lelandEOD
SILVER Star
What a nightmare.
So, I wanted to document my troubles and tribulations for the sake of others who may find themselves in similar circumstances with their 3FE.
The bottom line is this: The CEL will LIE to you.
My truck, over the course of several months, was running poorly and no amount of troubleshooting on my part seemed to make any difference.
Symptoms:
Loss of power during acceleration in the mid-rpm range.
Exhaust smelled rich (verified by a Gunson Exhaust Analyzer @3.0% CO), but the spark plugs looked lean (white chalky appearance).
Slight miss at idle.
Over the course of many months, I replaced the following parts:
Fuel pump
Pulsation damper
Fuel regulator
Coil/igniter
Plugs
Ignition wires
Cap/rotor
Distributor
ECU
AFM
O2 sensors
TPS
I also used the FSM to run all the diagnostic checklists and bench tests of individual components.
None of my attempts to solve the running issues were successful and the engine was not throwing codes.
The one thing I was confident in was that the problem involved the O2 sensors. When I tested the O2s using the procedure prescribed in the FSM, I was getting no voltage on one of the O2s (VF2). I checked all four wires for continuity to the ECU and ground, which checked out. I had a good ground, ~2.5v to the heater element, and continuity on the signal lead to the ECU. WTF!
I figured out what was wrong with the O2 sensor when I cut some harness tape back from the connector and found that the heat shrink tubing used when I spliced a new connector on the loom (the old one melted at some point before I got the engine) got too hot from the manifold and allowed all four wires to ground together. Bingo!
So, I fix the issue with theO2 sensor wiring but after test driving it, I found that the truck was running worse than before. The truck would run but I noticed the engine developed a hesitation that was not present before I fiddled with the wiring. In short order (one or two trips to work) this hesitation/stalling tendency grew in frequency and intensity to the point where the truck was near un-drivable.
This past weekend I spent all day trying to figure out what I had goofed up while trying to diagnose the O2 issue. I back tracked and re-swapped all the components I had changed out, trying to find the one that was causing the hesitation/stumbling under load. I changed out the ECU, the distributor, coil/igniter, AFM, etc, all to no avail.
As a last ditch effort, I decided to clean the throttle body and bench test the throttle position sensor (TPS). When I got it on the bench, I found that the TPS was out of spec on the two resistance tests with the feeler gauges. I had a spare TB/TPS on the shelf that did check out and after reassembling everything, the stumbling/hesitation problems are gone.
Theory: the only thing that makes sense to me is that the faulty O2 sensor was causing the ECU to compensate in such a way that it concealed the mis-adjusted TPS. Once I fixed the O2 sensor, the ECU appropriately adjusted the mixture, causing the mis-adjusted TPS to make it's presence felt. Wild.
Lessons learned: I had no idea the TPS being out of whack could create running problems like I experienced and I wrongfully assumed it was working correctly because I did not get a CEL or code for a TPS error.
Additionally, I did not get a code for the O2 sensor that was grounded out in the wiring loom so I now know a lack of error code is not equivalent to the absence of a problem. The CEL will lie to you.
I still have a rich running condition and subtle miss at idle to figure out, but at least the truck is drive-able. That was fun.
So, I wanted to document my troubles and tribulations for the sake of others who may find themselves in similar circumstances with their 3FE.
The bottom line is this: The CEL will LIE to you.
My truck, over the course of several months, was running poorly and no amount of troubleshooting on my part seemed to make any difference.
Symptoms:
Loss of power during acceleration in the mid-rpm range.
Exhaust smelled rich (verified by a Gunson Exhaust Analyzer @3.0% CO), but the spark plugs looked lean (white chalky appearance).
Slight miss at idle.
Over the course of many months, I replaced the following parts:
Fuel pump
Pulsation damper
Fuel regulator
Coil/igniter
Plugs
Ignition wires
Cap/rotor
Distributor
ECU
AFM
O2 sensors
TPS
I also used the FSM to run all the diagnostic checklists and bench tests of individual components.
None of my attempts to solve the running issues were successful and the engine was not throwing codes.
The one thing I was confident in was that the problem involved the O2 sensors. When I tested the O2s using the procedure prescribed in the FSM, I was getting no voltage on one of the O2s (VF2). I checked all four wires for continuity to the ECU and ground, which checked out. I had a good ground, ~2.5v to the heater element, and continuity on the signal lead to the ECU. WTF!
I figured out what was wrong with the O2 sensor when I cut some harness tape back from the connector and found that the heat shrink tubing used when I spliced a new connector on the loom (the old one melted at some point before I got the engine) got too hot from the manifold and allowed all four wires to ground together. Bingo!
So, I fix the issue with theO2 sensor wiring but after test driving it, I found that the truck was running worse than before. The truck would run but I noticed the engine developed a hesitation that was not present before I fiddled with the wiring. In short order (one or two trips to work) this hesitation/stalling tendency grew in frequency and intensity to the point where the truck was near un-drivable.
This past weekend I spent all day trying to figure out what I had goofed up while trying to diagnose the O2 issue. I back tracked and re-swapped all the components I had changed out, trying to find the one that was causing the hesitation/stumbling under load. I changed out the ECU, the distributor, coil/igniter, AFM, etc, all to no avail.
As a last ditch effort, I decided to clean the throttle body and bench test the throttle position sensor (TPS). When I got it on the bench, I found that the TPS was out of spec on the two resistance tests with the feeler gauges. I had a spare TB/TPS on the shelf that did check out and after reassembling everything, the stumbling/hesitation problems are gone.
Theory: the only thing that makes sense to me is that the faulty O2 sensor was causing the ECU to compensate in such a way that it concealed the mis-adjusted TPS. Once I fixed the O2 sensor, the ECU appropriately adjusted the mixture, causing the mis-adjusted TPS to make it's presence felt. Wild.
Lessons learned: I had no idea the TPS being out of whack could create running problems like I experienced and I wrongfully assumed it was working correctly because I did not get a CEL or code for a TPS error.
Additionally, I did not get a code for the O2 sensor that was grounded out in the wiring loom so I now know a lack of error code is not equivalent to the absence of a problem. The CEL will lie to you.
I still have a rich running condition and subtle miss at idle to figure out, but at least the truck is drive-able. That was fun.
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