Too Lean Codes, Stumped.


Sep 26, 2003
Got a '96 GT with a 2001 Cobra Motor which then got setup for the 2003 Eaton terminator supercharger setup using all Ford parts from a wrecked '03. Been through about six custom street tunes this year. My tune guy has the equipment to write custom tunes and re flash the ECU. The problem, looking back, has been tuning on top of failed or failing parts. Replaced the IAC unit, TP sensor, replaced a failed injector(39lbs), finally found plugs to fire the mixture, NGK5s, etc. Both down stream O2 sensors are turned off and I have a wide band, fuel pressure gauge, boost gauge now. Also a new Aeromotive 340L pump.

Everything was working fine for about three weeks in August, then threw a code for the Evap system at the same time as the P0174 too lean code bank 2. We worked to clear up the Evap code by replacing the Evap solenoid, then the hoses going to it and away from it down the fender to the metal line, which then goes to the fuel tank. Finally replaced the Evap sensor and the code went away, but NOT the P0174 too lean code.

I have researched this code so exhaustively I could write a book on the subject. So yes, we've checked for vacum leaks, injector leaks, exhaust leak before the upstream O2 sensor, etc. I get 40lbs of fuel pressure with the line off the regulator and 30lbs with the line attached. The car runs and sounds great and holds idle perfectly. The wide band gauge, which USED to read at 14.7, now reads at 13.2 consistently any any part throttle RPM. The car runs great under boost!

I suspected the MAF sensor was under reporting the air, since the O2s were reporting a lean condition, and the fuel trim was compensating with more fuel. Both O2s report .7-.8 volts and I trust them. We tuned to a used 90mm MAF from an '01 lightening so suspecting it I checked for a base voltage, got .85, and the sending voltage gradually increased like it should.

Even still, I switched it out for a '03 Cobra MAF, got the same results and code, switched THAT out for a new Hitachi 90mm Lightning MAF, and still, got the same results, too lean bank 2 P0174. I always hated to see that code so i always cleared it out, but then i started to wait, and got the P0171 bank 1 too lean code as well. I will always get both codes if I wait for 4 driving cycles instead of 2. Now convinced this is a SYSTEM code, not a literal specified side of the engine determinant. Even replaced the PCV valve.

Disconnecting the battery to reset the ECU results in rough ide and 15s and 16s on the wide band. until the ECM "learns". Then the i,le smooths out and the A/F goes to 13.2 again. My LTFT always maxes out at 25%, then the code. Liove data is an increasing LTFT, 15, 20+. The car starts up trying to stay at 14.7, but then gradually drops to 13.2. The car runs fine, but I want my 14.7 back! It's confusing bc 13.2 is too rich...for normal driving, but the O2s are reporting too lean, and the ECU is "over compensating."

My tune guy says he set the fuel tables accurately and they don't degrade. But if he tuned the car on top of a failing Evap system would that cause the lean condition now that the EVAP is fixed?
Would reflashing the ECU help?
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Active Member
Nov 29, 1999
not sure if you found issues yet..

however..these codes I've had to fight also..

what I found..

pcv hose going under blower to lower intake..

this hose will harden...

mine would not pull off nipple,yet it spun around lose..

I put a hose clamp on it..there isn't any clamp to begin with..


SN Certified Technician
Aug 14, 2009
Houston Texas
My tune guy says he set the fuel tables accurately and they don't degrade. But if he tuned the car on top of a failing Evap system would that cause the lean condition now that the EVAP is fixed?
Would reflashing the ECU help?
My vote based upon your description is no. Why? Because the failed part is the EVAP purge flow sensor. Which won't allow air to enter or change the AF ratio. A failed open EVAP purge solenoid would allow extra air to enter the intake.

So unless the EVAP solenoid has been failed open the entire time AND the original tune was established to compensate for the extra air AND now the EVAP solenoid works, that is the only way I see the EVAP system playing a part in this.

Let me ask you. At the time the original tune was set up were there any outstanding DTC codes? If not I don't see an EVAP "issue" as the cause.

Of course another possibility is the original tune was done with a vacuum leak that that has now been corrected.
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