CEL - Code 172

[$1Explorer]

Hello All!

My 1993 Sport is giving a code 172 in continuous and KOER - a 111 in KOEO. After reading the threads in search, I have:

Cleaned the MAF (really dirty)
Cleaned the throttle body (i know the decal says not to, but the bottom tube was so filthy it wouldn't pass air)
Cleaned the IAC
Checked the vac lines
Cleaned the PCV valve
Replaced the Fuel Pressure Regulator.

It runs and idles a whole lot better now, but I can't get rid of the 172. Any ideas? I'm down to a month to get it to pass smog.

Cheers!

John

 

[MrShorty]

By the book, the first test for an O2 sensor code is a fuel pressure test: make sure the fuel pressure isn't too low.

Have you checked the wiring to the O2 sensor? Not pinched between the tranny/engine or otherwise damaged?

Some people think of the O2 sensor as a maintenance item. How many miles on the sensor? To some, especially when there is only one O2 sensor being flagged and that sensor has a lot of miles on it, they like to replace the sensor, and then diagnose further if that doesn't fix it.

172 is for the O2 sensor indicating lean. From the way the engine runs, condition of the spark plugs, and exhaust, would you say the engine is running rich or running lean?

 

[cjones101812]

It runs and idles a whole lot better now, but I can't get rid of the 172. Any ideas? I'm down to a month to get it to pass smog.

Cheers!

John

Did you clear the code after doing the repairs??

also, with just 172 (lean bank 1) check for intake leaks, vacuum leaks, plugs, wires, etc. on that side. bank 1 is where the # 1 cyl is so passenger side.

 

[$1Explorer]

OK, I've checked the fuel pressure. With the key on, it reads 38-40 psi. After engine starts, it drops to 30 and stays exactly there, whether you rev the engine or not.

When it cools off, I'll check plugs and the O2 wiring.

A little background: I just got the car. I changed the transmission in it back in September for the girl who owned it (she bought the trans and I donated the labor). It had a completely blown out muffler which I convinced her needed replacing, but I think it took her a while to get around to it.

The 172 code refers to the right/rear O2 sensor: I think that would be the one closest to the blown-apart muffler. Would being that close to outside air cause it to fail and not the left/front one? Or am I way over my head in the guessing department?

Thanks for the reply, MrShorty. I'll report back after I've done everything you've suggested.

Cheers!

John

 

[$1Explorer]

Did you clear the code after doing the repairs??

No, the 172 is in the continuous memory, and also in the Key On Engine Running test. When you drive the car, there is no CEL for the first couple minutes, then it comes on and stays on until you shut off the car.

Cheers!

John

 

[cjones101812]

an exhaust leak at the manifold or where the pipe connects to the manifold could cause a problem but i've never seen a blown muffler cause an O2 problem. especially when there is a CAT between them.

 

[$1Explorer]

an exhaust leak at the manifold or where the pipe connects to the manifold could cause a problem but i've never seen a blown muffler cause an O2 problem. especially when there is a CAT between them.

OK, that makes sense. Back to the testing...

Cheers!

John

 

[$1Explorer]

I've visually checked the exhaust system looking for leaks; it looks pretty good (although even with a lift you can't see the passenger manifold very well.) I pulled the middle cylinder spark plug on the passenger side and it looks great, with nice tannish-white deposits on the tip.

The O2 sensors both are probably original, as they still have the ford numbers on them (and they also say BOSCH). I think I'll replace the right one and give it another try...

Cheers!

John

 

[$1Explorer]

OK, I replaced the right O2 sensor with a new Bosch one, cleared the codes, and took it out for a ride...

Still CEL 172.

It doesn't matter if the engine is cold or hot when you start it, every time you drive about two minutes the CEL comes on. If you turn it off even for just a couple seconds and then start it again, the CEL goes out for about two minutes before it comes back on.

Let's see, what else:

You asked if it was running rich or lean. There is a considerable amount of water vapor at the tail pipe, but no hydrocarbon smell or oil in the exhaust. Lots of power and no pinging. It seems to me to be running great, but the CEL sure doesn't agree with me!

Any more ideas?

Cheers!

John

 

[MrShorty]

Have you checked the wiring to the O2 sensor? Not pinched between the tranny/engine or otherwise damaged?

Fuel pressure holds a steady 30 psi, even when revving the engine? One would expect the pressure to jump up when the throttle is opened and engine vacuum decreases. What does the pressure do if you remove and plug the vacuum line to the regulator? 30 psi isn't necessarily too low, but if the pump can't increase the pressure when the vacuum decreases, it does suggest the possibility that the fuel system isn't quite keeping up with the engine.

 

[$1Explorer]

Fuel pressure holds a steady 30 psi, even when revving the engine? One would expect the pressure to jump up when the throttle is opened and engine vacuum decreases. What does the pressure do if you remove and plug the vacuum line to the regulator? 30 psi isn't necessarily too low, but if the pump can't increase the pressure when the vacuum decreases, it does suggest the possibility that the fuel system isn't quite keeping up with the engine.

The wiring to the O2 sensor is free and not pinched. I also checked the entire perimeter of the bellhousing/block junction and nothing is trapped between them.

The pump puts out 38-40 psi whenever active with no vacuum to the regulator (either KOEO or running with vac disconnected/plugged). It tries to put out exactly 30 psi when running, although if you blip the throttle you can get it to spike a little. But steady state at idle is 30 psi, and steady state at 2500 is 30 psi. That seems to me to be what a regulated system should do (unless the spec is some other number than 30!)

Maybe I should take off the upper manifold and see if there's anything that needs to be cleaned? I guess something could be hindering flow to one bank? (Depending on where the injectors are, maybe). Yep, I'm grasping at straws...

Cheers!

John

 

[$1Explorer]

Ok, I think I have a brainstorm (or more likely, a brain dribble):

While looking at the upper intake, it occurred to me that as ugly as it looks inside, it and everything connected to it (EGR, IAC, etc.) would affect both banks, not just one of them.

So I started thinking, what would affect just the right bank, causing a lean condition? Maybe a clogged injector! I've been running Techron fuel cleaner since I got the Explorer, but that hasn't even been a full tank of gas yet. I think my code reader will initiate the cylinder balance test; hopefully that will point out one or more cylinders on the right bank not putting out.

Wish me luck...

Cheers!

John

 

[cjones101812]

you checked the middle plug but how about the other two?? it only takes one to cause a problem.

you are correct, 172 indicates a problem with the right bank. as i said earlier, check for things that affect that bank, exhaust, plugs, plug wires, injectors, intake leaks (on that side), etc.

 

[$1Explorer]

you checked the middle plug but how about the other two?? it only takes one to cause a problem.

you are correct, 172 indicates a problem with the right bank. as i said earlier, check for things that affect that bank, exhaust, plugs, plug wires, injectors, intake leaks (on that side), etc.

While I was putting it back together to do the cylinder balance test, I read your post and figured checking out the other plugs was a good thing. After I discovered that removing the upper radiator hose makes getting at plugs much easier, and that the only way I can get the rear boot off is through the wheel well, the front plug (#1) looks as good as number 2, and the worst is #3.

Here is a pic:

This was the worst of the bunch, yet it still looks pretty good to me. The gap is about .055", and the color is tan and white.

I'm starting to think that maybe it's just running a little lean, and that getting the injectors clean might do it. Off to the balance test, I guess.

Cheers!

John

 

[$1Explorer]

Well, it passed the balance test, but now it idles too high; I get a 411 along with my 172. I guess I didn't hook up a vac line or something when I took off the throttle body the last time...

Back to the drawing board.

Cheers!

John

 

[$1Explorer]

The solution

I re-examined the removed passenger side O2 sensor, and saw that the wires had been crimped at some point (probably when the clutch was replaced by the previous owner). Apparently while crimped it shorted out the corresponding circuit in the computer; replacing the O2 sensor didn't fix the faulty circuit. Replaced the computer, and it passed smog with flying colors.

Hope this helps someone down the road, and that it doesn't take them 4 months to figure it out.

Cheers!