Reman 4.0 SOHC -- Problem with P0340 error and +42 STFT error -- SOLVED FINALLY!

[Fire]

I'm writing about this problem and resolution in case anyone else installs a remanufactured engine to replace an aging 4.0 SOHC. Maybe save some headaches.

TL -- Spent a week troubleshooting and found 2 wire connectors (FI and cam sensor) swapped that were near each other and keyed the same (apparently to maximize the chance of mismating!). Check those 2!!

Ok, my 2002 Sport Trac with about 202000 miles had broken rear timing guide that had progressed from rattle to rockcrusher sound. Pulled the pan and found no metal but a bunch of plastic pieces. Weighed my options and decided to install a $3k reman engine myself (how hard could it be??)

Well it was hard. Lots of cussing, sweat and time. Pulled the old engine over about 2 days with my father-in-laws help, then when the reman engine came in, we spent a day to transfer everything (valve covers, starter, intake, alt, etc). Mine did come with an oil pan and front harmonic balancer installed so that was nice.

Ok, last week over 2 days we installed it and connected everything, --and it ran like crap. On my cheapy OBD2 scanner I got a P0340 camshaft position sensor system error, and the short term fuel trim (STFT) was crazy high on bank 2 (drivers side cylinders 4,5,6) at +42% above baseline fuel volume.

On mine, the cam sensor is right on the DS valve cover, so took it out of the vehicle to check it. It is a magnetic sensor so moving steel/iron across it is supposed to generate a voltage disturbance that the ECM sees. Checked it with voltmeter and it signaled fine, but put in a new one just in case. Checked continuity of those cam sensor wires in the wire harness going to ECM big connector on the firewall. Did the same for the crank sensor and all 6 fuel injectors, and all good. Checked the O2 upstream sensor readings and they were varying like they should.

Talked to my repair shop that I use and trust. They were backlogged and couldn't look at it for a week or so, but agreed a mistimed cam was the likely cause based on the readings.

Talked to engine mfgr about how it seemed like wrong timing, and he asked for pics. Pulled the bank 2 valve cover, rotated crankshaft to TDC on compression stroke and checked position of cam. It is hard to tell if it is in exactly in the right spot without the Ford cam alignment tools, but I took pics and sent them in. The mfgr techn agreed it was hard to tell, and said he thought it would be better to send me a replacement engine than try to retime the camshaft(s) correctly in the field. He was great and they pay for labor and such for the replacement.

So I had the replacement engine order placed as of last Thurs. Planned to take it to my repair shop, who is backlogged more than a week, and have them replace the motor when the replacement came in. Likely a lot of waiting, maybe 3 weeks.

But I kept looking at the cam position and reading threads and watching videos, with the uneasy suspicion that the cam was timed ok (I took a primitive measurement showed maybe .3 degrees off which seemed negligible), and that they might replace the motor and it would behave the same crappy way.

So I kept trying to troubleshoot it this weekend.

• Swapped the O2 sensors between bank 1 and 2, but problem stayed on bank 2, so the O@ sensors were not the problem.
• Thought about how when installing the new engine with transmission attached, we banged and wedged the wire harness, the corner of the ECM flange, the FI vacuum diaphram on the end of the fuel rail. Maybe damaged something?? (--I wouldn't install an engine with transm attached again. Yes lining up engine to transm is pain, but installing together Much worse IMO.)
• Checked through pretty much the entire engine harness, found some cracked and broken insulation, and taped it, but didn't fix the performance.
• Read about how a bad alternator (diodes) can generate excessive noise on the crank and cam signal lines, so rerouted sensor lines and also ran with alt disconnected, no change. Still got same code and such.
• Vacuum check showed maybe a minor leak, but my gauge is old may not have read perfect anymore, but checked all vacuum lines for leaks or damage. Broke one hard plastic line, just moving it around so fixed it. No change. (Did not pull the intake manifold off, but was in the back of my mind that maybe those o-ring gaskets were misplaced, and would have to pull eventually to check).
• Thought I maybe had clogged fuel injectors on bank 2, so not flowing enough. Didn't know how that would tie into the P0340 error, but maybe the fuel trim issue. I was reusing them from my old engine. Had to clean the outside of them a lot when I moved them over and thought maybe some crap got inside. Went back and pulled the fuel rails, and all the injectors. We built a flow tester with carb cleaner routed into the FI (see youtube), then energized the FI with 9V battery, and tested them for clogs and spray pattern. All 6 were fantastic.
• Yesterday I bought a timing light to check for spark on bank 2. It was from Harbor freight. Got weird readings, where bank 1 ok, but bank 2 no flashes or too many flashes. Thought I had a bad coil pack. Picked one up today at PullNSave, and pulled an entire engine harness from a 03 Sport Trac just in case mine had breaks internally, and since it was there. Looked at ECMs in case that was the problem but didn't pull the trigger yet since would have to get the key redone for the PATS.
• Then did a compression test on the bank 2 cylinders, and they ran 130-140 psi so seemed good, so no bent valves from cam mistiming. But no solution either. Cylinders 5 and 6 a bit black, cylinder 4 grey/white like it should. Hmm, only cylinders 5 and 6 getting too much fuel, but fuel trim setting in ECM affects all the FIs in the bank.
• I had already checked the wire harness for continuity from ECM to the sensors and FIs, but decided to backprobe the connected fuel injectors when key in ON position to see if they were getting the 12V expected on 1 wire (12V constant and ECM drops the other wire to ground to energize the FI coil to spray). And figure out why cyl 4 wasnt getting as much fuel. Saw cyl 4 FI only had .03V, and same at ECM connector so it wasnt a harness defect. Noticed different type of connector and wire colors on that one FI, but I thought maybe it was repaired way back when with a generic connector pigtail. (Thinking bad ECM, oh crap.)

But, I noticed the cam sensor right next to the FI, so back probed that for grins and giggles. It was getting 12V. I couldn't remember if it was supposed to get 12V, but noticed that the wire colors were similar to 5 of the 6 FI connectors. Then it hit me, could they have been swapped? Yes, they connected either way! I had been installing the 2 connectors swapped the whole time since I installed the engine. They look different on the outside, but the pin spacing and keying is the same, so they just go right on when switched. The ECM wasn't seeing the cam pulses, and cylinder 4 wasn't getting any gas.

The engine runs fine now.

Moral of the story: Although I labeled and separated all removed bolts, and took some pics of complex areas, I did not mark each side of electrical connections. Do it. Even if they seem like they are so different they wouldn't go together, -- cuz they do sometimes, and ruin your life for a week.