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Old 03-19-2013, 04:39 PM
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check engine light, and violent bucking

If I drive the truck for more than a couple hours at a time, it will start to miss or buck, sometime violently. enough to kick off the cruise control. when this happens it triggers a P0194 code. which is a fuel rail pressure sensor.

Im going to try to change the sensor myself. any help appreciated.

the dealer told me that it would not help. stated it was probably a high pressure fuel pump. $9600.00 for the repair. Thats $10 thousand dollars.

2008, H&S mini maxx, dpf delete, mild tune. 114k
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Old 03-19-2013, 04:55 PM
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Frp sensor is an easy job that will take you only a couple hours . It lives under your passenger valve cover over cylinder 2.. I don't think it's your issue either, though... How long have you gone on the fuel filters? What brand are they? In short, it sounds like your hmfc is lagging and not supplying enough fuel, or you've got an issue with flow elsewhere..
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Old 03-19-2013, 05:05 PM
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The filters have about 2k on them. they are motorcraft. Its funny, it runs good for a while "about 2 hours" and then starts missing. let it cool overnight and it starts all over. I changed the filters last time it did it. I did a post on this a couple months ago, without any feed back. So, I did a DPF delete and tune trying to fix it then. I also added a bottle of hot shot secret in december also. that seemed to help, back then. WTH? I can not afford a 10 thousand dollar repair right now.
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Old 03-19-2013, 05:20 PM
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Stoopid, maybe... But when it 'does it', kill the engine and take off the fuel cap, start it up and drive it.. see if it still happens..

My thinking is that the lift pump is fighting for fuel.. maybe against vacuum, maybe against obstruction.. the time limit suggests vacuum, though.. your tank vent maybe obstructed.. maybe a simpler test is just taking the cap,off after such an event, and listening for a huge gasp of air..

Last edited by drewactual; 03-19-2013 at 05:26 PM.
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Old 03-19-2013, 05:25 PM
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will do ill try anything at this point.

I just joined powerstroke nation. there are several posts there with the same problem, but no resolutions.
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Old 03-19-2013, 05:35 PM
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will the H&S mini maxx show fuel pressure?

how do you check for obstructions in the vent tube?

Last edited by dnifong; 03-19-2013 at 05:38 PM.
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Old 03-19-2013, 05:39 PM
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And it could just flat out be your lift pump... losing prime or overheating, or some such.. another thing to check when it happens, is grab the wiring harness going into the lift pump and see if its warm.. then the pump itself could be warm, but should not be hot..

If the hpfp is making enough pressure to run you for 1hour 50 minutes, it shouldn't be taking a dump on you afterward.. I doubt it's your frp sensor, because the BIT will diagnose it bad in the first half second the ignition is on, and you'd NEVER have cruise control as a result of a limp home logic.. just for grins, though, bring up on your mini max the ICP gauge.. get a good reading before and while it happens to you.. you should see the pressure drop if what I'm thinking is happening.. icp is fuel rail pressure, by the way..
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Old 03-19-2013, 07:18 PM
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yes you can monitor fuel rail pressor with the mini, I would do that right away and see what it looks like. And something else to think of is the 08's had the wiring issue that rubbed the top of the hpfp and would cause issues to. Hope it's not a hpfp but if it is get ready to drop some coin
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Old 03-20-2013, 04:20 AM
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OK, thanks, do you know where that harness is, and where the trouble spots were?
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Old 03-20-2013, 05:32 AM
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I'm finally at a computer and can type instead of struggle with the dang phone...

it could be the HPFP harness... most often, though, the harness shorting pops a fuse- no?

the part about your issue that is confounding is that it does it around the same interval- or around two hours into run time.. that, imHo, helps you track it down to either an electrical issue (heat and expansion on a feeble area of wiring NOT specifically fuse protected), or a flow issue in the pump or lines.. if it were mechanical, it should be present no matter how long the engine has been running.. that is just my opinion, though, and maybe misguided..

going with the flow issue, there are a bunch of places where it could happen..

http://www.backglass.org/duncan/ps64...ply_System.pdf

regardless of the code given (P0194), I have a hard time believing the HPFP works and then all of the sudden decides NOT to.. that thing is either good to go, or it isn't..

that leaves the HMFC and the fuel cooler, the lines between, and the source.. and it simply sounds like a vacuum issue..

but, then there is this, too:

the FRP/ICP sensor is the only sensor that figures fuel pressure- it can't be triangulated with other sensors.. it should have some sort of redundancy, but I reckon that ain't doing us no good to know right now.. what happens with that thing is this:

when the key turns, in less than half a second a Built In Test (BIT) is ran, and all key sensors are given a diagnostic check.. either they check out, or they don't.. if they don't, a CEL is ignited..

question- if you clear the code, does it come back on before you start the engine?

the way the FRP works is with swinging voltages.. IIRC, it is looking for a 5vdc swing to determine FP.. it's pretty sensitive to voltage, as these things can range from 20kpsi to nearing 30kpsi during normal operations..

if the FRP fails, and I know this from experience so it isn't just conjecture: the PCM resorts to a different operating logic.. since it can't determine what the fuel rail pressure is, it can't determine how to inject in terms of duty cycle (how long injector is open).. it will still run, but the PCM limits how much power it applies to the drivetrain, and it eliminates functions such as cruise control.. it does this, by my own figuring, by operating on a pre defined script that calculates between throttle position and response.. if you start out slow, it will drive okay- if you apply a lot of throttle, it will buck-n-fight you.. all of this is if it fails outright.. usually indicated in a P0192/93 code..

there is another area on these rigs where chaffing of harnesses is a problem.. it is NOT fuse protected, either- or, at least, not directly fuse protected.. it is the harness leading to the FRP sensor itself..

that bundle is under the passenger valve cover, and is evident as it connects to the FRP.. it bundles with some other wires while still under the vc's.. it comes out of the front of the head in a round bundle before rejoining yet another bundle.. point being: if you remove your vc's and see an evident chafe, you've likely found your huckleberry.. if you don't see it, and replace the sensor and shorter harness and the issue still persist, it could be in the harness exterior to the heads, which exits the heads, and twists back toward the firewall between and below the battery tray and the air box.. my guess is that it is the shorter harness leading to the FRP sensor, though..


my figuring, fwiw, is either a delivery issue that is between your tank and your HPFP, or a short on the FRP sensor harness under the valve covers.. If you go buy a new FRP sensor, make sure and get a new harness, too..
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