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Discussion Starter · #1 ·
1999 1500 5.9L 160k miles. My beast has been running fine until a few weeks ago. I started noticing a minor misfire at idle and at speed. Tested and found no vacuum leaks but the problem kept getting worse, so I figured the old girl was due a tune up ... cap, rotor, plugs and wires. Now it is even worse. No check engine light. Got out the scanner and along with a P0171, it showed every single cylinder misfiring. Started looking at O2 voltage and short and long fuel trims. In closed loop, upstream O2 is not oscillating siting low but it does change with rpm changes, so I was suspecting that might be the issue. Downstream O2 pretty much echoes the upstream .. so now I'm wondering also a possible cat issue? But I looked and short term trim is maxed out low and long term trim is maxed out high...is that possible?!? Maybe the ECM has lost it's mind? Any help would be wonderful.
 

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Discussion Starter · #3 ·
I can't say for absolute positive, but I looked down the throttle body and did not see any oil and I don't have an oil consumption problem. But I guess that doesn't rule the gasket out...hmmm. Maybe I can rent a boroscope and look into the runners or something.

But what would make the fuel trims go in opposite directions? Short term going lean and long term going rich? That part really has me puzzled. Thank you for the advice. I will look into that deeper.
 

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Well mine had oil in it but couldn't really see where the leak was. The leak throws fits with the sensors. I put a timing light on mine(at idle and with a little rpm) and you could see the spark would cut out for split second and when it started it again it was not where it should(well before). This is what give the back firing threw the tb that many experience.
P0171 1/1 Fuel System Lean
Does this have 4 02's?
 

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No, just 2 O2 sensors.

And I can find no oil in the intake

A neighbor stopped by and suggested pulling a battery terminal for a while to clear out any 'learned' bad habits, (DOH! Why didn't I think of that?!?) so I did that. I started it and let it warm up, and the idle was smooth as could be. After 5 to 8 minutes mainly idling with a few revs to 2k now and then, It ran like a new truck! I looked at the fuel trims and they looked normal. Just slightly pulling to rich on the short term ( 5% or less ) and pretty much 0% on long term trim. I took it out and put a couple of 10 mile trips on it and it ran great until the last 4 or 5 miles. Then it started doing the same thing as before.
I can't say for sure at what exact mile or how many minutes passed when it started acting up, but it ran great for such and such a period of time and slowly I started noticing a slight miss which steadily started getting worse until just before getting home it barely topped the last small hill without having to really nail the gas. Just over about 2500 rpm it cleared out and I noticed a HUGE sudden jump in power...like when you hit the power band on a 2 stroke dirt bike. Got it home and hooked up the OBDII again and sure enough. O2 sensor 1 was not responding and was staying low and the fuel trims were like before, short term pulling full lean and long term pulling full rich.

So now it seems that something in the system is going whacky over time or prolonged heat?

I HATE throwing parts at a problem, but I am stumped. I am going to go ahead and replace the upstream O2 sensor today and see what changes, but I am growing more suspicious about the ECM.

Thank you for your help.
 
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