New member / old Bronco / big problem...

260K total miles. Been driving this truck a long time (120K), & it's normally pretty reliable daily driver. Factory long block & rebuilt E4OD at 210K.

Lately (w/ out warning) it developed the following problem: When started (hot or cold), the engine idles up, then drops down slowly (as expected), then starts very rough idle & operation from a lean condition. Need to drop into neutral at stop, & feather - to keep it running. This lasts for 5 - 10 minutes of operation - then smoothes out & purrs like a kitten. Once it smoothes out, it runs fine for hours.

While driving (before it smooths out normally) it will smooth out, chirp the tires, & take off like a scalded dog - if given enough throttle.

A MAP sensor was replaced before scanning for codes - no change. We put a scanner on it, but during both rough & smooth (drove around & monitored) operation, the only code was for low coolent temp @ 156 degrees.

Idle air valve / no change. Fuel pressure seemed low & jumpy at 22 - 26lbs running, but holds 32lbs (for 15 min at least) when powered off. I'm not sure about the pressure gauge we were using, but a fuel filter, fuel pressure regulator, & fuel pump (all new) were replaced one at a time - no change. No fuel leaks. Pinch off the fuel return, & pressure pegs 80lbs / idle smooths out. Voltage is good at the fuel pump, & injectors.

If vacuum is removed from the regulator, it helps a little - but not much. I have not had the opportunity to check manifold vacuum yet, but starting to wonder. Have looked for a vacuum leak, but found none.

Also starting to wonder if the plenum (throttle body) was not sealed - or torqued correctly during the engine replacement 50K ago. Any special tools required for this?

Any thoughts or suggestions would be greatly appreciated, as this problem is starting to bug me... Thanks in advance!

Regards - GCF

 

My first thought was either thermostat or ECT then read again and it is doing it hot. How about the HEGO, HEATED Oxygen Sensor? I think you can check be sure the heat power circuit goes to the O2 sensor before replacing it.

 

Thanks for your input.

1. HEATED Oxygen Sensor - is there another name for this?

2. Is it possible for this sensor to malfunction, without thowing out an error code?

GCF

 

Forgive me for not welcoming you here. You seem quite knowledgeable. I am not but, remember changing the oxygen sensor made a huge difference in drivability.

1. Just the oxygen sensor.
2. All sensors can malfunction to some degree. The codes show up when the sensors get farther out of the expected range.

 

Well, thanks for the welcome, & also the vote of confidence. Although I perform electronic equipment field service for a living (self employed, contract tech), I am not trained in modern automotive electronic controls.

When I was a young fella (25+ years ago) I wrenched my own stuff, but that was old technology - & another time.

Your point about "All sensors can malfunction to some degree. The codes show up when the sensors get farther out of the expected range" was a good one, & sheds a little more light on the subject.

I guess the question is, what specific sensor could be "out of range" enough that it would account for a very lean operation - until the engine warms (5 - 10 min), but still NOT show an error code on repeated scans?

Do I understand correctly that when you refer to the Heated Oxygen Sensor, you are NOT referring to the O2?

 

Maybe I misstated...the O2, HEGO, and Heated Oxygen sensor are all the same. They added a 12v heater to it several years ago get the sensor up to temp faster. The Haynes manual tells how to test for the power to the connector. Who knows you might get lucky by cleaning and dielectric grease the O2 connector. If the oxygen sensor has not been replaced you might want to try it. Some say every 20K but that seems too often to me. Suggest Motorcraft since your fuel/air mixture depends on it.

 

I don't know of a way to test the O2 sensor but the ECT and ACT are easy. Remove them (remember where they go) put them in the oven beside a good thermometer. Read resistance. Raise over temp a little and read again. Repeat. Record your readings at each temperature and compare them to the chart. Ford Fuel Injection » Sensors