Service Engine Soon

Ok, I know this has come up before, but I have been hearing different things fro different people. The engine has over 300,000 miles on it. Yes that is on the original motor with no rebuild. The only thing that has been done is I replaced the intake manifold gaskets. It is a 1996 with the 350ci motor. The engine still runs strong but burns about 3/4 a quart of oil every 1,000 miles. A recent thing I did to it was the K&N intake (FIPK gen 2). It was fine for a couple days, then I got the light and decoded it to reveal P0171 lean bank 1. One of the exhaust manifold bolts by cylinder 1 is broken off, and some guys are telling me that is what is causing the code. Is that possible? The service engine soon light has turned off for about a hundred miles and turned back on. This process seems to continue being the trend. I haven't tried the elctric parts cleaner yet, but I did read about it and will try it. Also does anyone have the torque specs for the exhaust manifold bolts. Thanks for the help.

From my Chilton Manual, the torque spec on the outer bolts is 20 ft-lbs and 26 ft-lbs for the center ones. This says it is for the 88-95 models though and the 96-98 models is not shown. Sorry, thats all I have...

Just found the note for the 96-98 models. Step one is 15 ft-lbs, step two is 22 ft-lbs. Hope that helps!

I think you know already but I suspect MAF sensor contamination from the K&N (oiled right ?). It may have just tipped the F/A ratio scales a bit.

I'm guessing your O2 sensors may not be exactly new either. BTW, They should last 100K miles on this engine.

Thanks guys, I will probably go out and get new O2 sensors. I don't think they have ever been changed. I'll tell you what though, that motor still runs strong. My dad bought it new and I bought it off him. I'm amazed at how well the motor still runs.

So just to clarify, you don't think that broken exhaust manifold bolt could have anything to do with it, right?

I always go to the GM Shop Manual at local library when diagnosing codes but I 'suspect' a lean code is thrown by an O2 sensor reading ( O2 does a trim +/- 128 ticks, when out of range I believe rich/lean code is triggered).

If the O2 sensors have not been change in 130K miles it may not be accurate as life expectancy is only 100K for the type in our vehicle 5.7l. If much over the 130K miles then the O2 sensor is probably not working perhaps not at all. In this case I don't know how a lean situation would be detected.

Vehicles can run fine and often pass smog without any O2 sensor, I ran a porsche 911 for ~100K miles without one working in California. If no O2 signal then the ECU will default to a preset table with no trim from O2. I had a 1991 Range Rover that ran ~80K miles with dead O2 sensor and no codes passing smog.

The exhaust manifold bolt has been broken awhile <I suspect> with no code. The change was K&N filter. Over oiling can contaminate MAF sensor wire which would tell ECU that less air coming in than actual and to reduce fuel causing a lean condition. There could be air leak(s) down stream of the MAF that resulted during your modification.

So, verify what is causing code (GM service book is great on diagnostics). The error could be a sensor (O2 or MAF) but start with the error detect sensor (O2). Replacement is not absolutely necessary but requires measuring the electrical signals at the O2 sensors needing an adapter to get to the connections OR you can measure the resistance and get some idea if they are working at all.

There are 4; bank 1/2, up/down stream of cat. They are heated also.

Ok, I know this has come up before, but I have been hearing different things fro different people. The engine has over 300,000 miles on it. Yes that is on the original motor with no rebuild. The only thing that has been done is I replaced the intake manifold gaskets. It is a 1996 with the 350ci motor. The engine still runs strong but burns about 3/4 a quart of oil every 1,000 miles. A recent thing I did to it was the K&N intake (FIPK gen 2). It was fine for a couple days, then I got the light and decoded it to reveal P0171 lean bank 1. One of the exhaust manifold bolts by cylinder 1 is broken off, and some guys are telling me that is what is causing the code. Is that possible? The service engine soon light has turned off for about a hundred miles and turned back on. This process seems to continue being the trend. I haven't tried the elctric parts cleaner yet, but I did read about it and will try it. Also does anyone have the torque specs for the exhaust manifold bolts. Thanks for the help.
An exhaust leak before the O2 sensor can cause the O2 sensor to read a false lean condition and the PCM will compensate by attempting to adjust the fuel trims.
If the PCM cannot compensate for the false lean condition, the MIL will come on.
I would fix the exhaust leak first..then retest.