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Old 11-09-2008, 12:04 AM   #1
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Exclamation Strange cutting at WOT... Fuel pump?

About halfway to Chicago, my car began to cut out at WOT at higher RPMs... This was intermittent at first, but now it happens every time I try to get on the throttle at high RPM (above 4k). The cutting out feels/sounds exactly like traction control on wet roads, but I have no wheelspin. Oh, the car also backfires immediately after cutting out.

-I do have a J&S, it is active whenever this happens.
-My fuel filter is pretty new (about 8k ago, in March?)
-I haven't noticed the A/F drop on the wideband when this happens, but then again, this happens very quickly.
-My car is a 2002 SVT with JRSC and BBK, currently running FR BBK tune (SCT PRP is on the way)
-Currently use 42# injectors, have 60# injectors for after PRP comes.
-Other known problems: Last compression test (3 weeks ago) was 90-120-120-120, I have bad rings in #1. I have new rings to install, but I think I will do the piston while I'm at it. The job to fix this is in the planning stage, will happen pretty soon.

My current theory is that my fuel pump gets weak & causes the mixture to lean out, causing knock, which the J&S catches, then cuts timing to save the motor. Solution: New fuel pump or booster?

thread that says similar symptoms point to fuel pump

Am I right? What else could it be? I double-checked all my connections and got different fuel, with no change.


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Old 11-09-2008, 10:15 AM   #2
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Like i said log your fuel pressure for a week or so then we will know if its the fuel pump
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Old 11-09-2008, 10:25 AM   #3
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Ok, check this theory out.

Low compression in one cylinder causes some unburnt fuel to show up in the exhaust. The ECU reads this as a rich condition, and leans the fuel trim. Now you might not see it on a wideband gauge because that is not reading one cylinder but reading the whole engine. The computer, and your gauge are telling you that nothing is wrong, however you're running lean on all cylinders and the increased readings are a result of incomplete combustion in one cylinder. The cutting out you're feeling is the knock sensor doing it's job, and the ECU cutting timing back 10 degrees.

I've never heard of replacing one ring on a DD, but ok.
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Old 11-09-2008, 10:28 AM   #4
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The pcm will not change fueling in open loop, it runs off the base fuel table. So i'm inclined to disagree.
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Old 11-09-2008, 10:49 AM   #5
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ok I'm just gonna give in and finally ask...... what's WOT?
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Old 11-09-2008, 10:57 AM   #6
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Old 11-09-2008, 05:27 PM   #7
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Oddly I can't find the Fuel Pressure PID! Anyone using XCAL2/Livelink with the SVT Focus? Do you have a fuel PSI PID? I remember reading it when I had a predator... but now can't find it.

Maybe I need new firmware/software... I'm looking into that.

I did do a few datalogs with what I can read (including wideband)... the spark advance freaks out when this happens, from the J&S i'm sure. Another thing to mention, the J&S does not always go off when the cutting out occurs.
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Old 11-09-2008, 07:30 PM   #8
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come over when youre done at the lab we can find it. you dont even need your laptop we can use mine. But i would like to take a look at those datalogs.
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Old 11-09-2008, 10:00 PM   #9
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ok so we logged it and basically found out it is the fuel pump, or need for a boost-a-pump. The pump is pegging at .480 duty cycle.

Can anyone else confirm? Does Matt need a new fuel pump?

Here you can see 2 instances of the problem. Here is a color code list:
dark orange:throttle mode, 1 is open loop, 0 is transient, -1 is closed throttle
light orange:throttle position absolute
green:fuel pump duty cycle
yellow:fuel pressure (and you can see this on the y-axis)
blue:rpm


First one, its pulling good and hard until the fuel pump duty cycle pegs at .480, and then fuel pressure drops. This happens at about 5600 rpm. The screenshot spans about 5 seconds.




Second one, same thing basically, but at much much lower rpm. about 3600. This screenshot spans about 3 seconds.





PS matt you should have put this in FI chat.
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Last edited by illinipo; 11-10-2008 at 12:06 AM.
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Old 11-10-2008, 08:30 AM   #10
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moving it to FI chat.

side note. doesn't the SVT us a Dual Fuel pump setup? I was under the impression that it does. so maybe it needs to be replaced.

I am not for sure.
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