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Thread: Leaning out under light/heavy throttle.

  1. #1

    Default Leaning out under light/heavy throttle.

    Hello All. I'm at whits end with my Atomic EFI. I have had the unit since they came out (5 - 6 years old). Over the past two years (no trouble previously), I have noticed that under light throttle the A/F ratio will go lean (14-20:1+ above 2,000 RPM) and the MAP will increase. (1/8-1/4 throttle). When all this happens, the car falls flat on its face. After many calls to the MSD tech line I have done the following to attempt to sort out the issue I have done the following one at a time:
    - Cleaned old MAP sensor first (no change).
    - New MAP sensor (No change).
    - New WBO2 sensor (No change).
    - New throttle position sensor (no change).
    - Complete Tanks Inc EFI fuel system (no change).
    - Complete new fuel return line system (no change).
    - New fuel filter (no change).
    - 1/2 carb spacer (no change).
    - Updated firmware last week (8/22/2020) (no change).

    What is there left to do other than to send it back? It starts good, idles good. I'm getting the appropriate amount of vacuum at the manifold. IAC hovers around 10 - 15%. As soon as I give it any gas while in gear, it goes way lean quick and I notice the MAP sensor climbing. I've been told that under any throttle, the MAP sensor should decrease not increase. I have taken the unit apart two other times to make sure the MAP sensor was installed correctly and no pins were bent (none were).

    What I have:
    Ford 302 (in a '66 Mustang)
    Cam: Comp Cams 291THR7 - LSA 107°
    Aluminum heads
    9.5 CR
    5 speed transmission
    Anyone have any good ideas?

  2. #2


    What's the fuel pressure when the problem occurs, if you briefly clamp the return line closed the maximum fuel pressure should be produced? If it stays low, suspect a weak fuel pump, pump power or gound issue, blocked fuel filter or in-tank sock. The MAP sensor measures the pressure in the intake manifold, so the reading will increase as the throttle is opened. To test the accuracy of the MAP sensor, with KOEO the reading should display the BARO at your altitude. To calculate manifold vacuum, BARO - MAP at idle = intake manifold vacuum. Compare with a vacuum gauge and see if they match.
    Last edited by tcm308; 09-02-2020 at 09:33 PM.

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