3FE- Loss of power/mild stumbling/at mid RPM range (Fixed it Thread) Whew. (1 Viewer)

lelandEOD

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Aug 21, 2012
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Northern Nevada
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.
 
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Joined
Aug 26, 2009
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down in a hole.
man I can't count how many times I've reminded people that the tps needs to be calibrated...glad you got it worked out, as for the stumble miss, have you done a valve lash? also, check that the idle suffers from pulling the dipstick out while running and that the same happens when the oil cap comes off while running. check your air hoses between the ICS valve and the throttle body and the air cleaner housing are all intact without cracks or holes...
 
Joined
Nov 9, 2014
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66
Location
Huntsville,Texas
Good to see documentation of probs. I just recently re-soldered the connections on my 3FE's MAF I should have photoed that and posted it to save folks cash. It's a pain in the donkey but worth saving the 2-300 to put some where else on my rigs. I haven't resealed the top of the MAF and may still do a photo how to if I can find the time.
 
Joined
Sep 25, 2011
Messages
258
Location
El Dorado Hills, CA
I thought I'd add to this thread. My son has a 1988 FJ62 that we've been slowly modifying and making it the kind of truck my son wants. When we bought the truck he drove it 60 miles or so home, in addition to an extensive test drive before buying it. Through all this it ran great. Then, the truck started to run rough on the highway and from a stop, seemingly getting worse very slowly over time.

Thank goodness for ih8mud forums as we were able to narrow down a target list of possibilities. First we replaced the thermostat (old one was tweaked pretty badly) and flushed and replaced coolant. That fixed an issue with the temp gauge not being responsive but didn't have an effect on the rough running 3FE. We didn't think this was the primary cause of the rough running engine but it was on the list of post-purchase maintenance catch-up items and the first thing we did from an engine perspective.

Next was the TPS. It's an after market unit but before deciding to replace it we set aside time to clean and adjust the TPS. The TPS was on pretty loose with neither top nor bottom screw holding it snugly. We ended up centering the TPS adjustment range on the top screw, tightening it down and the engine runs great now. We'll fine-tune the TPS setting the next time he has the truck at home.

Apparently the TPS was working itself out of adjustment, likely moving slightly and randomly over time

I have a new respect for TPS and, admittedly, didn't have a clue what this was for until now.

We still have a healthy list of maintenance items to complete but it's nice to have this issue solved.
 
Joined
Aug 26, 2009
Messages
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Location
down in a hole.
it is a great place...
you should get the gaskets and take off the ICS and Throttle body to properly clean it and then mount the TPS and adjust it while the TB is off the manifold, or rather, while the ICS is clear out of the way. as important as it is to properly adjust the TPS to the ACTUAL position of the TB at the 3 points of calibration, none of that will matter if the TB is dirty and the plate sticks too far from closed...the whole process needs to be evaluated, right? and make sure to set it to within specs, although, if you can set it by feel, more power to ya!
 

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