I have posted before with not many responses. So I will try again. I have a '94 f150 with a 302. Last summer I got hesitation/backfire/check engine light. Computer showed code 172 (o2 sensor reads lean) I checked for vacuum leaks. Didn't find any. So I replaced o2 sensor (and the egr and pvc while I was at it) Truck ran beautiful for 2000mi. Then....hesitation/backfire/code 172 again. Checked vacuum lines again. Said hell with it and replaced them all. Same. Replaced the new o2 sensor. Botta Bing! Ran like champ...for 2000 mi. Replaced the second o2 sensor! Ran like a champ for...you guessed it 2000mi. What is going on?
Maybe the o2 sensors change slightly over time and you reach the critical point at 2000 miles because another component in your engine is the real culprit.
As a simple example, you put a new serpentine belt on and it goes for 2000 miles before it gets chewed up. You replace the belt again and it goes for another 2000 miles before it gets chewed up. The real problem is a misaligned pulley in this example but you see the results as the belt wears away until it gets to the point where it shreads or breaks.
Does this make sense?
Did you examine the sensor for evidence of contamination?
Yes, as you posted I have looked for the "other culprit" to the o2. No I did not do a conversion. I have a stock 94. I just used the term Maf. Did not really know there was a diff.
Anyway, I have the SD unit apart right now. It "looks" clean. Yeah I know it can look clean, but be dirty. What do I clean with? Brake cleaner and Qtip? Alcohol and Qtip? Air comp? How much is a new unit? And I notice to disassemble I have to have a torx bit with the little safety nipple to do so. Thanx Ford
Thanx in advance for your efforts. I am sure I will eventually eliminate the source. Why diagnose when I can throw parts at it.
The page you referenced had as one culprit: external oil spillage. coincidently I had the leaky oil pan gasket syndrome, and the leak got down onto the o2 harness. Hmmmmm. I had wondered if there was a connection. (get it!) I have fixed the leak, and replaced the o2 in the meantime, but still got the o2 failure. It took a little longer to "blow" the o2 (5000mi) that time. Maybe I will spray more more brake cleaner into the harness and blow out with comp. air. Again. I guess oil in the harness plug would reduce the amout of volt. getting to the computer; although the inside of the plug "looked" clean. One possibility.
If your code 172 (which you say means lean) is not happening because of extra air introduced into the engine then the other side is that not enough fuel is introduced into the engine to mix with the air. The fuel injectors (if thats what you have) may not dumping enough fuel into the mixture because the computer is getting bad information from the air flow sensor or if the air flow sensor is correct then the fuel pressure is too low or injecter(s) are partially clogged.
Yes, the o2 sensor reading lean code only tells me just that: I'm lean. If the injectors were clogged/not pressurized or the fuel pump was weak, I would still get the hesitation/backfire/code 172 even after I put in a new o2. But I get 2000-5000mi of beautiful running engine. During that 2-5000mi period it runs PERFECT.
Another line of thought: I know that my o2 sensor is the 4 wire type meaning it has the little heating element in it to make it read to the ECU faster rather than wait for the exhaust to warm it up. I have wondered if there is a problem in that circuit leaving the little element on all the time? Possible? I know: grasping at straws.
What I am trying to say is that the new O2 sensor may not indicate a lean reading until it has aged a bit and its characteristics change.
Here is a link I used to suspect the fuel injector or fuel pump problem. It describes your o2 sensor pattern.
Question: 1995 Ford F150 5.0 mileage: 232,000. I had a long block installed at 204,000 miles. About 5,000 miles ago my check engine light came on, the code indicated it was the 02 sensor, so I replaced it. The light came on again 1500 miles later, again the code said it was the 02 sensor, so I replaced it. About 1500 later again the same story, so I got a different brand. This one lasted about 2000 miles and again the light is on and the code says it is the 02 sensor reading lean. What else could the problem be? Could it be giving me a wrong code? The code is 172.
Answer: The 0≤ sensor reading lean is probably caused by a malfunction in the fuel system, possibly a plugged fuel injector or a weak fuel pump.
I found the problem with mine....a bad ecu....there is suppose to be between .1 and .9 volts...there was none...got another box and the problem went away...and gas milage almost doubled from 10mpg to 18mpg...
__________________ 1995 F-150 Eddie Bauer
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