Stumped on 6.4. Fuel rail pressure issue.

bigfords · Feb 24, 2017

Alright guys got another 6.4 that I'm a little stumped on.
Guy brought it in for bucking/missing under acceleration, figured it was a misfire but its not looking that way. Monitored Power Balance and test drove, sure enough around 50 mph with converter locked, under acceleration the truck started shaking and bucking/surging. Power Balance was like a wave, with no clear specific cylinder dropping out like normal.
Monitored Fuel Rail Pressure, the High Pressure VCV and also the PCV while duplicating customer concern. The graph for these PIDs all started to be sporadic, and the rail pressure would rapidly go up and down, but now more than I want to say 1000 psi up or down if I recall correctly. The PCV and VCV following closely, and I imagine they were trying to match the FRP changes.

I thought possibly a FRP sensor, but I replaced that with no change. Its either an injector loosing pressure or the pump, am I correct?

Any experience would be appreciated!

Truck is tuned and ******d.

thanks!

jimdawg185 · Feb 25, 2017

There's also a hpfp wire that can get tweaked. I believe there's a Ford TSB on it. It's been a while since I've seen that because most were fixed by the dealer. But if you're going that deep into it mine as well replace the pump and make sure the wiring is fine.

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bigfords · Feb 25, 2017

Yes, I think your talking about the harness that comes off the gasket and goes up to the passenger valve cover area. Usually they rub through and short out, blowing a fuse.

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jimdawg185 · Feb 25, 2017

bigfords said:
Yes, I think your talking about the harness that comes off the gasket and goes up to the passenger valve cover area. Usually they rub through and short out, blowing a fuse.

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Yep! I've seen them make the pump do funny things and not blow the fuse too. Either way, sounds like a pump issue.

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webb06 · Feb 25, 2017

I vote for the pump too. Had one a few months back doing the same thing.

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bigfords · Feb 26, 2017

I guess what kind of has me a little concerned is that while monitoring the FRP, VCV, and PCV, the fuel rail pressure graph starts to fluctuate just before the PCV and the VCV do on the pump. So it looks to me like the PCV and VCV are seeing the FRP fluctuate, and then trying to compensate for the loss/gain that the rail is getting.

My main thing is that I don't want to finish putting the pump in and have it do the same thing! lol

I think I'm going to throw the under valve cover harness on it before pulling the cab just to be safe. Should have done the harness when I did the FRP sensor...

Anyone ever see an injector cause this?

jimdawg185 · Feb 26, 2017

bigfords said:
I guess what kind of has me a little concerned is that while monitoring the FRP, VCV, and PCV, the fuel rail pressure graph starts to fluctuate just before the PCV and the VCV do on the pump. So it looks to me like the PCV and VCV are seeing the FRP fluctuate, and then trying to compensate for the loss/gain that the rail is getting.

My main thing is that I don't want to finish putting the pump in and have it do the same thing! lol

I think I'm going to throw the under valve cover harness on it before pulling the cab just to be safe. Should have done the harness when I did the FRP sensor...

Anyone ever see an injector cause this?

An injector could do this too. As well as the harness. But if it was an injector I think that would show itself on the power balance. The harness might give you that wave I suppose without a clean power balance break.

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bigfords · Feb 26, 2017

Do you think a potentially bad injector would show as a spike or loss in rpm on the Power Balance? It seems like #2 is coming down too much, about -20 rpm, and #8 is spiking too much at the exact same time, basically doing what #2 is just up instead of down. This is when the graph is not doing the wave, but very close to the time the truck starts bucking.

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White_monster · Feb 27, 2017

Truck have ported rails?

bigfords · Feb 27, 2017

White_monster said:
Truck have ported rails?

No it does not. Stock everything except for ******s.

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jimdawg185 · Feb 27, 2017

bigfords said:
Do you think a potentially bad injector would show as a spike or loss in rpm on the Power Balance? It seems like #2 is coming down too much, about -20 rpm, and #8 is spiking too much at the exact same time, basically doing what #2 is just up instead of down. This is when the graph is not doing the wave, but very close to the time the truck starts bucking.

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Generally a loss, but if it's hanging open it could be the other way around in theory. The event after the loss is typical from my experience. I wish Wayne or someone that's seen more of these would chime in. I'm a hobbyist, the full time techs see a lot more screen time. They get used to the little nuances and the root causes of them. Like I do with oil analysis.

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bigfords · Feb 27, 2017

Test drove the truck again today while monitoring the fuel rail pressure and fuel pressure control valve. It was colder today and much worse. Vilely shaking the truck I really want to do a fuel pump I'm just trying to rule out the injectors before I pull the cab. Anyone else have any similar experiences? It seems like only the pressure control valve and fuel rail pressure are the ones that are fluctuating. The volume control valve seems to Hold Steady.

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