2006 4L Mountie, repaired PCM arriving Monday, need advice

[marvelousbison]

Hello All! Researching this forum has been incredibly helpful for ferreting out my Mountie's issue, I just wish I had found it sooner.
Truck was taken to the shop about 2mo ago for rough idle and stalling at idle, tended to only do it on warm days, garage sold me on a new timing chain and cassette for $4.5k (i love the truck), didn't resolve the issue. After taking it back to the shop multiple times for continued rough idle, stalling, things they forgot to do, and another $300bill, I gave up and started researching and trying to diagnose myself.

At this point codes were P0135, P0141, P0155, P0161, P0403, P0443, all 4 O2 sensors failing is what caught my attention and lead me here, and lead me to believe the PCM was at fault. Sent it to one of those rebuilders on Ebay, tech notes say "unit failed for heaters and egr. Replaced control chip and reflowed transmission chip. Unit tested good." They are suggesting I verify all components on fuse 42 have proper resistance and/or amp draw and verify wiring is free of shorts. This last sentence is my issue. I am an inexperienced DIYer, out of money, and without a trusted shop or mechanic.

I tried testing for shorts in the wiring with a multimeter at the battery, but am unsure if the 0 reading is accurate without the PCM installed?

I've visually checked all the wiring I can find above and below the engine bay and back to the O2 sensors, could not find any bare wires or groundings.

I guess my main question is, why do these PCMs tend to fail? Is it likely I have some fault in wiring somewhere, or a bad part solenoid that caused a power surge to the PCM to damage it? Or are these failures aged PCM based and I'm likely safe just plugging it back in and rolling when it gets here?

Please forgive and correct anything painfully naive I've said, as I mentioned I'm pretty inexperienced but trying to learn.

 

[marvelousbison]

Today I found three "ghost in my machine" type faults in wiring to the intake manifold, throttle body, and I think radiator fan. Thanks again to this forum for the tip of pulling the sheathing back to check for faults, id only checked bare wires next to connections and where the wires were near heat before.Radio capacitor wasn't grounded, hard to think that might matter though.

 

[J_C]

Okay so your codes are for the o2 sensors, EGR valve and EVAP purge valve.

You might have just had a vac leak or stuck purge valve and nothing wrong with your PCM despite the service stating they found something, when they could have just read off those codes you listed and used that to make a case for repair validity.

I would hook up a scan tool capable of live data and look at the long term fuel trim. PCMs don't usually fail, they can be are more often pointed to when a technician can't find a fault elsewhere. I've heard of technicians trying to sell a PCM replacement service then sneakily fixing the real problem. It even happened to me, but for a driver's door wire for the window or lock of all things. They wanted to bill for a PCM and just ended up soldering a wire back together.

I don't understand your checking for shorts at the battery. The way to check for shorts to the PCM is you get the pinout for the connector, for each of the signals you wish to check. You use multimeter resistance mode between the two (or more) wires to the sensor in question to see if you get a reading expected for that type of sensor. If neither wire is a ground then you check for resistance between chassis ground and each wire. If one wire is a ground you only check resistance to ground on the other wire(s). There should be high resistance to ground except whatever the sensor may cause, or if sensor is unplugged then it's out of the equation.

You can also check resistance of the wire from the PCM connector to the respective pin on the sensor connector it goes to. Usually Ford specs the max resistance for the wire run to be under 5 ohms and even that high is a potential sign of frayed wire or connector corrosion.

While I'm thinking about it, another thing to do is unplug, inspect contacts, and replug the two bulk wire connectors on or about the firewall in the engine bay, one to the PCM and one going through to the fusebox, IIRC.

I'm feeling like this is about the limit of what I should write because it may not even be the PCM or wiring at fault, but yes you can check for shorts of everything besides the PCM itself, without the PCM installed.

I can't believe a shop had the gall to charge so much for the timing components job yet couldn't (or didn't want to?) even figure out the rough running problem. The disturbing part is that after they did all that work, they were (IMO) kind of obligated to make sure the vehicle ran correctly even if it had come in with a problem, just to verify they did the work correctly.

What is on fuse 42? Since I suspect it wasn't the PCM (but I could be wrong), I would plug it back in and on first startup, already have a scan tool connected that can display real time data.

You could really have a fault in your wiring, but it is not likely to have caused all those codes, but the poor running condition could have been detected by the PCM, especially the O2 codes. That the PCM was able to get the motor running is a sign it may have just been reading the systems of another problem, like a vac leak, possibly purge valve as mentioned previously.

The thing is, now that a shop has been in there doing timing components, there could be even more wrong. It really ought to have had the existing problem repaired before such a major job as pulling engine for timing components.

It shouldn't be something like a bad solenoid sending a surge. Their coils aren't that massive and the PCM should have a protection diode isolating it from anything significant, that is large enough that the PCM is controlling a relay to that, not through it.

I reserve the right to be wrong, as I have a certain method of steps and there are multiple unresolved variables about what has happened so far.

 

[marvelousbison]

Thank you so much for your well thought out and well written response!
It did have a vacuum leak in one of the egr valve hoses I repaired when the pcm was gone, in addition it was missing the gasket between the egr and throttle body so I replaced that.

I was checking for shorts in wiring at the battery based on this video-


The news today though is that the repaired PCM is in place, check engine light is off, and the truck seems to be running well so far. Took it for a 50mi test drive yesterday, and then 10mi drive to work today.

 

[marvelousbison]

And yea, I'm super disappointed in the repair shop, especially because they replaced the transmission in the truck about 3yrs ago and that went smoothly, so at the time I thought I'd found a good place.

 

[J_C]

Glad to know it's working again. I'd hook up a code scanner and if any codes remain, clear them and after a full driving cycle to fully warm the engine, see if any returned and check the long term fuel trim since the vac leak was present.

Although the video had minor errors, that method can be used to find certain undesirable/fault parasitic drains, not necessarily indicating shorts. For example a bad wiper motor (internal) switch can cause a parasitic drain but isn't a short. Same with a stuck relay,. faulty switch, and other potential problems.

When it showed the transmitter and receiver, that was just strange and not needed, an inferior way to do it that doesn't always work, when the same multimeter he was using previously, can measure resistance between the two wires, or voltage drop, or current flow between any two points.

However, none of what he's doing in the video, would detect a short that would damage the PCM. Such a short would be in a sensor line to it. A fault in the power feed to the PCM is not going to damage it, just make it malfunction... completely, not just generate a few codes, and this would be checked by seeing if the PCM is getting steady 12V power (while connected, a high resistance fault like a mostly frayed wire would droop under a load) and has a good ground.

 

[rmeline]

Thank you so much for your well thought out and well written response!
It did have a vacuum leak in one of the egr valve hoses I repaired when the pcm was gone, in addition it was missing the gasket between the egr and throttle body so I replaced that.

I was checking for shorts in wiring at the battery based on this video-


The news today though is that the repaired PCM is in place, check engine light is off, and the truck seems to be running well so far. Took it for a 50mi test drive yesterday, and then 10mi drive to work today.

Where did you get your PCM repaired and how is it doing?

 

[marvelousbison]

Your Part Source on ebay- Security Measure

I'm delighted to report this was a good fix with no further related issues.

 

[rmeline]

Your Part Source on ebay- Security Measure

I'm delighted to report this was a good fix with no further related issues.

Great! Can you share the name and location of the company that repaired your PCM?

 

[J_C]

Great! Can you share the name and location of the company that repaired your PCM?

Didn't you... quote it?

 

[marvelousbison]

Sorry I wasn't clear, the company is called "Your Part Source," I'm not sure where they're located but they are in the US, I found them through eBay, I put a link to their eBay account in my last comment. It's a common repair so they had an ebay listing already up to purchase the service, but I believe they say you can message them with your specific codes to see if it's a known issue that they repair. You send them your pcm, codes, and symptoms, and they repair it (if they can) and send it back to you.