1995 F-150 Code 173 & Poor Drivability - Ford Truck Enthusiasts Forums

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1995 F-150 Code 173 & Poor Drivability

Old 01-03-2006, 03:25 PM
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1995 F-150 Code 173 & Poor Drivability

This is my first post on this site which I discovered while searching the web for a solution to an EEC IV code 173. I am ASE certified with a college degreee in Automotive Technology and I am fairly well equipped to work on these systems, but I don't have alot of experience (I was an aircraft mechanic in my first life). I followed all of the steps in the Ford repair manuals and it took me a couple of days. Anyway, I fixed the problem by replacing the only part I could find bad - the PCM. I went through ECU Direct because ford no longer supplies the part. I originally sent it in for R&R ($210), but it turned out that the PCM was unrepairable due to corrupted data in the CPU inside the unit. They ended up sending me a different remanufactured unit for a total cost of $345. I don't know how the data got corrupted, but the truck owner told me that there has always been a slight problem with idle since new, and now the truck runs better than ever. May have been a manufacturing defect that progressively got worse with time. I just thought I would post this in case anyone else was having similar problems. I am going to cut and paste a letter I wrote and sent along with the bad PCM to ECU Direct. Hope this helps....

From: Ken Brown (Automotive Technician)
To: Whom it may concern at ECU Direct
Subject: Additional Information Regarding ECU R&R Service for Autocraft Direct Confirmation # 87059 with EEC IV OBD I Code 173.
1995 Ford F-150 (Eddie Bauer edition), 5.8L, SFI, California Emission Controls, 4WD, E4OD Automatic Transmission, 104509 Miles, VIN # 1FTEF14H3SNB61314, PCM Part Number F5TF-12A650-AAB
Customer Complaint -
When starting truck, engine races at high RPM for a few seconds and MIL flashes 4 times. While driving at cruise @ 65 MPH engine starts to stumble, but recovers when throttle is goosed. This occurs several times during trip to work and back. Poor performance while climbing a grade usually followed by MIL illumination. Poor gas mileage. Truck had been in shop for this a few months prior. They replaced both O2 sensors, PCV valve, fuel filter and something else on the engine (customer canít remember but it had 3 letters in the abbreviation) probably the EVP sensor (it looks new). He says it ran better for a couple of weeks and then started acting up again.
My Observations and Actions Ė
Same as customers on startup. At idle engine slowly decreases RPM almost to the point of stalling and then slowly increases to a smoother normal idle and then repeats this slow cycling continuously. Scan data shows this to follow changes in injector pulse width. This also happens while driving around town and is sometimes accompanied with a MIL for a few seconds or more, but the MIL does go out. Code 173 (EGO not switching Ė Rich Bank 1) was stored in continuous memory. Scan data agrees and shows HO2S 1 not switching normally, but HO2S 2 was. (HO2S 1 wandered around very slowly between 350 & 600 mv with O2 sensor connected or disconnected.) As a quick check I swapped O2 sensors and the problem remained on bank 1. I also scoped out the TPS because from the customers complaint description I thought it may have had a problem even though there was no code for that. It was fine.
So here we go. I got into the ford maintenance manual and followed all of the troubleshooting steps for Code 173. I have my Snap-On scanner, EEC IV breakout box, Fluke 98 scope meter and a DVOM all hooked up at the same time. I cleared codes did several KOEO and KOER tests and a couple of power balance tests. KOEO always tested good as did the power balance tests, but the KOER test almost always popped a Code 173. I did get a Code 176 once (EGO not switching Ė Lean Bank 2) but that was still switching fine. Also I was having some trouble running the KOER test. I was getting no response sometimes and would have to re-enter the test up to 3 times before it would run. Fuel pressure was within specifications, resistance tests on all the fuel injectors were good. According to scan data, Closed Loop is being achieved at @ 96 degrees (F) and all other available scan data looked good except for HO2S 1 (There was no scan data on the MAF sensor or STFT 2). I had a good waveform on HO2S 1 at the O2 sensor and at the O2 sensor harness connector behind the battery (R/H side), but not at the breakout box pin 44 (the signal wire for HO2S 1). (PCM on this truck is lower LH side firewall) I was getting a good signal at breakout box pin 43 for the bank 2 HO2S 2 signal wire. (These are 4 wire O2 sensors and share the same grounds.) Book says to locate the short to power and fix it. So now Iím wiggle testing away and watching scan data both engine running and not running with no results. So I went into the wire harness and visually inspected everything and did more wiggle testing. PCM connector and pins look good and have no damage. Still canít find anything wrong with the wires either.
Then as I was standing in front of the truck scratching my head with the engine running and scanner in KOER Wiggle Test mode, truck going through itís crappy idle routine, the engine stumbles and 5 codes appear on the scanner. 543 fuel pump ckt open batt to ECA, 173 as usual, 538 cylinder balance test fail, 536 BOO switch, and 632 transmission control switch. I wasnít touching anything on the truck when this happened. So I tested all the relays and they were good.
I still wasnít getting a signal to pin 44 at the breakout box, so I decided to back probe pin 44 at the PCM connector. Finally I had a perfect waveform on the bank 1 O2 sensor through the whole harness to the PCM. I read through the PCM connector at pin 44 and even both sides (harnesses) of the breakout box at pin 44 just to make sure. They were good. I double checked all of the PCM power and ground wires and found no problems. I figure the problem must be inside the PCM and the breakout box was sensing that problem or short.
I donít easily condemn a PCM. I fully realize that they rarely fail unless an output device/actuator fails and causes a driver or something to go bad in there. I havenít found a good cause for this yet, but when I sum it all up: 1) I have a good signal all the way from the Bank 1 O2 sensor to pin 44 at the PCM, but the PCM doesnít see it or recognize it, 2) Trouble entering the KOER tests and 3) those 5 unrelated codes popping out when they did for no apparent reason. In this case I really feel itís in the PCM and thatís why Iím sending it to you. Have you ever seen this particular problem before? I did some research on the internet and read some horror stories about people trying to solve a Ford Code 173. Nobody really has any sound advice to offer them and it took me a few days of troubleshooting to reach the conclusion I have on this one.
Please let me know what you find on this and possible causes if you can, so I may correct the cause before re-installing this PCM. Hope this info helps in your diagnosis. Feel free to contact me with any questions you may haveÖ.
Old 01-03-2006, 03:42 PM
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Wow. I'm speechless. Great job. It's a shame that more mechanics aren't this thorough when fixing people's vehicles.
Old 01-03-2006, 04:46 PM
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Old 03-15-2016, 03:58 PM
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Did the PCM solve the problem? I'm having some of the same issues on a 1992 F150 5.0.
Old 03-15-2016, 08:43 PM
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He said it did.
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