It isn't a truck, but it is OBD II related, and 4.6 related; This is the place to go for the best help!
1995 Thunderbird, 4.6
I bought a scan tool, one that gives live monitoring. Engine runs great for about 4 minutes, scan tool showing no codes, none stored either. Pulled PCM out to where I can see it, start car and wait: PCM gets hot, then the car goes to fast idle, runs really rich, and fan kicks in. No pwer when you step on the pedal, just bogs down. I thought it was the PCM, but I replaced it today with a new one, and it does the exact same thing. What does the scan tool say when this happens? It instantly says 'operating error' as soon as the car acts up, no mode will work. What the heck is this? A bad ground wire? I've been messing with this thing for a week now, in every spare moment I have. I have a new fuel pump, filter, and regulator. I have a new IAC motor. I cleaned MAF meter... What gives?

 

Could be a bad pcm ground. See what the actual coolant temp is reading when it goes into limp mode. I'm wondering if it thinks the engine is overheating and going into cooldown mode.
I also had a towncar that had a bent pin on the pcm that made it run real rich and ran real bad after a few minutes.

 

Hmmm, good question... With the scan tool on, before an event, it would get as high as 186 degrees before it would start acting up... Another time, I watched it get 210 and stay there as I tried to record an event, but then it didn't happen at all after 3 attempts to record (each one lasting about 2 minutes).
In limp mode, would it still cause an operating error on the scan tool?

 

I'm not totally sure. Have you checked alt output to make sure it is around 14V?

I'm just grasping at straws here, just ruling things out right now.

 

Haven't checked lately, but the alternator is only about 9 months old, and battery is two months old. Cranks up very fast, anyway...

 

Just a thought that maybe voltage was dropping or something.
Might have to bite the bullet and take it to a Ford dealer and pay an hours labor for them to scan it with the Ford tool to see if they can find anything before throwing any more parts at it.

 

I am really hoping I don't have to do that... Someone just told me the heated PCM is a result of a shorted component in the engine management system, so I will try to unplug sensors one at a time while the engine is still running good... Not sure if Ford's scanner will pick up what mine doesn't... I paid almost $200 for mine, though I know Ford's is likely thousands$$$...

 

That's a good idea. You might start with the injectors. Could have a winding in one the is getting hot and shorting out. Then I would move on to the coil packs if you can get a cheap good one to swap out with.

 

I tried each injector, before and after the event. They all stay at 17 ohms. The resistance on each injector wire (unconnected from injector) is between 1767-1800, not much variation... I probe red to red, and black to whatever color the other injector wire is.

Also, what happens to your 4.6 when you unplug the MAF meter? Mine when running normally at idle, just picks up a few rpm, but that is it. Shouldn't it run real crappy? Disconnecting TPS has no result. I did this before an event (limp mode, I guess?) occurred. All I have to do is let the engine run a few minutes to cause the event.

 

Well, The one that I had idles better with the MAF unplugged until I replaced the PCM.
Does it still go into limp mode with the MAF unplugged?

Try unhooking the temp sensor. Maybe??

 

I tried unplugging temp sensor, MAF, and TPS, one by one when the engine was running good, and it still went into limp mode eventually (not immediately). It just seems a matter of time before the things starts acting up. When you unplug temp sensor, the scan tool reads -40 degrees, so I know it is registering right, since plugging it back in registers engine temp around 185-190. When you unplug the MAF, it gives you a constant reading of 2.1. Plugged in ranges from 5.5 to 5.9 GR/SE

Frustrating as HELL, I tell ya! I am a person who HATES to go to a garage or dealership when I can do all I need to myself, and this is one of those things I may have to fight for a long time, as inconvenient as it is for me.

OK, update: O2 sensors 12 and 22 were monitored for several times, and their voltages range from .065 volts to .09 volts... Aren't they supposed to be between .1 and .9 volts? The sensors 11 and 21 are fluctuating within that range... I thought a low voltage like that would trigger a code? Maybe not on an early OBD II system like the '95 Thunderbird?
I also can't find info on which position these sensors are in... I am thinking they are both on the same side, front and rear, but I am not sure... Anyone know this?

 

I think 11 is passenger side front and 21 is drivers side front.
12 is passenger side after cat and 22 is drivers side after cat.
The after cats should vary a little, the front ones will be more up and down.

 

I'm thinking that the PCM heating up has something to do with it. It seems like something is drawing too much current. More thoughts when I get done with work....

 

I would start by looking at the outputs of the PCM which is what would cause the heat. I see that you tried checking the fuel injectors with a meter but I would see if you can get your hands on another set of them to at least test with. Also check the IAC which you can probably disconnect and see if the ECU still gets hot. Also check the various emissions solenoids and whatnot. Just a couple places to look.

 

I went to a 37 year Ford tech who is supposedly the best around these parts. Well, first of all, he destroys my kick panel, breaking it in two directions! (I should have left it off when I removed it the last time). Then he hooks up to his Snap=on scanner and tells me the puter goes offline, gurantee the PCM is fried. Even though I replaced it once with a reman, he figures I got a bad one, and recommended i get one from another store. I did that, at twice the money of the last one, and NO RETURN policy. ($200)
Plug it in, let it run for 30 minutes (the longest it's ever run since the problem started), and it goes into survival mode again. Back to square one. I am freakin' pissed!
I am thinking now about putting new O2 sensors in, just the 11 and 21 main sensors. When the trans place first got my car, they told me there was a code for a bad O2 sensor but their tool would not tell them which one. They cleared that code because I have yet to get a code. Could it be a shorted O2 sensor, either the sensor lead or the heater circuit? That is about the only thing I can see to try that is something I can afford to replace, since it is good to have fresh O2's after so many miles. (160,000) And, it is the only code ever seen since before trouble started. Thoughts?
By the way, I can't see a way of removing those sensors without dropping the exhaust from the manifolds. Anyone agree with this?