17 counts of knock, 105% injector duty cycle at autocross
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Joined: Dec 2014
Posts: 31
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From: Georgia
17 counts of knock, 105% injector duty cycle at autocross
About 8 seconds after launching my X in the 4th run of a local autocross race, the car hesitated, 6 seconds later it got worse with the service light flashing so I coasted back. Also there was a fuel smell in the cabin. Next run I risked going out again, pushed it hard and everything was perfect. Also the first 3 runs were all fine.
Weather was 80 degrees (Atlanta). There was a delay before that run, I had to idle the car in the grid for at least 10 minutes. There was a long left turn before the issue occurred.
At home I took a look at the logs and noticed at both knocking events I had more than 100% injector duty cycles. See Accessport log attached.
Any ideas what may have caused it? I can't have bigger injectors due to class rules. I checked older logs, went back 2 years and I have had a few occasions with more than 100% IDC (most was 108%) and I have had some knocks here and there (7 knock sum was the highest) but never together like this, or this severe.
Car has K&N Typhoon intake and UICP, Cobb 3 port EBCS, Tomei Expreme Ti cat back exhaust, AMS high-flow cat, AMS FMIC, Cobb LICP, Tial QRJ BOV with 1.5psi spring
Fuel pump relay was upgraded and spark plugs replaced 4k miles ago. No boost leak to my knowledge.
Tuned by DBW Motorsports on a dyno.
EDIT - no DTC was thrown
Weather was 80 degrees (Atlanta). There was a delay before that run, I had to idle the car in the grid for at least 10 minutes. There was a long left turn before the issue occurred.
At home I took a look at the logs and noticed at both knocking events I had more than 100% injector duty cycles. See Accessport log attached.
Any ideas what may have caused it? I can't have bigger injectors due to class rules. I checked older logs, went back 2 years and I have had a few occasions with more than 100% IDC (most was 108%) and I have had some knocks here and there (7 knock sum was the highest) but never together like this, or this severe.
Car has K&N Typhoon intake and UICP, Cobb 3 port EBCS, Tomei Expreme Ti cat back exhaust, AMS high-flow cat, AMS FMIC, Cobb LICP, Tial QRJ BOV with 1.5psi spring
Fuel pump relay was upgraded and spark plugs replaced 4k miles ago. No boost leak to my knowledge.
Tuned by DBW Motorsports on a dyno.
EDIT - no DTC was thrown
Last edited by tade256; Apr 28, 2025 at 07:25 PM.
Evolved Member
Joined: Apr 2011
Posts: 870
Likes: 301
From: WI
How much fuel was in the car when this happened?
I would pull and check the plugs to get a sense of what was going on, are they in good shape, burnt up, etc.
The fuel smell is interesting, I would probably look over all the fuel lines and such as well as check fuel pressure just to make sure thats all good as well.
Lastly, you could get the injectors cleaned and flow tested, its possible one is a little dirty.
Edit: one other thing you could do but would require a re-tune is bump the base pressure a bit to bring down the IDC % in the future.
I would pull and check the plugs to get a sense of what was going on, are they in good shape, burnt up, etc.
The fuel smell is interesting, I would probably look over all the fuel lines and such as well as check fuel pressure just to make sure thats all good as well.
Lastly, you could get the injectors cleaned and flow tested, its possible one is a little dirty.
Edit: one other thing you could do but would require a re-tune is bump the base pressure a bit to bring down the IDC % in the future.
Thread Starter
Newbie
Joined: Dec 2014
Posts: 31
Likes: 7
From: Georgia
Thanks for the ideas.
Half tank of gas, I never had fuel starvation issues with this much fuel, not even on track
I checked the spark plugs, they seem fine to me (attached).
i had a fuel smell couple of years ago at Road Atlanta, that was a very hot day. We couldn’t reproduce it, best guess was the charcoal canister…
I’ll probably send the injectors to FIC for cleaning.
Half tank of gas, I never had fuel starvation issues with this much fuel, not even on track
I checked the spark plugs, they seem fine to me (attached).
i had a fuel smell couple of years ago at Road Atlanta, that was a very hot day. We couldn’t reproduce it, best guess was the charcoal canister…
I’ll probably send the injectors to FIC for cleaning.
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