the result of a shorted sensor; usually they cause a code, but not always. Sometimes they mess with other components. I had a test cam sensor in my tool box for just such an occasion.

Other silly part that can cause all kinds of havoc with totally unrelated sensors --- A/C pressure transducer -- In the line compressor to condensor---to rule it out, just unplug it, and check to see if the coil code goes away, and she runs right.

Also, check and load test the wire from coil #1 to the PCM. If that is good, ASD relay output should be load tested, at the fuse panel, and coil--but you mentioned you had 13.7 volts at the coil so its probably good.

Lastly, PCM, if possible, and you have another LH, swap that one in to test if its compatible. Its very rare for a PCM to cause problems on these cars. It is also super rare for wiring problems---usually that happens if the car has been in a wreck or tampered with---scratched my head for a week on one, replaced engine harness, bam, ran perfect.

BTW, you cant have a used PCM flashed to your current mileage --- cluster mileage is not stored there --- and that is illegal anyways. Depending on options in the car the used PCM may or may not work (SKIM system) if the BCM detects that the vin doesnt match. On lower level cars -- SE etc, you can sometimes just swap the PCM in. If you get a used one, get a BCM out of the same car to match it just in case.....and that will change your mileage; to whatever was stored in the BCM, which you cant legally change-- again..... So use this as a last resort.
 
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