Picked this code up last week on my '94. Had what I thought was a bad fuel pressure regulator due to my fuel pressure not changing when I pulled the vacuum. Replaced that and its still there. Most likely a bad O2 sensor??

Larry

 

Replaced the FPR with an AFPR, and swapped in a friends set of O2 sensors. Still same thing. The car eventually throws this code and it sounds like it is operating in "limp" mode or something. Anyone have any other ideas? Are there any reset buttons anywhere I might need to trip? Thanks

Larry

 

Maybe O2 sensor or could be an exhaust leak.

 

I've swapped in a different set of O2 sensors and went over the header bolts on the passenger side up top and where the y-pipe bolts to the header. Which side is right side, the driver or passenger? Is it right side sitting in the car, or right side standing in front and looking at the engine? I can never remember that one.

Larry

 

All right and left orientation is from behind the wheel.

 

Cool. Ya, thats right. Anybody have any ideas what else I need to check?

Larry

 

The O2 sensor is most likely the culprit if you don't have an exhaust or intake leak. If you have more than 60,000 miles it wouldn't hurt to replace them both while you are under the car.

 

Car has 35,000 on it. I already pulled both O2 sensors and replaced them with a friends to see if they were OK. Still got the same thing. They were used sensors I swapped in so I guess there is a chance they were bad also.

Larry

 

Make sure the wiring is not contacting the manifold and in good shape. Did you clear the code after you replaced them? It could also be one of the following causes:

Wiring
Lean Injectors
Fuel Contamination
Fuel pressure
Exhaust leaks
Air Pump operating in closed loop

 

Wiring(will check that)

Lean Injectors(new Lucas 42lbers)

Fuel Contamination(wont rule out, but car is garaged so I dont think anyone could mess with it. Have had the same tank of fuel in it for about a month with no issues before now)

Fuel pressure(new pump and AFPR. Pressure looks good with a gauge.)

Exhaust leaks(will check again. I did have to tighten the rear bolt at the head on pass side and one of the collector bolts yesterday)

Air Pump operating in closed loop(no AIR pump)

Thanks for the suggestions.

Larry

 

Well, upon closer inspection of the wiring, I found #8 plugwire contacting the header and burnt through the silicone sheathing. Rerouted it and it runs great. No SES light and the power is back to 100%. Must have been shorting out on the header and not making it to the plug at all. Cant figure out how the cylinder not firing would show up as a lean condition since raw fuel would be going out the exhaust. Maybe because it wasnt mixing with the air for that cylinder, that cylinder was mixing straight O2 into the exhaust mix making the sensor read a lean condition?? Anyways, thanks for all the help Lyle.

Larry

 

Very easy to explain why a misfire causes a "lean exhaust" indication. "Misfires" was conspicuously missing from the list of typical probems posted above.

When a cylinder misfires, both the unburned fuel AND the free air (20+% oxygen) go out the exhaust. They go to the O2 sensor, and since it is an "O2 sensor" it can only see the unburned air (oxygen). It can not see the unburned fuel. Seeing the excess of oxygen, it sends the PCM a low voltage level signal.... the PCM says "lean, gotta add fuel".... and now we have all 4 cylinders on that bank of the engine getting way too much fuel and running rich - maybe even enough to have more of them misfiring. But the unburned O2 from the misfiring cylinder keeps passing the O2 sensor, and the O2 sensor keeps saying "lean". This continues until the long term fuel corrections (BLM's) for that bank of the engine reach 160, and the code gets set.

Looking at fuel pressure issues wasn't logical, because weak fuel pressure would have affected both banks of the engine and also set code DTC 44 for the left bank. You needed to concentrate on the things which affect only one bank of the engine, typically, exhaust leak, misfire or bad O2 sensor.

 

Thats what I was eluding to when I said the car was kind of running in "limp" mode. It didn't really sound like a misfire, but in hindsight, it could have been. It hunted for idle a bit. The cam is pretty big and has allot of overlap at idle anyway, but the car just felt down on power overall. BTW, cam is .600/.619 with a .050 duration of 248/254. Not huge, just allot of duration. Oh well, thanks for all the help and explainations. Learned something today and thats what matters.

Larry

 

That was a very good and clear explanation. I learned something new. I'm glad you got it fixed.