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Lean While Idling

murkinstock

Well-Known Member
Joined
April 8, 2013
Messages
308
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10
City, State
Salem, Oregon
Year, Model & Trim Level
'99 Explorer Sport
Hi everyone. Ive had this issue with my truck for forever. It even has a completely different engine now, but it's still running lean while idling. (according to the long term fuel trim)
The slightest but of throttle, and it disappears.

Otherwise it drives normal, sounds great, just worse MPG than I believe it should have.

I know I know, you're thinking, aha! Vacuum leak! Well that's what I've thought too, but I've tried EVERYTHING to track down vacuum leaks, to no avail. The only thing I haven't done, is replace the maf or idle control valve, because I want to be sure it's the problem.

Tried smoke, I've sprayed everything! I've removed vacuum lines, and plugged them to see if the fuel trims would go normal, and nothing. The canister purge on the throttle body, nothing. I've cleaned the throttle body and maf, nothing. I've reset the ecu countless times.

I'm beginning to think, it's just kind of its thing, and there's nothing that will fix it. Maybe a complete new wiring harness, and ecu? Idk.

Any thoughts? It is a mystery!
 



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Does the long term fuel trim still indicate lean when driving a steady speed, faster than idle? If so I would suspect a vac leak or MAF sensor is dirty (but you stated you already cleaned MAF).

If not, I'd take the IAC valve off, clean it out with carb cleaner, put a drop of light oil on its internal shaft right where it enters into the bushing there and work the oil down into it, take the felt filters out of the plastic cap and clean them too, then reinstall.

Some IAC seem too worn out to give a good 2nd tour of duty after cleaning but if the IAC is the problem then it should improve the situation for at least a little while. I would expect yours to need cleaned by now if it is the original, or even if a replacement that has seen a few dozens of thousands of miles.

I first noticed my IAC needed attention because RPM would drop at idle, every time the A/C compressor cycled on. However this was years ago, I didn't check for a lean condition at the time because cleaning the IAC was free, quick, and easy.
 






I actually cleaned it awhile ago, but I don't recall if I oiled it. Seems like something I would do, but I don't remember. I do think it works okay, but it still might not be closing 100%. Probably time to check it again. Also I didn't know about the filter! Will definitely check that too!
 






While driving, the fuel trims seem to almost always stay a little lean.

It'll be 10-11 while idle. Around 6 under moderate throttle, and about 3 under full throttle, while driving. Seems to get closer to 0 at high rpms

I think my previous idea of the lean going away under a tad throttle, is while it's still colder, or because it wasnt long after resetting the ecu.
 






A fuel trim of 6 or lower isn't necessarily bad for an aging engine. How about the things that carried over from the old engine? Same fuel pump? Fuel filter? Might check fuel pressure, could be low. Same 02 sensors? Might be reading low. Then again since your fuel trim numbers aren't that high, you might have a tiny vac leak.
 






That's what I'm thinking, either so tiny it's near impossible to find, and or it's just old

As for the fuel pump and filter. They're both Motorcraft, replaced about 10k miles ago. The o2 sensors have less than that, and they're Bosch. The engine has all new gaskets. Intake and everything is swapped from the old engine.

I'm thinking about just trying to forget about it now. Haha
 






You stated "it even has a completely different engine now". What year engine was installed? Does the engine that was installed use the same "calibration" as the original engine?
 












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