No closed loop HEGO switching at warm idle

The vehicle is a 1988 Ford F-250 5.8L EFI, EEC-IV, 4X4 Manual transmission. Absolutely stock with no modifications whatsoever. The exhaust system is new. Engine is in excellent mechanical condition with good compression for 93K miles. There is correct transition from open to closed loop after cold start and warm up. The HEGO is new and is fixed at 0.0volts during closed loop warm idle but responds to a propane enrichment. The warm idle is slightly rough with a sputter through the exhaust suggesting a lean misfire. The primary and secondary ignition functions show no faults and timing is on spec. KOEO self test is a code 11 pass. KOER self test shows code 41 for lean and no HEGO switching. Bank 1 and Bank 2 injector duty cycles are 95% at warm idle. Manifold vacuum is rock steady at 16" Hg. I have tested for vacuum leaks and found none. All hoses and plastic tubing are tight and hold vacuum when applied . TPS, coolant sensor, charging air sensor are all withing spec. The exhaust air injection system has no leaks and the control solenoid valves function as they should to direct pumped air in open and closed loop.The EGR valve is closed tight and does not leak into the intake manifold. PCV is a correct new Ford item. At warm RPMs above 2300-2500 there is some HEGO switching and injector control but it stops and HEGO goes fixed lean when returned to idle.

This is a FORD EEC IV speed density OBDI system so no data stream is available for reading through a scanner. Each input and output circuit has been checked and verified independently for input and output signals. I'm stumped by the high injector duty cycle together with warm idle fixed lean HEGO at 0.0 to 0.05 volts . I suspect a vacuum leak somewhere but I have disconnected all vacuum related accessories and not found any vacuum leak. The engine sounds like ones I have encountered with vacuum leaks in the past.

I know this group can solve the problem!

You're really stressing my memory on this one! :silly: :blink: I used to run into these trucks every so often when I worked in a rural area, but less so these days.

BUT, I'm reasonably sure a data stream is available? Snap-On scanner use the Ford-1 or 1A adapter. Last couple I worked on had data, anyway. Maybe that was only on later years...

Anyway, my first thought is fuel pressure. These trucks have two pumps, low and high pressure, and both are known to fail. Though, usually it's the high pressure pump that fails first. Any chance of doing a fuel pressure test?

I'd switch from measuring duty cycle to pulse width to get a better idea of what the PCM is doing with the injectors.

Tyler, thank you for your interest in this problem and the helpful reply. I neglected to mention in the original post that the fuel pressure is steady at specified 32-35psi. while running and 40psi on initial charge of the fuel rail when the key is turned on. After the 5-8 second turn off by the processor, the pressure holds at 38-40psi for several minutes without an engine start. Pressure holds at 30-35 psi for several minutes after engine is turned off. Fuel delivery volume is within specs. Both in-tank lift pumps and the high pressure pump to the injector rail are new. This year has the screwy fuel tank diversion valve that is fed by the lift pumps. That unit was replaced as well and functions as it should. Fuel delivery does not seem to be an issue.
I can check the pulse width in milliseconds and report the values.

OK, the ECU is working the injectors hard. On the injector ground circuit at the ECU I am getting average152ms pulse width ,a 96.5% duty cycle with a frequency reading of 6.3Hz.

Remember this is a Ford EEC-IV that is MAP controlled and fires the injectors 4 at a time. MAP signal is within specs and HEGO is still fixed lean at warm idle but will respond to a transient propane enrichment. Idle is rough with random "spitty" lean misfire sound through the exhaust.

Hi Cactus Jack,

There is something i can't catch. 6.3 pulses per second, means you have the injector open 957/1000 ms (96.5% duty cycle). Is it at idle?

If so, at 2300 - 2500 for sure you will have a huge misfire (not a little HEGO oscillation)

I think maybe you are seeing inyector ground, 96.5% of the time is at 12v, which means you have 96.5% of time a no energized injector.

Try this,

Measure duty cycle or pulse width between the two terminals of injector or.

The same test points you are using, but when you increase engine rpm, you will see a decreasing number in pulse width or duty cycle (which would be correct).

If I understand correctly the operation of the injectors, on this vehicle the injectors are "batch fired" 4 at a time. 4 injectors are energized simultaneously when their 12Vv constant power supply is grounded by the ECU. I interpret the 96.5% measured at the ECU injector harness grounding pin for each of the two grounding circuits (4 injectors per circuit) to mean that the ECU is grounding (energizing) each bank of 4 injectors 96.5% of the time.I assume it would be the same measured at an individual injector as it is for the whole circuit of 4 being grounded (energized) simultaneously. These same 2 pins were showing an average 152ms and 6.3 Hz, both banks of 4 behaving similarly. Maybe I am not interpreting the data correctly or I am misunderstanding what the values represent. Each batch of 4 injectors is wired in series so they are all grounded (energized)by the ECU at the same time. In effect, 4 combined injectors act as one large one when energized ,since all 4 are energized at the same time.

I supplied 12v batt power to my DVM and attached the other lead to the injector harness ground wire at its ECU connection pin. I thought I was measuring the circuit grounding activity of the ECU on the injector harness and therefore the time (duty cycle) of injector activation. I did this for both ground harnesses, each serving 4 injectors. The values measured the same for both harnesses.

Thanks for your suggestions and I will check the duty cycle and pulse width at an individual injector.

It is right the way you measured (positive test lead to 12v and common one to injector ground pin). Now i'm sure here you have 96.5% duty cycle apply to injector).

Is it at idle? If you have 96.5% of injector time capacity at idle, it doesn't make any sense at 2500 you don't feel misfire because injector can't supply more fuel than at idle.

Now i'm stressed too.

I made a mistake describing the fuel injector circuit. The injectors are wired in parallel, not series as I mentioned in my previous post. They are fed a constant 12v power supply with the ignition switch in the "ON" position. They are alternately grounded in 2 batches of 4 when energized to deliver fuel

Guafa,
Thanks for you interest and suggestions. The HEGO sensor is fixed at 0.0v at warm idle with the duty cycles , frequency and pulse width values I posted earlier. There is no switching of the HEGO at warm idle. If I rev the engine above 2800 RPM the HEGO is active and there is switching to indicate ECU control of the injectors at the elevated RPMs. There is no hesitation or stumbling on revving, and a WOT snap is crisp and no backfire. There seems to be no fuel starvation when revved. This era of Ford engines has two built in diagnostic routines for the ECU and the sensors/actuators, and will set two digit OBDI DTCs of the era. I have run both self diagnostic routines at least three times and there are no codes generated, and the system signals "system pass". The engine runs great during the self test diagnostic routines but resumes its lean idle condition with a random "spitty" exhaust when they are concluded.

The idle air control valve is new and functions within spec. This vehicle has an elaborate exhaust air injection system because it is rated at over 8500# GVRW, The air injection system augments exhaust flow both upstream (in front of the HEGO) and downstream into the catalytic converter. Upstream augmentation occurs in open loop with air injected into both exhaust manifolds and cylinder head exhaust valve ports. There is no ECU control of the injectors in open loop, and they run off of a baseline pre-set table of values for mixture enrichment during cold start and warm up . In closed loop injected air is then diverted downstream to the catalytic converter rear chamber away from the HEGO , and it controls the injectors based on the content mixture of the non -augmented exhaust gasses. I thought there might be a malfunction of the air injection system that was leaking pumped air ahead of the HEGO in closed loop causing it to read mistakenly lean. Such is not the case, as the air injection system is functioning properly according to commands from the ECU and there are no leaks in the hoses or pipes.

I'm starting to feel like Capt. Ahab.

It is possible that HEGO heater is failing and at idle doesn't have enough temperature to react.

What do you think?

First, let's fix the issue.

Then we can talk about those "no sense" duty cycle numbers. They are still spinning around my head.

OK and thanks again for offering your thoughts. I'm currently running a second new replacement Bosch HEGO. I tested the HEGO heater circuit for continuity and voltage and both were satisfactory for HEGO function, but that doesn't necessarily mean that the heater element is functioning. I back probed the HEGO signal wire at its connection to the vehicle wiring harness for these measurements and that is where I continue to read the HEGO sensor output values while operating the engine. There could be a problem in the harness between the HEGO sensor signal wire connector and its pin at the ECU. I will check that wire for continuity under a resistance load. I will provide a separate 12v power supply to the HEGO heater parallel to the harness wire and see if there is any change in response of the HEGO to another source of power for the heater element. I will also check both the 12v power supply harness wire to the heater and the HEGO heater ground circuit harness wire under a resistance loads as well .
As you mentioned, there could be a problem with the heater preventing the HEGO from warming fully to operation temperatures. If that was the case, there would be minimal or no voltage output activity measured at idle such as I encounter. At higer engine RPMs the exhaust gasses would be sufficient in volume and heat content to activate the HEGO even if its heater was inactive. At lower RPM less exhaust gas volume and therefore less heat would be applied to the HEGO.

HEGO sensor signal wire to ECU and heater circuit wiring all test good under an applied voltage load. HEGO will respond to transient propane enrichment at idle and does generate appropriate voltage and switching at RPMs greater than 2500-2800. It stays fixed at 0.07v at idle with no switching. Checked for possible upstream air being supplied to the exhaust and HEGO by the air injection system, but that air supply is cut off appropriately by the ECU when going into closed loop at which time air is supplied only downstream of the HEGO to the catalytic converter. There's no question that the engine goes into closed loop, just that at idle in closed loop the HEGO is fixed lean and reporting what I now believe is truly a very lean exhaust condition. I even swapped in a known good ECU as a diagnostic aid and there was no change in the HEGO value of 0.07v still with no switching at idle in closed loop. The question is what could be causing the excessively lean condition at idle that the HEGO is accurately reporting. I can hear what sounds like a random lean idle misfire "spit" through the exhaust. I had the injectors cleaned and spec'd and they are a matched set.

Intake manifold vacuum is rock solid at 16" and I cannot find any external vacuum leaks, fuel pressure 35psi at the rail test port, MAP and TPS sensors test OK but I will check them again. KOER self test is a pass (code 11 SYSTEM PASS) with no DTC codes on the scanner. The throttle body bores and butterflies are clean but I still have an excessively lean idle that "rolls" a little even with a new IAC.

Thanks for any suggestions.