I've been gettin these codes for awhile. I cant figure out why my truck seems to think its running rich and lean at the same time. I notice that my check engine comes on right when the thermostat opens. It will stay on for around 30-45 seconds then it will go off. All the sensors have been replaced along with all the vacum lines, EGR valve soleniod, IAC, fuel pump, fuel pressure regulator and fuel filter. The EGR valve hasn't been replaced but has been cleaned. I relpaced the injectors with a set I had out of another intake due to the tips being gone off of most of them. The set i put in had the 4 small holes in it instead of the tips not sure of any other differences but i still have the same problem. Any input will be GREATLY appreciated I'm at my wits end on this.

 

My Ford Service Manual has this listed for having both 172/173 codes:

FUEL DILUTED ENGINE OIL

Possible Causes:

-- Fuel injectors.

-- HO2S.

-- Secondary Air Injection (AIR) system.

-- PCV/Hose.

-- Vacuum.

-- CANP.

-- MAP sensor.

-- Electronic Ignition Coil Failure.

You have already replaced a few of those items. Have you pulled the plugs to see if they are gas fouled? If the truck has been running rich the O2 sensor could be gummed up as well. O2 sensors do have a finite life span, how old is yours?

 

Might want to get this out of the way first the truck is a 91 with a 5.0L E4OD 4X4. The O2 sensor "bosch brand" has around 6k miles on it. The plugs "motorcraft brand" have around 1k on them. Also have replaced TFI module "autozone brand" and pick up coil "autozone brand". And of course new Accel plug wires, distributor cap, and rotor button. MAP sensor "standard brand" was replacd around 8k miles ago. I have 36psi fuel pressure with vacum line hooked to regulator, and 42psi with it unhooked.

 

So that leaves the CANP solenoid, PCV and MAP. And you never answered my question of what spark plugs look like. When you replaced the vacuum lines did you inspect all the vacuum reservoirs and verify they still hold vacuum?

 

The spark plugs were clean looking not burnt not carboned up all uniform looking. Like I said earlier the MAP sensor has been replaced. What is the CANP solenoid. My vacum can is good no leaks soaked it down with ether the idle never varied when spraying.

 

The CANP (Vapor Canister Purge) solenoid is near the throttlebody. It's the item circled in this photo below:



It allows fuel vapors from the fuel tank(s) into the intake at certain times. It could be stuck open or partially open.

Have you checked the cruise control vacuum reservoir, located behind the driver side headlamp? How about the little plastic reservoir located on the blower motor case on the passenger side? Those little devils are frequently overlooked.

As the Ford Manual states the oil can be fuel diluted if you had a rich condition, but now it may be repaired. Has the oil been changed these symptoms/codes started?

The air injection system itself has a lot of vacuum lines, hard lines and check valves. Any unusual "exhaust" leaks?

 

The cruise control has been unhooked the vacum lines going to it have been capped off. The oil was changed around 1500 miles ago it wasn't unusually thin nor did it smell gassy. I have pulled the vacum line off of the purge solenoid going to the vapor canister with engine running couldn't feel any vacum on it. Not sure on which engine state it is supposed be functioning whether when warm or cold. As far as the other vacum reservoir on the heater box I will have to check later I'm at work now. I have new air pump. The hose from it to the diverter valve is in good condition. No more hard plastic vacum lines all have been replaced with 5/32 rubber vacum hose. The PCV valve and hose has been replaced. All new exhaust "no cats".

 

The purge solenoid is typically closed. As the Ford manual states about how it works:

The system works by capturing a very high percentage of fuel vapors that might otherwise escape from the fuel tank. It then directs these vapors to the fuel vapor canister where they are stored while the engine is not running. When the engine is running, and conditions are acceptable, the vapors are purged out of the fuel vapor canister and into the engine. This "clears" the fuel vapor canister and allows it to accept more fuel vapors the next time the engine is shut off.

It's pretty cryptic. I would go the other direction, make sure the solenoid holds a vacuum when closed.

When did the 172/173 codes start to appear?

From your statement: All new exhaust "no cats"....Where is the O2 sensor now located?

Have you traced the MAP sensor signal back to the PCM? It does have a large bearing on fuel/air control. You may have replaced it, but if the wiring is somehow fubared 12 new ones will not help. Another good thing to checkout, if it has not already, is the connector to the PCM as well as the grounds. I did work with a guy who got rich codes only when the headlights were turned on. The root cause was a bad connection at the grounds on the radiator support.

 

I found this reference to error codes 172/173 and others: 1983 Ford Bronco TSBs & FSAs (Recalls) for '83-96 Broncos & F150s picture | SuperMotors.net

This is a TSB issued by Ford to address damaged catalytic converters. The codes referenced do relate to your issue. Yes, I know you do not have converters anymore....

The TSB specifically states to check the O2 sensor ground which is a splice that eventually makes it way back to the PCM Pin 46, Signal return.

The Signal return is the to the right side of the O2 sensor in this schematic below:


Here is the splice in the middle of the schematic (referenced as BB):


I do not have a EVTM for a 1991 truck to refer to for where Splice 137 is located. My 1992 EVTM shows this as the location: Engine control sensor harness, near T/O to brake warning resistor/diode assembly

That leads me to believe it is near the brake master cylinder? Which makes sense because the connector for the code reader is located there as well, which also shares the same Signal Return on Pin 46 of the PCM.

 

I unhooked the purge solenoid it had vacum pulling through it plugged it back in still was pulling vacum. To my knowledge when you pull the plug on a solenoid it should be closed. I'm guessing since it was open both ways it is bad. As far as the location of the 02 sensor it is still in stock location.

 

Correct, the purge solenoid should be closed with no power applied to it.

 

Ordered a new purge solenoid see what happens next.

 

Put new purge solenoid on today pretty sure that's it. I had the vacum line capped off and it didn't through any codes.

 

That is good news

 

I know this is an old thread but tracking down the source of a 172 code as well. With the CANP solenoid unplugged on two different trucks, I’m getting 18 in Hg vacuum through the solenoid. I’m pretty sure, by reading the comments, that these solenoids are bad. Correct? Did your code clear up after the replacement?