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Thread: AFR too high on WOT.

  1. #1

    Default AFR too high on WOT.

    I've been having some issues with the tune, which had been quite stable. The AFR has been going into the yellow & red zone on acceleration and when I try WOT the engine bogs. Could this be something as simple as a blocked fuel filter? The fuel filter is upstream of the pump near the tank, so I need to have the car up on a hoist to check. Is there anything else that could affect the actual AFR?

  2. #2


    If it was me, I'd start by looking at the datalogs to see what was going on. If the fuel map is calling for fuel, but the pump can't deliver it, then you have a fuel pressure problem. Low fuel pressure at WOT could be a fuel pump problem or a clogged filter or a fuel line that's too small or kinked. If the fuel pressure is steady at WOT, then you have a different problem. You might just have a bad fuel map.

  3. #3


    If the fuel filter was blocked, I wouldn't expect you to have too high of AFR, but the opposite.
    Did this problem just start, or has it been like this since you put the system on?
    We really need more info; engine specs, cam, etc, a data log file and tune file.
    You're asking a question that has two dozen different answers.

  4. #4
    Join Date
    Jan 2010
    Brisbane, Australia


    High AFR meaning high numerically, i.e. 17:1 instead of 13:1?
    Better to state lean 17:1, versus rich 11:1.
    So is this the fault you're chasing, a lean condition? Gary
    Last edited by Gaz64; 02-14-2019 at 03:25 AM.
    Regards, Gary

  5. #5


    Sorry to the last poster. I tried to edit my last post and ended up deleting it. So here it goes again.
    Engine specs:
    Chrysler 440 bored out to 452.
    Fresh engine, about 1,500 miles old.
    Compression - 9.3:1
    Cam - not sure, but more than mild.
    Original heads with some porting.
    Headers & dual 2 1/2 exhausts.

    The car is a Jensen Interceptor and I have done around 1,000 miles in it with the Sniper installed. The first problems happened around Christmas when I was 100 miles into a 210 mile drive. The engine started to lose power and run like a dog. I noticed that the fuel flow was >70 lb/hr when I had been cruising at a fuel rate of around 35 lb/hr. The conditions were cold & wet.

    Subsequently, the Jensen ran well until a couple of weeks ago when under acceleration the AFR went lean anywhere from 17:1 to 20:1. The more throttle the leaner it went. The fuel flow also went high in the >60 range. Timing didn't change. We have had a very warm spell with temperatures of 32°F plus (90°F plus) and coolant temperatures have gone to 200°F plus.

    I changed the fuel filter to see if that would make any difference, but no change. It does run well for the first 15 minutes. I have changed back to my last good fuel map as the "learning" with the high AFR messed around with the latest map. I have not changed the timing map as it seems fine.

    Any advice appreciated. I can try a new WBO2 sensor to see if the original is faulty.
    I've had difficulty with retrieving the datalogs, so will have another try this weekend.

  6. #6
    Join Date
    Nov 2014
    Ont. Canada


    Sounds like fuel delivery to me. I would look at the fuel pump first.

  7. #7


    For closure on this thread, the Holley fuel pump was faulty. Warranty replacements are a long way and long time from New Zealand, so I bought a FiTech pump. Problem solved. I rolled back to an older map that had no AFR issues and the car drives as it should!

  8. #8


    Not a surprise. Another Sniper problem that wasn't actually a Sniper problem.

  9. #9
    Join Date
    Dec 2009


    And most relatively new fuel pumps don't die, they're murdered (meaning something caused their failure).
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

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