Hey all, nice forum you got here. Just joined!

Maybe someone smarter than me can help systematically diagnose this problem lol. I might've read 100 similar threads and it seems the root cause could be literally anything: fuel, exhaust leak, vac leak, EGR, TPS/maf/iac/O2, bad ground, bad ECU, you name it. I'm hoping someone can help me figure out specific diagnostic tests to pinpoint the problem.. sorry for long post, just trying to provide any info that might be relevant

My vehicle
95 f150 xlt 5.0L automatic
1FTEX15N6SKxxxxxx

Symptoms

• Loss of torque especially at low speeds when feathering the pedal. To the extent that it's not roadworthy.
• When it's *******, holding down the throttle will eventually downshift and gives a jolt of power
• The overall behavior is intermittent. I cannot find any pattern (cold vs hot, closed loop vs open loop, doesn't seem to make a difference). Clearing kept memory has not made a difference either. For about 40 miles today she drove great, then bam she's in a bad mood again, so bipolar
• Sometimes she idles great. Other times a bit shaky and you can see rpm needle not steady. Cold idle is a little worse than warm idle.
• Sometimes at a stop the truck will shift a gear. Everytime it happens I think wtf was that
• Check engine light comes and goes. CM code 172 (Lack of Oxygen Sensor (HEGO) switches, indicates lean, Bank 1). Also occurs during a KOER.
• Also code 334 (EGR closed V higher than expected). I installed new EGR parts and believe I used the wrong seal. After changing the seal to a thicker one, seems to have resolved this code.

Steps so far

• Fuel pressure looks good. 33ish PSI at idle. 40 at WOT. Maintains 40 even while the power loss event happens. Tank selection makes no difference. Ive never replaced fuel filter so that might be good idea in general
• Cleaned MAF
• Intake vac 16ish inhg (5000 ft). It is steady when idle is steady but otherwise it fluctuates +-1inhg. I know I should probably search for vac leaks elsewhere but not really sure where to start. Measured vac at EGR, if I remember correct it was 0 at idle and 5inhg with a bit of throttle
• Looked at the computer, circuit board/ caps in pretty good shape. I'll note that I had a similar power loss/limp mode issue 12 years ago and the mechanic at the time replaced the computer and that had fixed it
• Threw some cash at the problem (surprise! It didn't work) new IAC, new O2 sensor, new EGR valve sensor and solenoid. Original o2 sensor/connector/harness was caked in grease /oil. I do have a bit of an oil leak down there. I cleaned it up and applied dialectric. I did not immediately find any shorts in the harness
• Checked O2 heater fuse. Good.
• Cleaned MAF
• Sensor voltage good at the test port. Have not checked any individual voltages
• A couple months before this issue ever occurred, I replaced alternator, starter, starter relay, battery
• Have not checked spark plugs but I figure ill probably replace em tomorrow, it's been a long time. Maybe I'll do cap/rotor/coil and check timing.

Any advice is greatly appreciated 😅 thank you

Side note: sometimes I have trouble getting the ECU into test mode, either with the tool or using the jump method. Pulling ground usually resets it. Not sure if this is common or what to make of it. Test port does have 5V

 

I'm guessing you have two problems. Vacuum leak or an exhaust leak, causing the lean code. And a bad TPS sensor. Easiest way to diagnose the TPS sensor is with an oscilloscope. You can use a multimeter, but it's not as good. Look for dead spots in your throttle sweep.

What does your KOER codes tell you?

 

Thanks for the reply. Good idea, I'll hook up my digital o scope today. I'll also take a garden hose or stethoscope and see if I can detect any leaks

KOER gave code 172 when I last ran it. No other codes except for the directional ones (10/111) KOEO passes but throws 172 (and previously 334) in CM

 

Ok quick update. Potentiometer looks really good KOEO (screenshot below). Closed pos: .84+-.003, open pos: 4.535 +- .002. Was idling really rough this morning, wanting to bog down pretty bad. I might take for a drive with oscope logging just to rule out some funky tps signal while driving. But at this point idk, I might re-check the o2 sensor harness, then after that I suppose I will focus 100% on trying to find vac leaks

 

Here's are the old spark plugs. Been years since I replaced them. They look decent to me overall ...


O2 heater 11.7v with batt 12.05. .35V drop seems substantial to me. I'll log O2 sig during a drive when I get back on road

 

Did you get this figured out?

 

I fixed this exact problem by replacing the Fuel Pressure Regulator despite it testing and looking good.