Runs Lean, no codes

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Mar 22, 2005 | 02:53 PM

j1mk

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Runs Lean, no codes

I hope I can make some sense of this. My 1993 Bronco/4X4/5.8 EFI/Auto has developed a mysterious problem. At 205,000 miles I had the engine / trans / transfer case rebuilt. With the rebuild I also replaced every sensor, TPS, AIT, and distributor. For the next 20,000 miles it ran wonderfully. One day it began running lean on a trip from New Mexico to Arizona. It got so lean, it would barely run, and it toasted the catalytic convertor. Since then, it will do it on almost every trip out of town. The only code it has ever thrown is for the EGR, twice, not related to the lean run, and that has been replaced (twice). The mysterious part is, it runs great for short trips, and around town but begins leaning out after 50/60 miles at freeway speeds. A re-start will clean it up for the next 50 miles. The only thing I can think of is perhaps the throttle body is worn at the shaft and if you get it in the exact position, you get a vacuum leak. Does this sound reasonable?? The no code part has me baffled.

Mar 22, 2005 | 04:52 PM

greystreak92

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Joined: Sep 2002

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From: Gateway to the West

Could be a bad fuel tank vent. Short trips would allow the poor venting to compensate while the truck was shut down. Could also account for the "resetting" (letting you run another 50 miles or so) after you shut it off.

Mar 23, 2005 | 08:56 AM

j1mk

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Jeez... I never gave that any consideration. Now that you mentioned it, I did replace the little vacuum hose at the top of the canister before this happened. it was cracked and pretty ugly, was probably leaking..

Thanks.. great idea. I will try that tomorrow when I go to Flagstaff..

Mar 23, 2005 | 10:31 AM

kameronth

Cargo Master

Joined: Nov 2003

Posts: 2,000

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From: North Dakota

I had a problem a little while back with mine running lean. The only code I got was a code that told me the engine was running lean. So I replaced my fuel filter and it still was lean. Finally I replaced my fuel pressure regulator and that took care of the problem.