2004 Dakota with a 4.7 L engine V8. 118k miles. Started with a check engine light, the codes that were being thrown were for a third cylinder misfire primarily. Then it started losing power going uphill. Changed the plugs and wires (the plugs were toast, the person replacing them didn't understand how they were still firing because the gaps were so corroded), it was driving pretty smoothly, shifting well but we were still getting codes. My brother-in-law added a bottle of seafoam into the gas tank thinking that would help, immediately after that the engine started idling pretty rough. I took it to my local mechanic, After having it for almost three weeks and it suddenly not starting for them for no particular reason (then they told me it started again randomly a week later, still don’t know what happened) they did verify that the PCM was bad and I needed to take it to the dealership to have it fixed. The dealership could not fix the PCM because the part was discontinued through the dealership(?), so I took it home, went and bought a reconditioned from Flagship One and had my brother-in-law install it. There’s no more check engine light, however the idle is rough. When I start it up, it sits at about 6-700 RPM and feels good, however within a couple minutes it drops down to around 500ish. I don’t have a lot of faith in my mechanic and/or backyard mechanic and just wondering if anybody has any ideas of what I should be looking at to fix the situation and if I should be driving it the way it is. Thank you kindly!!

 

The 4.7L doesn't use a cap and rotor so no wires to replace. What codes were you getting? Check the MAP sensor and clean it with electronics cleaner. Do you have a scan tool? If so what are your fuel trims looking like? Have the O2 sensors ever been swapped? They are probably due by now if not. Generally I would get a second opinion whenever a "mechanic" states its a bad computer. Especially if they aren't able to showcase a good reason for it. Its usually the hail mary that some resort too even though they are often the last culprit.

 

You're right about the wires, I totally spaced that it doesn't have wires! We replaced the coils after the spark plugs. These are what the readouts were before pcm replacement.

oct 5 jiffy lube

oct 5 napa

oct 20 autozone


My mechanic printed out the wiring system diagram showing me where they located an issue in one of the wiring harnesses but I have noooo idea if that has anything to do with anything here. Just in case it's helpful, here's that image

I don't have a scan tool, don't know about the o2 sensors ever being swapped, still have my old PCM just in case...

I tried to drive it yesterday, my buddy told me since there was seafoam added to the tank I should drive the tank all the way empty but before I did that, I found and followed instructions online to re-learn the Throttle Position Sensor (key in on position, hold throttle for 15 sec until engine light blinked) , but it still drives super chuggy and NOW the check engine light is on solid. Took it to Autozone, code said the cat. convertor is bad. I have nearly a full tank so driving it to empty will take quite a bit of time, not sure if that's even advisable with the chugging/compression/fuel issue?

 

So starting from the top. Did you ever get that shift solenoid fixed? If not get it done. The evap is a minor code and should be worked on last. That wiring diagram is just looking at the power feed to the PCM. It looks like they found no power coming from the PCM to the starter relay. Probably a bad wire, relay, or connector on the relay/PCM. I would hinge my bets the PCM itself was fine but had some corrosion in the connector. That can sometimes be cleaned if its not bad enough. Regardless that shouldn't be relevant to the chug now. You mentioned they added seafoam to the tank. Do you know how many cans they added? Might have added far too much. The way the system tells the condition of your cat is pretty basic. It takes the reading from the upstream O2 sensor and compares it to the downstream O2. If there isn't enough of a change then you could have a bad cat or sensors that aren't reading correctly. Since you did spark plugs and coils its possible that one of the new ones was bad out of the box. Listen around the engine see if you hear a tapping or zapping noise. If you hear the noise start pulling 1 coil at a time till it stops. Inspect them for any burns or signs of arcing.