CEL: Running Lean only after 3hrs of highway driving?

[bigredscowboy]

I've had this problem for years now (since I bought it in 2003) and although I never checked the codes before I put a new-to-me engine in, I had the same problem before and after. After about 3-4hrs worth of highway driving over 70mph it never fails that I get the CEL (EEC-IV) 172 (sometimes i've also gotten 171, and sometimes bank 2 will pop up) Lean bank #1. I can drive at 60-65mph for days and CEL does not come on. Slowing down will make it go off temporarily but only letting the engine cool completely will make the CEL go away until the next 3-5 hr trip.

Perhaps pertinent:

• Engine was a junkyard find with 80,000 miles but the old one with 165,000 did the same thing
• Compression is 160-170 (although it takes about 15 cycles (about 15psi increment/cycle) to get it up on every cylinder)
• Sprayed brake fluid all over the intake and heads and no fluctuation in the idle.
• Had some rough idle starts recently (during cold weather), but usually fine after warms up (with one unusual stall in Neutral after a quick stop, using engine braking)

Not sure if the following matters but I'll list everything:

• Have a K&N but the problem also occurred before this switch
• No telling how old the MAF, fuel injectors, O2 sensors are. But from all the posts were these have been a factor, it doesn't appear that the OP's CEL is going away like mine.
• 15,000 mile Mobile 1 gets pretty dark after only 2-3000 miles.
• NGK plugs were clean with the slightest tanning (only 6,000 miles), although the gaps were over .060" -- I just fixed the gaps today, so I won't know how that affects anything for a while
• Usually use 89 but sometimes cheap out with 87
• Fuel Pump Relay recently blew after I redlined, but again, the problem is almost 10yrs old with no other fuel pump problems

Any suggestions, because even if I replace the MAF and O2, I won't know if I fixed the problem until my next long trip, and I don't foresee one until the summer.

 

[Big Jim M]

Seems obvious the problem is from heat-soak. I'd first be suspicious of the 02's. They can give that lean condition at will.
Might have soimething to do with the exhaust system as a whole being just a bit clogged...
Big JIm

 

[Yellowsplash]

Im gonna take an educated guess that either some of the oil from the K&N has gotten on your MAF element causing it to read lean over a long period of time thus changing your longterm fuel trims much leaner, or the possibility of exhaust leaks before the O2 sensors, or even a combination of the two...