mason: I'd love for it to be a vacuum leak, but none found. I continously look, though.

LTFTs = 0, STFTs = varied from about 4 to 45, no freeze frames stored, no exhaust mods. I went to pull the harnesses and do some electrical checks, and look what I found on the driver side pre-cat O2 harness:


The outer sheath is worn completely through. I'll have to pull the sensor off and investigate if there are any shorts, but I'd say it is probably about time to retire it. I doubt this is my main problem, but possibly the reason for my low O2 values.

I tried to get to the passenger side pre-cat O2, but the electrical connector is very difficult to access. Any hints on how to get to it?

 

Tonight's discoveries:
Suspect O2 sensor harness only damaged on outer sheath, inner wires intact. Sensor bench tested sensor good - applied heat and sensor returned values within switching range.
Tested wiring between O2 sensors and PCM connector. No open circuits or shorts to ground. Found continuity from sensor harnesses to PCM on all four connectors.

So if the O2 sensors return the correct values and there are no wiring problems, but the PCM doesn't report those values correctly while monitoring with my scanner...I think I've isolated the problem.

I do have a question that I haven't been able to find a definitive answer to through my searches. If I replace the PCM, will it work with the PATS key system without a visit to a dealership?

 

I'd like to know one more thing before you pull the trigger on the PCM. Do you have the reference voltage at the 02 sensors coming from the PCM at KOEO? The PCM/ECM is always the LAST call to make. You also need to make one more VERY IMPORTANT check BEFORE you install the new PCM. What smoked the PCM in the first place? It's usually an output device(outputs operate at higher current than inputs) that fails sending the current too high which lets the smoke out of the PCM. >1.2 amps for 6mSec is enough to smoke your PCM. You have to check all the ouput devices for resistance to make sure they are in spec BEFORE you plug in the new PCM so they don't over current your new PCM and take it out too. <70 ohms is suspect(current will go up if resistance is too low). You can compare your 02s to each other or to a know good 02. The other stuff will have to be checked against a spec you get from Alldata, Mitchell, or Helm.

 

Of the four leads going to the O2 sensors (B+, HTR GND, sig, sig rtn), across which two do I measure reference voltage?

I will double-check all the outputs, I know I've checked several. I think what smoked it was a very loose clamp on the negative battery terminal. I think the battery load was lost while driving, and the alternator fried the PCM. Just a theory, of course, but I will be replacing the clamp.

 

You've got two circuits in a heated 02 sensor. The heater circuit and and sense(signal) circuit. Look at your schematic on Alldata for the color code of the wires. You can compare the values you read on one sensor to the other one.

 

I'm familiar with the two circuits:

1. The B+ is the 12V heater supply that connects to the battery voltage through the PCM relay. The HTR GND connects directly the PCM - the PCM grounds that circuit to allow current flow through the heater.

2. The sig lead connects directly to the PCM. The sig rtn splices with the return of other sensors and connects to the PCM. The PCM measures a .1V to .9V difference across the leads to determine the oxygen content.

What I'm not clear on is the reference voltage you're asking about. I know some sensors have a 5V reference voltage on a 3rd lead, such as the TPS. But on the HO2S, are you wanting me to measure the 12V across the heater circuit? Or do you want me to measure the voltage across the signal leads in the wire harness?

 

 

I found a vacuum leak! I discovered that the hard plastic tube that connects to the rubber PCV hose is unhooked at the back of the engine. But I can't find where it is disconnected from. I have felt all around the back of the TB/intake, looked with a mirror, and cannot find any open ports to plug it in. Upon closer inspection, a short rubber connector on the end of the plastic pipe appears to be torn? Anybody have any pictures or diagrams of where this goes? I'd even settle for a really descriptive answer...

 

These engines are famous for failures of the rubber vacuum hoses. Many have 90 degree bends which dry out and crack. This happens to any manufacturer, but Ford uses some very tight bends and you'll find cracks on the outside of the bend and dimples on the inside of the bend. Sometimes you can't see the hole in the hose at the dimple because the rubber develops folds. The holes in the rubber hose can be pinpointed by opening up the hose where the fold is located. You'll find a hole between the folds of rubber. These holes which cause lean conditions do a very good job of hiding themselves.

 

That goes to a nipple on the back of the manifold underneath the IAC. My neighbor has a '99 f250ld that i changed the IAC and dropped one of the bolts so i pulled the throttle body and that elbow pulled in half when I took it off. Had to go back to Ford for the right one. Couldn't find one with the right sizes. The motor side of it is about half the size of the tube side.

 

Mine doesn't have the 90 degree elbow like most of these trucks seem to. Mine was just a straight connector, but it is twice as large on the engine side. The small side was torn completely away from the large side. I'm pretty sure it wasn't torn all the way off earlier when I felt back there for leaks. I pulled the IAC off a couple of times - I might have knocked it apart then.
Instead of going to the dealership to get a new one, I inserted a small section of 3/8" hose into a small section of heater hose and clamped it together tightly with a hose clamp. Truck is running great, now.

 

Keep up the good work!

 

Thanks for your help, eallan! Vacuum leaks sure can cause some bizarre problems.