My engine's been throwing a code 173 - HEGO rich for the last two weeks. No other codes. My fixes below have led to some improvement, and its now mainly hesitation/bucking in cruise.

1994 F150 - 4.9L 300

Things I've checked and replaced:

O2 Sensor Replaced and Tested
Air filter Replaced
Fuel filter replaced (motorcraft)
PCV Valve Replaced
Oil Change and filter
Rotor, Cap, Plugs, Wires
ACT Sensor (Tested bad, replaced - aftermarket sensor, tests good)
EGR Solenoid (had a leak, replaced)
Fuel Pressure (50 KOEO, 56-59 KOER, holds 50PSI after shutdown for over 5 minutes.
Canister Purge seems good. No leaks.
Vacuum Reservoir appears good. Holds vacuum and check valce works.
EGR holds vacuum and appears to be opening properly and when requested by computer. Doesn't open at all when solenoid is unplugged.
TPS checks good at idle and while driving with multimeter sticking in the windshield. (.98v at idle)
Cleaned IAC - Slight miss still occurs at idle. Unpluggged IAC and still experienced the bucking/hesitation in cruise.
Checked timing - Perfect at 10Degrees, which I had checked last year.

Anyone? Help?

 

So, I got some new results today and need help interpreting them.

After replacing the spark plug wires, cleaning the coil up, I re-tested the H20S. Had the multimeter picked up to the H20S signal and the return wire. At KOEO, I was getting about 1.4v from the harness alone.

Engine running, the voltage quickly went down to 0 and flatlined. I induced a vacuum leak from the main tree and was able to manipulate the voltage reading from the 02 sensor. With the leak fully open, the H20s read about .86v, the engine ran fine (although high at idle (around 1200RPM, and about 800 when put in gear.

Ran the truck up to 45mph and cruised about 5 miles, and all symptoms were gone. Got back and repeated the fuel pressure leakdown tests, running, KOEO, Timing the leakdown after idle, and everything passed fine showing no leaks.

I had been suspecting a leaking injector at this point, but now I'm not so sure. Any ideas? Can the intake be clogged so bad that my O2 sensor would read full rich even with all the leaning strategies done by the ECU?

 

Check the vacuum hose coming off the fpr, to make sure the diaphragm is not torn leading to the introduction of gas straight into the manifold via the vacuum line.

Also, does it blow smoke? Smell? Show symptoms of rich?

 

Checked that as part of the fuel pressure test.

Pressurized system, ran it 10 seconds, shut down 10 seconds, ran again, pulled the line off, no gas, pulled vacuum against the FPR with my mouth while running and engine off-no gas taste or smell. FPR seems totally sound per the specs.

Pressure is 50 KOEO, holds for quite a while after shutdown. The test I looked at specified -5 psi in 1 minute. Noticed no drop in 10.

 

No smoke. Maybe a slightly perceptible rich scent when you stand near exhaust, but not anything drastic like black smoke.

Put in new spark plugs last week, and after running every day, the two I pulled out look clean. No soot or oil/gas.

Just for kicks, I disconnected the MAP sensor vacuum feed and plugged the port. Cranked the engine and got horrible black smoke out the pipe and a barely running motor.

 

Did a critter nest in your air filter?

Need to check coolant temp sensor also.

 

No sir. Air filter is brand new and fresh as a daisy.

I'll get an ambient reading this morning before starting the truck and then after running on the ECT

 

Got some readings from the ECT:



KOEO - Cold - Awesome 63 Degrees here in New Orleans



(wrong polarity) KOEO - After CEL came on.

Sensors are factory from what I can tell. According the oldfuelinjection chart, the values seem dead on:

3.22v = Somewhere just south of 68 degrees.
.61v = 194 coolant temp after running at 45MPH for a while. Good enough to get into open loop.

Also, I've replaced the fuel filter about a month ago. I updated the top post for all the newer tests I've done.

 

What o2 sensor did you buy to replace the old one?

 

Bosch.

I bought the truck in 2013, and replaced the O2 sensor at that time with a Bosch. Thought it was a tune-up item and have since learned differently. Didn't have any codes at the time or since the last couple weeks.

Is this a case where an OE tolerance might be the issue?

 

The o.e. suppliers, like Bosch, are normally pretty solid choices for replacement. O2 sensors are very delicate and sensitive though, so it's not out of the ordinary for them to be contaminated rather quickly. If egr, ect, fuel system, etc are operating properly you are running out of things to be the cause.

 

So, I've pulled out the fordfuelinjection library, and this is what showed up:

The HEGO Sensor detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. A high concentration of oxygen (lean air/fuel ratio) in the exhaust produces a low voltage signal less than 0.4 volts. A low concentration of oxygen (rich air/fuel ratio) produces a high voltage signal greater than 0.6 volts. The 02 provides feedback to the computer (EEC) indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation. The 02 generates a voltage between 0.0 and 1.1 volts.

This would imply that my warmed up o2 sensor showing 0.0-0.2v is actually indicating a super lean condition, and that when i pull a vacuum leak, it's showing me to be rich. How is that possible when the EEC is throwing a code 173?

 

Bump for voltage question.

FTE 1987-1996 is littered with threads on rich codes like this that just die off with no answer, and I'd really like to close the loop and get it fixed.

Also, how could the A.I.R. Pump be causing the problem, and what's the best way to test it?

 

Are you getting those o2 readings driving, under load? Idle?

 

"Slight miss at idle"

Hm.. One thing about an O2 sensor, if there are ignition misfires, the sensor will indicate a lean condition, not rich, as one might think. They only measure the amount of O2 in the exhaust, not fuel.