5.8L O2 sensor stuck on lean
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5.8L O2 sensor stuck on lean
The O2 sensor on my '88 F-250 EFI HD is stuck on lean at idle and does not vary between rich and lean the way it is supposed to. The sensor is new and does respond to propane enrichment. Vacuum is rock steady at 16" and there are no vacuum or EGR leaks. Fuel pressure at the rail is right at spec and the MAP sensor tests OK. The O2 sensor does begin to switch at 2500 RPM and above, but returns to a steady 0.0 - 0.1 volts at idle. The vehicle is completely stock. Any ideas about the cause(s) of the problem?
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From: Ottawa, Ontario
The O2 isn't the problem then but it is showing you there is a problem elsewhere. The computer controls the mixture so pull codes to see if it has flagged anything, at this age the computer itself could be the problem too. There could also be 1 cylinder running rich because of a dirty or stuck injector, that will cause the computer to pull too much fuel out of all the others in an attempt to compensate. A look at the plugs will tell you if that is the case, also if fuel pressure drops to zero immediately after the engine is shut off that also suggests a leaking injectors... although there could also be an issues elsewhere in the fuel system but that won't cause a lean running condition.
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OK and thanks for responding with your suggestions. I have concluded that the engine is truly running lean and that the O2 sensor is accurately reflecting that condition rather than the sensor itself being the problem. Now to find the cause of the lean running engine. I cannot find any vacuum, EGR or exhaust system leaks which may be adding air to the upstream exhaust and fooling the O2 sensor. The duty cycle voltage readings at the injector harness connections to the ECU indicate that the computer is already enriching the engine considerably which adds to the confusion and mystery. KOEO and KOER show no fault codes in the system. I have changed the computer with two other known good units and still get the same stuck on lean 0.0 -0.1volt reading at idle. At the exhaust tailpipe there is a faint sputter sound indicative of a lean misfire. Fuel pressure at the rail is at spec and holds well after pump turns off. I had the injectors cleaned and checked/matched for flow and spray pattern. I will pull the plugs and see what they show. I am beginning to suspect an ignition problem that may be causing a misfire. Thanks again.
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Yes, I did a scan KOEO and the self check KOER. The ECU put the engine through the whole KOER sequence and the scanner gave a code for the fixed lean non responsive O2 sensor. That tells me the engine is truly running lean since the sensor is new and responds to propane enrichment. I think I might have an ignition caused misfires that are loading the exhaust stream with unburned oxygen which the ECU reads as a signal to increase the duty cycle of the batch fired injectors. The duty cycle readings suggest the ECU has early maxed out on its ability to compensate by enriching. I can hear a random spitty sound at the tailpipe during idle that suggests lean misfire. The IAC valve is functioning correrctly. I'm anxious to look at the plugs. The older EEC IV diagnostics cannot detect misfire faults that are ignition or injector caused like the newer OBD II systems can.
FTE Legend
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And you are positive the EGR is closed at idle? Any change in A/F ratio if vacuum line is disconnected from the EGR at idle?
What about the air injection system, is it all there and functioning with no leaks or partially/completely removed?
What about the air injection system, is it all there and functioning with no leaks or partially/completely removed?
Logistics Pro
Joined: Dec 2003
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From: Foothills of NC
My thought is to make a sheet metal block off plate for the egr and test it.
I made one for my truck many years ago, and never took it off. The truck runs good, I no longer worry about the vaçuúm solenoid leaking, as I left it in place and put a ball bearing in the hose going to it. The exhaust pipe is as clean as a 2020. No more soot buildup at all.
I made one for my truck many years ago, and never took it off. The truck runs good, I no longer worry about the vaçuúm solenoid leaking, as I left it in place and put a ball bearing in the hose going to it. The exhaust pipe is as clean as a 2020. No more soot buildup at all.
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Hi and thanks again for your suggestions. The Thermactor System (AIR) is complete, functioning correctly, and there are no leaks. The diverter valves do not leak and have tight connections to their hoses. and pipes. This is a completely stock HD 5.8L , so there is air injection upstream at both the exhaust manifolds and into the ports at the rear of the cylinder heads. There is also downstream air injection into the rear bed of the catalyst. The system is functioning correctly as commanded by the ECU. There is no upstream air delivery from the Thermactor system in closed loop engine operation. I suspected this might be a source of air to the O2 sensor but it is not. Closed loop operation delivers air only to the rear bed of the catalyst, downstream from the O2 sensor. The EGR valve is completely closed at idle. The pintle is clean of carbon rdeposits and seats properly to close the valve when not commanded open. When vacuum is applied from an external source to the EGR valve at idle the engine stumbles and will eventually stall. This is normal operation.
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Logistics Pro
Joined: Dec 2003
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From: Foothills of NC
The only epa test here for this model vehicle is a visual test. As long as everything is in place, it will pass. My egr plate is thin sheet metal sandwiched between the egr and manifold, trimmed to look like the original gaskets. Its basically undetectable.
As clean as the tailpipe is compared to how it used to be, which wasn't bad, but not clean like it is now, I'm glad all that exhaust is not being fed back through the cylinders.
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As clean as the tailpipe is compared to how it used to be, which wasn't bad, but not clean like it is now, I'm glad all that exhaust is not being fed back through the cylinders.
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