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Of course, the ECM's learning function has made my initial results less than stellar. I can now drive around the problem pretty effectively and the KFV is more stable than it was, but the actual problem persists.
What I have noticed as I am monitoring ignition timing is the timing spikes 5 degrees or more additional advance when KFV goes super negative. It should NOT be adding advance because the load is increasing, so there is some function in the ECM (I've heard the term predictive ignition timing in my searches) that is screwy with an aftermarket exhaust. One step colder plugs are next.
Other thoughts are:
1. Would water injection eliminate this and provide better tuning opportunities?
2. Does a small dose of E85 add enough cooling effect to completely mask this issue?
3. Is this still a fuel pressure sensor issue?
More to come...
Last edited by lobuxracer; Sep 17, 2025 at 07:46 AM.
For completeness, here is what I ordered for my fuel pump replacement project:
Along with this, you'll need the AEM pump, 50-1220, and the Spectra Premium STR125 (new strainer - prescreen).
AEM appears to control pricing on their pumps pretty tightly, I got mine from Summit for $145.95 earlier this month, but since then it appears tariffs have hit and the new price is $164.95.
The Spectra part is widely available. RockAuto has it for $29.79 as of today, September 17, 2025. Sellers on the 'Bay have it crazy cheap <$15 USD.
Of course, the ECM's learning function has made my initial results less than stellar. I can now drive around the problem pretty effectively and the KFV is more stable than it was, but the actual problem persists.
What I have noticed as I am monitoring ignition timing is the timing spikes 5 degrees or more additional advance when KFV goes super negative. It should NOT be adding advance because the load is increasing, so there is some function in the ECM (I've heard the term predictive ignition timing in my searches) that is screwy with an aftermarket exhaust. One step colder plugs are next.
Other thoughts are:
1. Would water injection eliminate this and provide better tuning opportunities?
2. Does a small dose of E85 add enough cooling effect to completely mask this issue?
3. Is this still a fuel pressure sensor issue?
More to come...
how much total timing are you seeing when it’s spiking? Mid to high 40’s?
how much total timing are you seeing when it’s spiking? Mid to high 40’s?
Yes, exactly.
I did an experiment today with manual mode and determined this really looks like predictive ignition timing problems. It shows up only at certain light throttle positions around the 1800 to 2200 rpm range. I was able to spike low speed KCLV from 21.7 to 23.5 just using a higher gear and a heavier throttle position coming away from a traffic light with final speed 60 mph (not breaking any speed limits or potentially getting an exhibition of speed ticket). I just focused on KFV, and it dropped to 0 during this run. At a lighter throttle position, KFV dropped to -6.x, and KCLV started plummeting as usual.
Now I'm wondering if there is a table the ECM uses for predictive ignition timing. If there is, it needs to be modified for sure.
Of course, the ECM's learning function has made my initial results less than stellar. I can now drive around the problem pretty effectively and the KFV is more stable than it was, but the actual problem persists.
What I have noticed as I am monitoring ignition timing is the timing spikes 5 degrees or more additional advance when KFV goes super negative. It should NOT be adding advance because the load is increasing, so there is some function in the ECM (I've heard the term predictive ignition timing in my searches) that is screwy with an aftermarket exhaust. One step colder plugs are next.
Other thoughts are:
1. Would water injection eliminate this and provide better tuning opportunities?
2. Does a small dose of E85 add enough cooling effect to completely mask this issue?
3. Is this still a fuel pressure sensor issue?
More to come...
In my experience E85 will mask some of it, but will not eliminate it.
