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omn-nom-nomnomnomnom
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2,293 Posts
Discussion Starter · #1 ·
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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Token Engin-nerd
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3,414 Posts
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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Premium Member
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21,259 Posts
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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Token Engin-nerd
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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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omn-nom-nomnomnomnom
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2,293 Posts
Discussion Starter · #7 ·
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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Token Engin-nerd
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3,414 Posts
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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Token Engin-nerd
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3,414 Posts
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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Dragon Warrior
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309 Posts
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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I AM DA FOCI FANATIC
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3,715 Posts
....ummm wow

The Fuel pump just stops working on the second one.
 

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I AM DA FOCI FANATIC
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Did you guys do another compression test? Still at 90?
 

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omn-nom-nomnomnomnom
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2,293 Posts
Discussion Starter · #14 ·
Yeah fuel pump and injector pressure both seem to bounce with RPMs right when the cutoff happens...

I want to buy a new fuel pump ASAP if it is the problem, can anyone think of anything else that could cause this? I hate throwing money at problems...

Did not do a compression test yet... I am at work, I'll have to do that when I get off.
 

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Token Engin-nerd
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3,414 Posts
....ummm wow

The Fuel pump just stops working on the second one.
the same thing happens in both of them, its just that matt took off the throttle faster the first time.

why it pegs at just .480 i dont know, could be a bad FPDM.
 

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Token Engin-nerd
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3,414 Posts
here is an explanation i wrote up in another thread:


the fuel pump doesnt cut off, the pump pegs and the fuel pressure drops off until the PCM just cuts fuel, then the pump can play catch up and get more pressure... only for it to happen all over again. This is all within about 2-3 tenths of a second.
 

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omn-nom-nomnomnomnom
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2,293 Posts
Discussion Starter · #18 ·
Not yet... Call me crazy, but I'm kinda hesitant about chopping a hole in the floor of my pretty car.

Which I'll probably do tonight or tomorrow.

&[email protected]%^#$%
 

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Token Engin-nerd
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3,414 Posts
we can get to all the wires without cutting dont worry about it

i think we're going to need 3 people though, one to drive, one to tell us when the fuel pump is pegged, and one to read the voltage at the pump
 

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I.S.A.I.K.I.
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5,413 Posts
^That's one way to do it, but now that I'm thinking about it does the cut off and volt reading go hand in hand?

I was going to mention that idea but was afraid you guys were going to giggle about it. lol

Small question..... how do the spark cable jackets look? No cracks or anything to cause an arc nearby a grounding point?
 
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