My '91 F150 5.0 recently started stalling at red lights and a "valve rattle" with moderate acceleration.


I read the codes and I got 41 (HEGO), 33 (EGR), 51 (ECT/ACT). I removed and cleaned the EGR (the position sensor passed testing), reconnected the O2 sensor which had became disconnected and replaced the ACT sensor despite passing tests. The ECT is maybe 3 months old and passed tests as well.

After this the driveability issue was resolved and KOEO test was clear. I disconnected the battery to reset the KAM. All was well for 4-5 driving cycles.

Today the truck did the same thing. I checked the KOEO code and got a code 51 again (ECT/ACT). I also get a code 44 (thermactor) with KOER test. However, the metal tube had rusted off long ago so I assume this is not a problem.

I tested the new ACT and ECT. Both were in range and had 4.6 volts to them. I'm at a loss of what to do next. Any ideas?

 

Was the engine at operating temp when you pulled the codes? If not you will always get the ECT/ACT codes and can ignore them.

The thermactor tube rusted off could be bad depending when it's separated, if it's upstream of the O2 sensor then it could allow fresh air into the exhaust system which will mess with the O2 sensor and result in a lean running condition.. which is what causes detonation under load aka.. that "valve rattle" you hear.

 

Yes. All the codes were run when the engine temp was warm.

The "metal tube" is the one that runs from the rubber hose and solenoids directly behind the engine to the downstream side of the O2 sensor. All I did was weld a seal over the opening on the CAT. This was a few years ago. I assume the CAT could become clogged as a result but the engine vacuum is around 52 and rock solid stable so I doubt my CAT is plugged.

 

52? really

 

How exactly are you measuring this sensor? One wire should be 5v and the other should be 3.xx volts when cold and drop as the engine warms up. This is with the sensor plugged in back probing it, if your getting 4.6v out of it when warm unplug it and see what you get, if it doesn't change you have a short, if it does change you have a bad sensor.

Edit: 4.6v is -50 that's why it's complaining

 

Good advise here. Thank you.

I followed my haynes manual for testing. To test voltage it says to remove the connector and test for voltage using both wires on the connector for + and -. I got 4.6 volts doing that. Then I removed the sensor and tested the resistance when it was in water that I measured with a thermometer, cold and hot. The values were accurate for the temperatures.

After reading your post I back-probed each wire while connected. (btw, I have to get a piercing probe! That was hard to back probe. I ended up using a needle lol). The gray wire was 4.6 volts. The green wire was 0.024 volts hot AND cold!

I replaced the sensor and ran the KOEO test. No codes now. The voltage on the green wire when the engine was hot was 0.9 volts NOT 0.02. So I assume that was the problem. Time will tell I suppose. At least advance auto replaced the part under warranty.

Thanks again. This taught me a lesson about testing these types of sensors. The procedure in the Haynes manual is clearly not conclusive.

Sorry. 52 cm Hg NOT in Hg. in Hg would be about 20. I thought a steady drop to zero would suggest a clogged CAT. Although we now know my test procedures aren't exactly accurate lol