I notcied you're quite knowledgable with Ford EEC systmes from your responses in the Ford truck forums. Wondered if you could help with mine. 1993 4.0 Exlporer. EEC-IV. Has R and L O2 sensors. Have been getting a left bank lean 176 code for 2-3 years now. The check engine light is driving me nuts. O2 sensors have maybe 30K on them. Just changed the plugs and wires - this got rid of most of the hesitation/missing I had. But the ever-so-slight miss still intermitently occurs.

All 3 Left side plugs all had moderate carbon build up when changed - were just 20K miles old. Right side doesn't appear to have any issues. 3 right plugs were all clean and white. No other symptoms.

I'm trying to understand what could cause the left side only to have an error and what typically drives the 176 code. Has to be something independent of the right side. Maybe it's just the left O2 sensor that is bad. But the plugs were sooty too. It's possible I guess that the bad wires allowed unburnt gasses to go by the O2 sensor which picked up the unburned oxygen and caused the PCM to give the left side more gas and ran it richer to cause the black soot. But does the PCM operate each injector independently?

I don't think I have the leaky intake manifold problem since 2 different dealers had replaced the intake gaskets with the new design while under warranty. If I did, I could see some extra oil buring in the cylinders to cause black soot. But then why the 176 lean code?

Any help is appreciated.

Thanks - JEFF

 

Where to begin. How about O2 sensor code 101:

O2 sensor is used to fine tune the mixture as calculated from the MAF data. True to its name, the O2 sensor measures the amount of oxygen in the exhaust stream (not fuel), and can output a signal between 0 and ~1 V. "Ideal" value would be about 0.45 V. O2 sensor controls a feedback loop. When the computer sees a lean signal (<0.45 V), it enrichens the mixture on that side (the computer can control individual injectors on yours. It's called sequential fuel injection). When the O2 sensor signal crosses 0.45 to where it's reading rich, it leans the mixture out. When the feedback loop is working as it should, the computer should see a signal that is constantly going back and forth from rich to lean (constantly crossing the "ideal" 0.45 V).
To set a code 176, the computer sees a lean signal (<0.45 V). It enrichens the mixture trying to get the O2 sensor to go above 0.45 V, but the O2 sensor never responds. The computer continues to enrichen the mixture until it eventually realizes that something is wrong. Once it realizes that the sensor isn't responding, the code is set, the CEL comes on, and it ignores the O2 sensor signal.
So, what we want to find out is why is the computer only seeing <0.45 V from the O2 sensor? Possible causes:
1) Electrical: bad sensor, bad wiring, bad ground, bad computer. Do you have a DMM? Backprobe the O2 sensor circuit and see what it's doing (might do this to both O2 sensor circuits so you can compare "good" and "bad"). Do you have access to a scanner? '93 appears to be the first year that the EEC-IV computer had the ability to send live data to a scanner. Would be interesting to compare the DMM output with the scanner's output.
2). Anything that would cause excess oxygen in the exhaust: Vacuum leak (you mentioned the intake manifold gasket, which is one possibility. Make sure it's tight. It tends to effect the left side, especially #5, more than the right side), exhaust leak, misfire (gas-oxygen never burn, so there's excess O2 in exhaust, remember O2 sensor only sees oxygen, not fuel), fuel delivery problem (low pressure would be expected to effect both sides, but you never know. Worth checking), bad fuel injector (fuel not getting into cylinders).
3) Other EEC-IV components: Special mention. Sometimes the computer doesn't see the fault in the other components (MAF, for example), and blames the O2 sensor. Make sure the MAF is clean and everything else is working. Can I assume the only other code was a KOEO 111 "pass"?

Did you perform the KOER test? If so, did it pass, or did you get an equivalent code in that test (136 I think)? It can be easier to diagnose from the KOER test, so be sure you include that test in your diagnosis. Be sure to warm the engine up before the KOER test. Cold O2 sensors don't work, and you'll get a potentially false KOER code.

Considering some of the things you've changed, it might also be prudent to clear the CM codes, and make sure that 176 is a "current" code, and not something left over.

 

Good points. Thanks for the O2 lesson - confirms what I just looked up on the web today. .45 V ans constant oscillation. I even got a test procedure where I can hook up my Fluke meter and see the real time voltage.

I have an Autoxray meter and have performed more KOEO/KOER/DCL capture tests than I care to rememeber. Codes all cleared multiple times. 111 on KOEO and KOER tests so systems check OK. 176 code comes out of memory after CEL comes on while driving. And it doesn't always come on. It'll come on maybe after 10 minutes of driving, stay on for 10 sec's or 5 minutes, then go off and might come back right away or not at all. Very erradic.

I'm not familiar with a DMM. Can you explain?

I did notice on at least one DCL capture test (while driving the car) that I got a "invalid" value every other or every third measurement point for the O2 sensor parameter. You know how the DCL capture grabs data every second for 20 or so seconds. The rest of the values were all overe the place from .9 to .1. Not sure why the meter, or PCM, would return an invalid reading.

Now that I've got a few miles on the new plugs I'll check them again this weekend and look for deltas between the right and left sides. But like I said, I'm still stumpted as to what affects all 3 cylinders on one side. Injector? but all 3 at once and all on the same side?

What about the EGR? It's on that side. But I thought it recirculates gas back through the entire intake so both sides would be affected. Could it be kicking something into the exhaust side?

I did notice that the O2 sensor itself had some black soot on it. The outside of it, not in the exhaust tube side. Liek there was some exhaust blow by the threads. So the side is definitely rich.

Maybe it's as simple as the previous misfires, before I changed the wires, clogged the sensor up so bad that now it can't read the exhaust right. Can these O2 sensors be cleaned?

MAF - I put a Granatelli unit on 2 years ago. I did have the dirty MAF/pinging problem and solved that but I wanted the less restrictive after market one anyway.

Also have Borla headers, flow master exhause and a K&N style airfilter.

Thanks for the help - JEFF

 

So, on top of having the trouble code, the fault isn't always there -- it's an intermittent fault. That's going to be real fun to truobleshoot.

DMM = Digital MultiMeter, sorry for not stating that earlier.

You say the O2 sensor is relatively new. I hate to bad mouth anyone, but I once bought an Echlin O2 sensor from Napa that went bad on me within a few months. Might have just been bad "luck of the draw" but it can happen.

When the computer outputs "invalid O2 sensor voltage," have you been able to have your own meter hooked up to the circuit to see what the sensor is showing?

 

That's what I mean. I have my Autoxray EZ-link meter hooked up to the truck's test port harness when I'm driving around. I first tried waiting until the CEL came on but by the time it did, the "lean" event had passed. So all the 15 or so parameters that the DCL capture test recorded looked normal. So I just started hitting the capture key at random and that is when I started seeing the O2 sensor "invalid" response. It really makes the O2 sensor suspect.

Digital multi-meter. Well, I do not have an oscilliscope if that is what you mean. I do have a digital Fluke volt/ohm meter. This is what I plan to use tomorrow to test the O2 sensor.

I just got done looking at the plugs and yes, the left side is black sooted and definitely running rich. The right side plugs look perfect, right out of the box. I changed them just last weekend. And I rechecked and the 176 lean code is still there.

I can't believe the root problem to be anything at all but a bad O2 sensor, telling the PCM things are lean and then the PCM giving more gas. The plugs always tell the real story and the damn thing is running rich, not lean. The left O2 sensor is the only single control component that can effect just the left side. This has got to be it.

Oh, by the way. I checked all the bolts for torque and the heads are fine. But the darn intake bolts to the block were about 50% loose. One wasn't even hand tight. The loosest ones were on the left side. Given the reports on these bolts lossening and the fact that the dealer last changed the intake gaskets in 1995, I don't think the bolt looseness is my problem.

Anyway - we'll find out tomorrow. I found my receipts from Napa too so maybe the O2 sensor is still under warranty.

 

That Fluke meter is a DMM. Multimeter means a meter that checks multiple functions (volt, ohm, amp are the basics. some will also do frequency and other stuff).

One thing I thought of that would be an interesting test: Switch the two O2 sensors (assuming they are the same). If the problem follows the O2 sensor (switches sides), then the problem is in the O2 sensor. If the problem stays on the same side, then it's soemthing else.

 

I checked the Fluke and sure enough is says right on the top Digital Multi-Meter - I always called it a volt/ohm meter.

Anyway, I thought about swapping the two sensors too and see if the problem moves to the right side. But getting to those connectors on top of the bellhousing is such a pain. And yes, it would have moved to the other side.

No, after more thought as my thread from last night elluded to, there can't be any other reason why all 3 cyclinders on the left bank are all running rich. The only thing that effects all three and only those 3 at the same time is the O2 sensor.

I replaced them both 29K miles and 4 years ago. Napa, Echlin parts don't you know.

I went down to Autozone this morning first thing and got a Bosch unit for $46. Came home and ran 2 pints of water through a vacuum port to clean out as much carbon as I could. Put the new sensor in and it's perfect! No codes, no CEL and the car remains in close loop operation while driving. It was in open loop all the time.

So I am very happy right now. The only thing I worry about is how much carbon have I accumulated in those combustion chambers and tops of the pistons running aroung so rich for the last 16K miles since the symptoms first started.

And a bonus to boot. I found that my Autoxray meter can monitor 20 or so PCM outputs realtime as I drive. This isn't even listed in the instruction book. The O2 output fluctuates from .9 to .1 and crosses over constantly. I have the meter set at 1/2 second sampling intervals so it's a little slow but I think the read outs prove the sensors are OK. The only drawback is that the meter only gives one O2 reading. I guess the PCM just averages the 2 sensors.

So on the failed 02 sensor, I mentioned befor that it was black sooty on the outside like there was exhaust blow by the threads. Well it was tight so that's not where the soot came from. In hindsight, I should have paid more attention to this sooty clue. Apparently, the sensor broke somehow to leak internally. The exhaust went through the sensor body and exited out the back end. There's a cap on the end and thus the exhaust was directed back onto the outside of the sensor body. So a mechanical failure.

Thanks so much for your help and ideas on this. Hopefully others out there will be able to pick up something from this experience too.