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03 lincoln ls 3.9L interesting fuel trim fix
Posted to Technical Tips Forum on 11/14/2012 8 Replies

Has P0108 active and P0401 stored codes. Both banks running lean at idle. Long term fuel trims maxxed out at +25%, short term switching around 0%. No lean codes active or pending. Need to get to pass OBDII emission inspection.

Clear codes at KOEO and P0108 comes back right away. Use mutimeter and scan tool to verify EGR MAP sensor voltage KOEO. High @ 4.8 volts, spec is around 4 volts. Tap EGR assembly with screwdriver handle and voltage comes back down to 4.1 volts. Connector and connections OK, added some stabilant. Also had P0401 previously, and the EGR assembly is a common pattern failure item. Remove valve and checked passages, nice and clean. Good vacuum to valve also. Replaced EGR assembly with motorcraft unit, verified new MAP sensor voltage KOEO, 4.1 volts, code did not reset on next key cycle, fixed P0401 and P0108. This car uses the MAP and MAF to calculate the BARO reading, MAP and MAF performance are crucial.

Interested on why this car was running lean? Here's what was checked concerning lean condition: air filter/intake restriction, vacuum leaks (checked with smoke machine, throttle body cleaner, propane EVERYWHERE on this engine, nothing), vacuum gauge was steady at idle on 20 inches, checked PCV system, ok (isolated pcv from intake/throttle body to vent crankcase to atmosphere for diagnostics), checked all hoses/plastic vacuum lines, brake booster, VCT system operation (verified it was not being duty cycled or that a cam gear was not stuck), lab scoped cam/crank correlations (both cam sine waves were between the 7 to 10 crank wave interval, so engine is timed properly and no jumped teeth), sealed and smoke tested plastic air intake assembly/tube to look for minute cracks in hopes of finding a leak, none. Smoked crankcase, sealed. Fuel pressure and volume, good. This car does not run rough or stumble at idle and has excellent power on acceleration. Here was the frustration concluder: when CEL came on, customer took to advance auto parts and they scanned it and told him that P0108 didnt mean MAP sensor it meant MAF sensor. He told the customer this car was only a MAF car. So the customer took the car home and ordered a MAF off of ebay. It was cheap and new! He installed it and the codes still came back. Little did he know that the MAF he just bought did not generate a proper voltage for the PCM. BARO was in spec though. Ford spec says 0.6 to 0.9 volts at idle is good. Well, his sensor only showed 0.6 volts at idle which was at the lower end of the spec. So with no more tricks up our sleeve at this point, ordered a reman sensor with actual ford markings on it. After installing it, fuel trims came back down and were correcting properly. Grams/M showed exactly the same on either sensor, the only difference was 3/10's of a volt and in computer language, that can small difference can translate into a big change. The new sensor had 0.9 at idle, high end of the ford spec. and that fixed it. Customer said he had the old MAF still, so we installed it and it checked out ok. Removed the one we purchased and returned to store.

So basically he had one problem, installed a second problem, then brought it to us. Gotta love diagnostics! Hope this helps someone with the MAF specs when looking at fuel trims on this car.

Tracy Marion
Stokes Tire
King, North Carolina, USA

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car Vehicle Data

2003 Lincoln LS LSE 3.9L

Engine3.9 L
Trans5-speed Automatic (Electronic)