I've got a strange issue on my 4.6 equipped 2003.

For an extended period of time, I've had repetitive (at least 3 times) failure codes which indicated that cylinder 5 (front cylinder - drivers side) had a hard mis-fire. I've replaced the coil-over as well as the plug several times, and each time the issue goes away.

Then most recently, I had the AC stop functioning, it would simply not turn on. As I consider AC work out of my comfort zone, I took it a shop I trust. He said the bearings were going out on the compressor and it needed a new one. Installed a new unit, and all seemed fine after picking it up at the shop.

A few days later we were headed out of town and the AC cut out again. Turned around and took it to the shop, and they looked it over. Thought maybe someone hadn't tightened up a fitting and it had leaked the refrigerant. But before I got there, it started blowing cold again, so that wasn't the case, everything was fine. They pointed out a relay that could be suspect, but they didn't have one handy. While on the long drive, it again cut out and stopped cooling. Hit a repair shop, grabbed the relay and slapped it on. It started working again immediately. But then stopped again shortly after.

Since that trip, the AC will sometimes work, sometimes doesn't. When it's not cooling, the compressor is off. Now the interesting part, when the AC stops cooling, I noticed a misfire, although it didn't seem like a normal one, as the pauses you normally feel with a hard misfire "felt" slower than they should have been. But at some point, it smooths out, and the AC starts working.

If it misses long enough, then the check engine light comes on, and the code is a misfire on cylinder 5. If it runs smooth long enough (and the AC is working) then the check engine light goes out and the code clears.

I thought maybe the electrical system was the culprit and had the battery and alternator checked. Both good. Although the first place that checked the alternator (while on the vehicle) had it pass twice and fail twice. Second place checked clean, and when off the vehicle it checked good.

So now I have three theories, but before I throw money at parts or a shop I thought I would run them by the folks on the site.

Theory #1 - The trucks computer is failing, and for some reason gets into an odd state which causes it to not properly fire cylinder 5, as well as turn off the AC. Although I'm not sure what programming logic would end up with that scenario.

Theory #2 - The computer is detecting the misfire, which is being caused by a bad coil or plug (although that many repeats seems suspect) and is turning off the compressor due to the engine not running correctly. Although I don't remember it ever doing that before, but maybe those failures were in the winter, can't remember for sure.

Theory #3 - There is some strange grounding/short circuit that is impacting both of these systems. I didn't find another write up on this site that sounds similar. In that case, it was the coil wire was shorting out on an AC line. In my case, I don't believe the AC lines run near the cylinder I'm having issues with. But maybe isn't actually a different one that is shorting out the AC.

Anyway, thanks for any ideas or if anyone has run into this before. I seem to remember being told that the computers on these trucks were prone to having issues, but that could have been a different one. If it is likely the computer, does anyone know of a good source to get a used one that will program it for my VIN/mileage?

 

Ok, follow up with a bit more data.
I checked what I believe to be both the high and low pressure switches. I'm getting power to both, as well as I'm seeing power on the far side of the switch when the AC is turned on. By that I mean on the opposite side of the switch from where power is applied to it.

AC compressor still won't engage however. I tried shorting out those connectors, and when I did that I do hear something strange, along with engine revs changing slightly. But again the compressor doesn't kick on.

Went and got codes pulled to re-verify the firing issue, just in case they aren't related. Turns out it's cylinder one this time, not 5. Going to replace that coil-over and plug today and see what happens. Although, interestingly enough, after pulling the codes, the engine smoothed out and didn't hesitate any more.

Again, any ideas or other theories appreciated. I did find a couple of places to buy ECMs, but holding off on that unless someone feels it is likely the issue.

 

You're throwing a bunch of money at this, coils ain't cheap. For the price of one MC coil, you could have taken it to the Ford dealer and had a diagnostic done and likely found the issue.

 

The A/C issue could be a bad low or high pressure switch that cuts out intermittently. The only way to know is to wire a few temporary test lights in the cabin and wait to see which one goes out after the A/C stops cooling.

As to the misfire...unless I missed it you never mentioned swapping out the fuel injector with the one next to it to see if the trouble codes moves to cylinder 6. Keep in mind that when the CEL is flashing, that it is a warning that the misfire is severe enough to ruin your catalytic converters of you continue to drive it that way.

 

Doesn't the PCM disengage the AC compressor when it detects abnormal engine conditions, like severe misfires and such?`In any case, the most important issue to fix is the misfire, focus on that first and go back to the AC after the misfire is fixed.

 

I replaced the coil/plug on cylinder 1, had to make a long drive this weekend, since then and no issues with running/codes/etc. So that looks good.

Question to one of the comments, what low cost diags would the ford dealer run that would be more extensive than pulling the codes on the system?

As far as the CEL, it wasn't flashing, just lit. Did have an issue once before a few years back on my older EXP started flashing, don't remember what it was, but do remember the higher level of concern on the flashing CEL.

As to the AC, still acting similarly. Although it occasionally kicks on, it is off most of the time. I was thinking the PCM might be disengaging it, but that's never happened before with the various component failures that lit the CEL.

Also, with several hours of driving this weekend, with CEL has not come back on, no hesitation/stutter of a mis-fire, etc. Truck has been running smooth for the duration. But the AC has only been working a small amount of the trip.

 

I never said it was a low cost diagnostic but one diagnostic fee plus repair is cheaper than throwing parts at something. We've all been there and I can with experience that the diagnostic fee is about the price of one coil from the Ford dealer.

 

Fair enough, I wasn't getting the coils from ford, so there is likely some cost difference there. Maybe it's just where I live, but I've never had much success with the ford dealers with regards to abilities or costs. But then again I probably haven't tried to use them for anything in years.

So let me rephrase my question, do the dealers these days have better diag tools that can help isolate what is happening beyond what I can do by pulling codes and digging into it myself? Or do they have better tools than taking it to a local independent repair shop I trust to try and troubleshoot the AC issue at this point?

My theory's are evolving on this truck as we go. I haven't had these types of issues before, and on all the previous fords I've owned I could handle most everything myself, and if I couldn't I could take it to an independent guy I use.

If the computers are giving the edge to the dealers now such that they can more reliably detect and fix the issue than doing it the way I have always diag'ed things myself, then maybe it's time to adjust what I do.

But if I'm taking it to a deal, and they are just pulling codes and walking a manual's recommendations, then I don't see the point in paying them the typical dealer premium for it. Of course, maybe that doesn't exist in the same way any more either.

Thanks for the input guys.

 

I also had a question about the test lights. When I was checking the low/high pressure switches via a multi-meter the other day, I was seeing the appropriate voltages output from those without the compressor engaged.

Can you elaborate on how wiring up test lights to monitor those could help beyond that? I would think that if I was detecting the correct output and the compressor still wasn't engaged, then I have effectively ruled them out. Is that not correct?

 

I think it's important to know that the computers t hat the dealers have likely cost in the 10's of thousands of dollars and are capable of diagnosing transmission issues, brake issues, evap systems, fuel systems as well as engine issues. Diagnostic fees are high and I hate them too but they are very likely to pin point an issue that a hand held scanner may not be able to detect.

I think the hand held scanners are fine for finding a definitive issue such as a bad sensor or a bad coil. My hand held from Auto Zone has actually led me astray a time or two and I have little trust in it.

 

You can always buy your own dealer-level scan tool, either a knockoff from China or the real thing, but you'll be spending a couple hundred bucks for it. A good middle way is an OBDLink MX which will run you about 100 bucks with FORScan, it gives you a lot but not quite dealer-level.

 

When they pull codes they get the same codes as you do with a scan tool. They do however have additional tools, such as video scopes to view spikes and irregularities that the average home mechanic doesn't have due to the cost of these tools. But in the end it always seem to come down to how good the mechanic is at diagnosing.

 

Dealers can do deeper diagnostics than a generic scantool, but it's not a magical device to cure all diseases. hehe. I have a chinese knockoff and it has been very helpful to have, but I find myself using the OBDLink MX together with FORScan more as it gives the most useful things already. Only when you start going into doing power balance tests or reflashing, you'll need the dealer tool. Even programming PATS keys can be done with the FORScan software, albeit only on the Windows version, not the mobile app.

 

Thanks for all the input guys. Looks like it might be worth a trip to a dealer to at least get an alternative look at this one.