Code 172 CM & KOER

[SenorNoob]

I've been getting this code at first intermittently now constant (since the swap) as soon as the O2 sensor kicks in. Even when the check engine light is on I can connect a voltmeter to the O2 sensor and it reads a fairly steady 0.8. When first started it's at about 0.05 then starts climbing immediately.

My biggest question is how can there be a LEAN code when the O2 sensor is reading rich?

I did have the O2 wiring wrong (signal and ground crossed) when I first swapped the engine in. Is it likely that I managed to burn out the ECU?
And if the ECU is burned why I am I not getting a code for system at adaptive limits. Doesn't that imply that the computer can adjust and cause the O2 to start switching?

I've tried to read the plugs and can't make anything out of them.
Can't find a vacuum leak anywhere. (Cause one and the O2 reads 0.01)
Cleaned the MAF.
New Motorcraft O2 sensor.


Other useful info:
1992 Explorer ECU
1991 Explorer Harness (From automatic)
1994 Ranger Engine (And original orange injectors)

Sorry for the LONG post.

 

[monsterchuck]

I had the exact same issue on a 3.0 taurus. O2 read rich but kept throwing lean codes. PCM took care of it. If you had ground on your signal wire, I can see that causing problems.

 

[SenorNoob]

I connected my voltmeter to the sensor being careful with the direction of the wiring and the signal is entirely negative!

So I found this:
---------------------------------------
Characteristic Shift Down
An oxygen sensor works by comparing exhaust residual oxygen content to an ambient air reference. This reference is obtained through the O2 sensor wires. If the sensor ceramic is cracked, and fuel and exhaust get through the crack, the reference side of the sensor will get contaminated. The O2 will then start working in reverse. The reference side will have less oxygen than the exhaust side, therefore, the voltage will be generated in the reverse direction. This can be observed at the sensor as a negative voltage. The oxygen sensor may continue to switch as the average voltage approaches zero and continues negative. This condition may clear up on it’s own, but if the ceramic is cracked, the Characteristic Shift Down (CSD) condition will probably return. CSD is more prevalent during the winter months since a lot more fuel used for cold-starts in cold ambient temperatures.

http://www.fordforumsonline.com/forum/engine-drivetrain/3578-air-fuel-ratio-control.html

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I've tried 4 different Motorcraft sensors and all go negative almost immediately after starting it. (The one that came with engine, the new one I bought when I had the wiring wrong, and 2 pulled from a 93 explorer that supposedly ran good.) The response from all 4 seems right when a vacuum leak is caused except for being negative. IE: the voltage goes to about -0.01.

Voltage has been checked at the connector for the engine harness to the main harness and at the ECU connector itself. Same results.

I have double checked the wiring to sensor. Almost seems like it's backwards but I know that's not the case..

The truck runs fine once it's warmed up but is rough and stalls easily when first taking off in the morning. One morning it stalled and seemed to blow raw gas out the exhaust when it fired back up after holding the pedal to the floor. (I was looking through the passenger side mirror.)

"IF" the lean code is ignored it seems like it runs rich first thing in the morning and then the computer corrects and it runs fine.

Might be normal but after running I find that everything in the intake behind the throttle plate is wet. Enough to wipe it with your finger and have it come out wet. I would think that with the injectors sitting below the upper would mean that it'd stay dry.

Mostly thinking out loud here. Sorry for another LONG post. Maybe if nothing else somebody else can find something useful here later.

 

[SenorNoob]

After too many hours of scratching my head and googling, I saw that there is now way for an o2 sensor to read like that so I thought Why not swap the wires back to the way it was? What could it hurt? That would seem to yield normal results. So I swapped the signal and signal return wires on the o2 sensor and now it seems to be working. I won't know more until I drive a longer distance. With it like this the wiring from the o2 to the computer is crossed but I can see it switching now almost immediately after starting...

 

[kimcrwbr1]

Have you checked your fuel pressure regulator. If the diaphram has a hole in it it will suck fuel direct into the intake and if the sensor gets wet with raw fuel it will give a false lean code also. If you smell fuel in the vacuum line to the fuel pressure regulator the regulator is bad. Just something to look at it can also flood the brake booster with raw fuel makeing it go bad.

 

[SenorNoob]

Thanks for the advice. I had the fuel pressure tested before I started this. Also checked for vacuum leaks using Walmart special version of PB Blaster.
I even went out and bought the "correct" (90tf-aa) injectors for my ECU and the rebuild kits for all 6. I just wanted to try everything else first.

I don't know why it works with wiring backwards but it does so far. I'm gonna watch it carefully tomorrow on the way to work and back. I will update when I get home tomorrow evening.

 

[FordRanger1991]

Ya seems like you got some wired things going on...
The swap would have been too easy had everything workout haha
But ya my thoughts would have been the "correct" injectors also but my truck has them with my swap and I am getting that same lean code!! My problem just deals with high idle intermittently... I have checked every sensor 8 times I am thinking I might try a new comp as a last result... The local jk yard has one for $65 I might get it and try it out.
Keep the updates coming!

 

[SenorNoob]

I had high idle with mine all of the time till I swapped the wiring around. It was high because it was running rich. Although my tach I don't think is accurate truck now idles at 1000 on the tach but sounds like any lower and it would die. It was idling near 2000.

I really should be asleep now. Gotta be at work in 8 hours, but The Ranger Station is addictive.

 

[kimcrwbr1]

The best way I have found to locate a vacuum leak is with a piece of vacuum tubing. Put one end in your ear and search around with the other end. The closer you get the louder the hissing will be. Be careful of moving parts. Did you sniff the vacuum tubing for fuel? My schematics only go up to 1990 for the 4.0 have you checked the pins on the computer to the O2 sensor? My book reads from the senser
orange/black stripe key on hot
black/ltgreen stripe ground
dkgreen/purple stripe pin 29 on ECM
orange is spliced with the black/white stripe pins 46 and 49 on ECM
It takes alot of time but it never hurts do do a complete pin out on the computer and check for shorts. If everything checks out maybe a bone yard ECM. I wouldnt pay 75 bucks for a used when you can get a good one for a little more. If you get the same code with a bone yard computer then you likely have two good computers. Odds are very slim two computers have the same faults. Just in case the ECM is transmission specific.
You definately want that code to go away how is your fuel mileage?

 

[kimcrwbr1]

Might be normal but after running I find that everything in the intake behind the throttle plate is wet. Enough to wipe it with your finger and have it come out wet. I would think that with the injectors sitting below the upper would mean that it'd stay dry.

When you said this it made me think of the fuel pressure regulator. The regulator may still be working ok but a small pinhole in the diaphram will suck fuel directly into plenum and when you turn the engine off the brake booster will suck in fuel vapors in eventually destroying the diaphram. By reversing the wires on the sensor its just telling the computer its running rich so if it gives you a rich code then your problem is fuel related pointing back at the regulator diaphram or injectors. Fuel in the plenum tells me its the regulator diaphram. What year is the 4.0 out of?

 

[SenorNoob]

No more code! Maybe just my source for the wiring colors on the o2 sensor itself was wrong. I had the 2 whites as heater, black as signal ground, and gray as signal. With gray as signal ground it works correctly. I actually did the sniff test and couldn't smell anything.

Thanks to all for your help.

Good luck with your high idle, FordRanger1991.

Edit: I can actually see the o2 switching now..

 

[FordRanger1991]

Fantastic man! Always good to hear someone fixed a problem!