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Old 08-10-2018, 02:25 PM
JayGunn JayGunn is offline
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Join Date: Jun 2018
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Default odd sensor readings for 2.3l with P0172

I foolishly bought a 2002 Ranger 2.3l in part because I thought it had no check engine light. Turned out that 8 bulbs in the instrument cluster were burned out including that one. Dumb purchase on many levels. But I'm trying to bail myself out by getting it to pass inspection.

Fixed the P0125 in no time--thermostat was stuck open. I used the trick on this forum of putting in a 2004 Ranger thermostat and adding a 1.2k ohm resistor to the heater wire connector to prevent a PCM problem. Works great.

The other code is the P0172 System Too Rich. I have read every post about it here. MANY possible causes.

First thing I did was clean, then replace the MAP sensor because its reading didn't change above 17" vacuum. Pump it to 25", no increase. Replaced with one from Rock Auto. Also verified good readings from the TPS. Replaced the air filter. Cleaned the MAF. Cleaned the IAC and put on a new gasket. Also cleaned the throttle body and put on a new gasket. IOW I did the easy stuff. P0172 returned in 20 miles.

Then I took it to a trusted mechanic who told me: "You need a new engine. You should get rid of the truck, maybe trade it in somewhere. How much did you pay for it? THAT much?!?" (I told you it was a foolish purchase--he agrees). His idea is that rings are old and leaky (184K), and the excess blowby confuses the PCM because it is 'air' entering the intake that is not metered by the MAF, so it screws up the mixture. Too much blowby, he says, is causing the System Too Rich condition. I don't get it. Shouldn't it be causing a Too Lean condition?

By the way, this mechanic coached me that if I plug the PCM entry to the intake manifold and remove that tube from the PCV, then install a breather cap on the valve cover I probably will be able to pass inspection. Odd because if he 'notices' the cap he'll have to fail me, and he's an upright but sympathetic guy. After doing that improper mod, including plugging the hole in the intake downstream of the MAF where the tube from the valve cover used to fit in, the trucks runs about the same, but the fuel trim numbers look a LOT more normal (they were very negative). Still, P0172 returned. Incidentally, off idle the truck is intermittently very hesitant, maybe a bit less so when fully warmed up, but it always smooths out and has decent pep as soon as it hits 2000 rpm.

Finally I bought an ELM327 wifi sender and checked my sensors on FORScan Lite. (What a fantastic thing this is, to watch your sensors on your phone!) So this post is my request for advice about what the sensors are telling me.

--Coolant temp and Cylinder head temp look normal, somewhat above 200F. I had put in a 194deg thermostat.

--TPS looks fine, runs from about 0.8V closed to around 4.0V nearly floored.

--Short term fuel trim once I hit 2000 rpm and above fluctuates between -5 and +5, averages near 0, maybe slightly negative. At idle it runs from -6 to +4, lots of gradual fluctuations, no sudden jumps.

--Long term fuel trim was quite negative until I did the breather cap trick, now also runs between 0 and 4 when running. Once in a while briefly goes negative. After running for a 10 mile drive I let it idle, and LTFT runs from +3 to +4 with a little fluctuating out of that range.

--MAP sensor runs from about 0.8V to around 4V.

--IAC looks odd to me. It runs in the 40-55% range all the time, even above idle. At 2500 rpm it is still open around 50%. Is this odd?

--MAF is the other one which looks very odd to me. Going down hill with the throttle closed it drops to 0.32 lb/min. At idle it runs about 0.36 to 0.42, and floored at 3000 rpm it jumps up much higher, maybe 4? 5? I didn't write it down. But the point is that my virtual meter in FORScan Lite has a scale from 0.00 to 0.14 lb/min. So even at idle I am way off the scale. I'd like to know whether those lb/min values are way high (bad MAF) or if the scale is wrong in the software.

--Finally looked at the heated O2 sensor of bank 1. Can't figure out how to interpret it. Its value of course fluctuates. Maybe I need to monitor some other aspect of the sensor(?)

Thanks in advance for looking at this. Eager for suggestions.

Chapel Hill NC
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Old 08-17-2018, 09:28 PM
KenHigg KenHigg is offline
Ranger Driver
Join Date: Nov 2014
Posts: 376
Default Re: odd sensor readings for 2.3l with P0172

This is a lot to address in a single post. I suggest you focus first on understanding the fuel trims. Then how the o2 sensors play into the equation.

In theory they is a perfect ratio of air to fuel, something like 14.7/1. However, while this may be great for an engine idling with no load, if the engine is under load at higher rpm it needs more fuel in the mixture. In the pcm these mixture values are permanently saved and when the other engine sensors report that the engine load/rpm conditions change the fuel/air mixtures change to what the saved values dictate. In a idealistic scenario both the short term and long fuel trims would remain at 0. Looking at long term trim first - as the engine wears, it may start to sensing more or less air than is actually making it to the cylinder and the factory mixture settings need to be tweaked. So if the o2 sensors report the engine is running rich the pcm will take away say 1% of the recommended fuel. Then it stores or remembers that instruction until conditions dictate other fuel mixture adjustments. When all of this goes as planned and adjustments are reasonable the system is in closed loop. If the adjusted fuel mixture gets out to an extreme, 25-35%, the computer leaves closed loop and goes into open loop operation and a default ratio is used. This is usually when a trouble code shows up.

If this is useful I can continue, if not I understand and won't waste my time
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Old 08-18-2018, 05:22 AM
tomw0 tomw0 is offline
Ford Ranger Driver
Join Date: Aug 2015
Posts: 903
Default Re: odd sensor readings for 2.3l with P0172

from a search:

P0172 System Too Rich (Bank 1)
Technical Description

System Too Rich (Bank 1)
What does that mean?

This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. Although generic, the specific repair steps may vary depending on make/model.

Basically this means that an oxygen sensor in bank 1 detected a rich condition (too little oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1.

Note: This DTC is very similar to P0175, and in fact your vehicle may show both codes at the same time.

You will more than likely not notice any drivability problems, although there may be symptoms such as a misfire.

A code P0172 may mean that one or more of the following has happened:

* The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
* There could be a vacuum leak.
* There could be a fuel pressure or delivery problem

Possible Solutions

Possible solutions include:

* Inspect all vacuum and PCV hoses, replace if necessary
* Cleanthe MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
* Inspect fuel lines for cracks, leaks, or pinches
* Check the fuel pressure at the fuel rail
* Check the fuel injectors, they may be dirty. Use fuel injector cleaner or get them professionally cleaned/replaced.
* Check for an exhaust leak before the first oxygen sensor (this is unlikely to cause the problem, but it is possible)

web site:

If the O2 sensor was dead/dying, it would produce low voltage, indicating a lean condition. Instead it is indicating voltage, so it must be working.
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iac, ltft, maf, p0172, stft

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