Ford 6.4 drive-ability

Heavywrecker · Oct 31, 2017

Hello, I have an 08 6.4L ford new to me....i have been battling a th. Issue since I bought the truck. I work on diesel cars and trucks for a living as well as run a towing service. My truck has been doing this thing where the frp will irrastically drop, truck will stumble and come rite back out of it. It will do it either when driving it normally out of a parking lot, down the highway or when the cruise is set. If you hold the accelerastor at a steady pace and for example at 70 mph, the frp will drop down from around say 11-12,000 psi to 2500psi, you can feel the truck loose power, it doesn't die but you can feel it stumble. Now if you give it a little gas or let out a little it will straighten rite back out and frp will return to normal levels. I tested hfcm, had 8 psi, so I replaced it with a brand new fass 165. Figured the rail pressure sensor was bad, also replaced that with an alliance part from my local international dealer in town. Truck also has a ******* tuner in it set at 250hp, tried a few settings and it does the same thing on all of them. Only thing I did notice is that there was a small amount of oil in the frp sensor connector, figured this was normal since she's under the valve cover. Other thing I noticed, after I put the frp sensor in it, I keep it in a garage overnight, I thought I had it fixed. I let it sit outside one weekend (2 days) while we had a decent amount of rain and it started to mess up the very next Monday on the way to work. Thinking maybe something got wet possibly? Other thing I noticed is that when it screws up at low speeds the wrench light in dash comes on and it's almost like it disables the throttle then if you mash is, truck starts to go slowly and blows quite a bit of black smoke in the process.

Any help appreciate, you know how it is...when you do this stuff for a living, you hardly have time to work on your own stuff lol

Thanks in advance!
Video attached

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cbrown · Nov 1, 2017

So did you pull the codes?

DEEZUZ · Nov 1, 2017

Change frp sensor

6.0 Tech · Nov 1, 2017

I change frp sensors and uvc harnesses as a pair. May want to change the harness as well. Ive seen a bunch have loose pins in the frp connector.

sootie · Nov 1, 2017

nothing wrong with 8psi low side fuel pressure. not sure why you added the fass. Any codes? Could be chafed hpfp harness too...

Strokersace · Nov 1, 2017

sootie said:
nothing wrong with 8psi low side fuel pressure. not sure why you added the fass. Any codes? Could be chafed hpfp harness too...

Chaffed hpfp harness was my 1st thought too.

White_monster · Nov 1, 2017

As was mine. Especially for that year too.

Heavywrecker · Nov 2, 2017

Only code is throws is for fuel pressure lower than desired I believe it is, been too dang busy to actually get deep into it to do d out what's going on, next easiest thing try is probably the under valve cover harness ?

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sootie · Nov 2, 2017

Easiest thing would be to find out what the damn codes are and post them up here ...

DEEZUZ · Nov 2, 2017

Firm believer in if no popped fuse #74 then no HPFP harness rub through

webb06 · Nov 2, 2017

Monitor vcv% and pcv% on the high pressure pump. Have a truck waiting to get the cab off and hpfp replaced because the vcv is hanging around 8% causing p0087 fuel pressure lower than desired. Vcv should be between 18-30% typically around 24%. The truck at our shop has low power pretty much all the time.

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Heavywrecker · Nov 4, 2017

I replaced the harness under the valve cover this morning, found oil in the connector, must be from the old sensor leaking before I replaced it. I had the fuel pressure lower than desired code in the truck. Only one it ever sets. Monitoring the fuel pressure, what's the lowest you should ever see It? It idles at 5,000psi, but while driving sometimes it will drop down by the 4,000-3800 Mark. Wondering if the valve in the pump isn't getting hung up like you say.... appreciate all the input guys. It's our busy busy time of year just haven't had time to get deep into it lol

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webb06 · Nov 4, 2017

Heavywrecker said:
I replaced the harness under the valve cover this morning, found oil in the connector, must be from the old sensor leaking before I replaced it. I had the fuel pressure lower than desired code in the truck. Only one it ever sets. Monitoring the fuel pressure, what's the lowest you should ever see It? It idles at 5,000psi, but while driving sometimes it will drop down by the 4,000-3800 Mark. Wondering if the valve in the pump isn't getting hung up like you say.... appreciate all the input guys. It's our busy busy time of year just haven't had time to get deep into it lol

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Basically should never go below 5000psi idling or driving.

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Heavywrecker · Nov 6, 2017

I'm assuming the hpd is dying lol

Life goes on

Who's got the best price on a kit?

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Heavywrecker · Nov 11, 2017

Did I get the Friday edition or do the bolts make threads on the way in?

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White_monster · Nov 11, 2017

The threads cut as you put them in. The holes are supposed to be that way. If you read the tech section in the library on this site you will find the torque procedure in the coffee table book.

Heavywrecker · Nov 11, 2017

Cool beans, we have Mitchell1 at the shop so should be covered there

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DEEZUZ · Nov 12, 2017

Should be 85 in lbs iirc

Heavywrecker · Nov 12, 2017

Got it all together, just have to test drive it when I get time....too busy with work lately

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Heavywrecker · Nov 12, 2017

Appreciate everyone's help! One more question, I understand these motors have high blow by and it's considered normal. What is considered acceptable?

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