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Thread: Part throttle stumble-afr goes lean on tip in

  1. #1

    Default Part throttle stumble-afr goes lean on tip in

    Hey guys, I have a ls2 (the in between 24x version with 1x cam) with ls6 intake manifold with the atomic ls efi system on it. Car starts fantastic, idles great, comes to a stop fine, Wot is no problem, getting up to highway speed normally is fine, it's the in between gentle cruising and WOT that the car will pop from the exhaust almost like a machine gun until you let off.

    I have idle afr at 14.7, wot at 12, and part throttle at 14.2. I have left the timing as is for now.

    Engine has a mild cam, a comp thumpr 275 and headers.
    Fuel is an aeromotive 340 in tank with a return using an aeromotive fpr set at 58psi. Injectors are siemens 840cc (set to 92.3lb at 58psi on handheld).

    I've tried new plugs, new coils. Compression test is good.

    Using mild cam setting, rest of the settings I've confirmed many times over.

    When I roll onto the throttle at about 60%+ throttle position my afr stays at around mid 14-15 instead of getting any richer. I believe it's a lean hesitation. It's like the ecu skips any transitional period between cruising afr and wot afr. The only way I've been able to get rid of this is set part throttle to ~13:1afr but then my steady throttle periods like on the highway im super rich and the car guzzles fuel.

    Any thoughts?

  2. #2


    What are your TPS and MAP enrichments set at?

  3. #3


    I have them set to 15 tps and 8 map. It seems to like it.

    The major issue I've noticed today when I was messing with it is below 2200rpm if I roll into it and it starts sputtering the timing is down to 8-13 degrees, then as soon as I get above around 2500rpm the timing jumps up to 25 or so and it clears right up. Why would my timing be going so low under part throttle conditions?

    It runs fantastic under all other conditions.

  4. #4


    Where are your other readings? IAT when it's acting up and when it clears up. Do you have any advance set into your timing for idle or part throttle?

  5. #5


    Iat is slightly above ambient, nothing wild. I bumped up part throttle timing +8 and idle +3. Operating temp hovers right around 200. IAC is at 0 in gear at idle. Should I raise this to 10-30? Could that be an issue? It used to be in the 60's and I adjusted it because it would stall at stops before.

    I reset everything and tried performance cam. It doesn't lean out on tip in as much and runs great. Just that timing dropping to 10-13 makes it sputter and burp until it ramps timing back up to 25-30 then it clears right up at around 2k rpm.

  6. #6


    My IAC is about 20 in gear 0 in neutral. My TPS is at 18 and MAP at 12. Mine runs much better on mild setting although the cam is alittle larger custom grind turbo. Just curious as to why such large injectors if your running N/A if your going power adder I would understand.
    I'm also running 14.9 at idle 14.8 at part and 12 at full then 11.2 under boost. I have no adjustments on timing.

  7. #7


    IAC shouldn't effect part throttle timing or fuel. Do you have the latest files installed in the system?

  8. #8


    Whoops, went to edit last post and it deleted.
    Yeah, I have the latest firmwares unless there is something newer than what is listed on the msd website.

  9. #9


    I do have a file that is newer than the 1.3.0 that I believe is on the site. I have a 1.3.67 that an MSD tech had sent me. It did help a few issues I was having. But I'm not sure why it had not been put on the site.

  10. #10


    Could you please email it to me? I'm on 1.3.0 is my email.
    Much appreciated

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