I've been trying to get my truck running properly for a time. New injectors were a huge help, then a new filter (again), but it still it would just go flat at times and I'd get a clacking noise in the fuel line occasionally. Yesterday on the way home it was a turd, no power at all, barely idled, no smoke at all and it was louder than usual. I checked fuel pressure at the schrader valve, and found negative pressure there. So, I finally decided to put on the lift pump I got two months ago. Now it runs cooler, quieter, and uses less fuel.
I know that fuel pressure can have a huge effect on the timing in the pump, but why is that? I am assuming the timing advances more with low pressure, as seen by the higher coolant temps, louder knock, roughness and low power. Would there be a positive effect to upping the input pressure?

 

I don't get how fuel pressure (outside the IP) would impact timing at all. Maybe the low pressure affected performance because it was starving for fuel to fill the IP reservoir.... not necessarily affecting timing. Just my thoughts...

 

The IP has a small vane pump which will suck fuel in once the pump is running; it'll run just fine from a gravity feed. If it's drawing a vacuum however... that's where things get iffy.

Thinking this through: The IP housing is kept around 5PSI during normal operation. The Cold Advance solenoid drops this pressure to 0, which advances the timing due to less back-pressure against the advance piston. If you were getting little enough fuel into the housing that it wasn't building up to the 5 PSI operating pressure, you'd easily get some advance.
No smoke and low power is also a symptom of not getting enough fuel.

As far as input pressure goes, it will affect timing somewhat. The most important thing is to try to keep the pressure constant throughout your range, so the timing doesn't vary.
When you time the pump, you effectively compensate for whatever pressure you have going in - it's affect is taken into account in the overall timing.
If you change the pressure, you will affect the timing, and so you will want to re-time the pump. Or, if you're trying to get different timing, just move the pump housing.

Unfortunately, I don't have a graph or anything of how input fuel pressure affects timing; I'd love to play around with a test bench someday and graph out a bunch of factors with these pumps.

 

Interesting. I just did the e-pump swap, and I did notice a slightly greater increase in smoke when I hammed on it. Too bad I don't have a timing gun to see how greatly it has affected the timing. Are we talking about a small change (+/- 0.5* BTDC) or a big change (+/- 5*BTDC)? Would a greater increase in fuel pressure advance or retard the timing?

 

Not sure exactly the amount, but it should be more incoming fuel pressure = more advance.

My dad has an E-pump on his truck, with a fuel pressure gauge. It's a decent Faucet pump, but while it has an idle PSI of like 7, it will drop to 1 PSI if I floor it(More fuel use, constant pumping speed = less pressure). I'm not sure the full effect on things, and how worn his IP is, but I know that I had to advance it a lot to get good power while hauling a trailer. When /not/ hauling a trailer, it's way too advanced, clatters horribly and is a bit hard to start.
This is why I like mechanical fuel pumps(fuel flow is directly proportional to RPM), though I'd really like to see a /good/ mechanical fuel pump, instead of the cheap crap we have.
Either that, or a massively oversized E-pump; something that can handle all the fuel the IP gets, with a regulator to keep the pressure at something low but constant.

 

A free flow e-pump hooked directly into the input of the mechanical pump works. basically acts as a primer to the mechanical pump. kinda the best of both worlds

 

Until the membrane on your lift pump fails and you pump your crankcase with Diesel....

 

the transfer pump in the IP will be unaffected by increases in base line fuel pressure, IE no additional HP, you can look forward to advance curve deviations from design parameters when you provide the IP with more than 4 psi,, most fuel E pump designs have built in fuel pressure regulators in the form of spring loaded bypass or diaphragm springs, as in the manual lift pump that comes on your IDI.. As pressure drops volume increases in all of these pumps,, most of the time when your lift pump FAILS it not broken,, it just has a bit of debris in one of the check valves as ford did not install a prefillter to block debris from tank except the shower head.. protect your lift pump with a cheap 100 micron filter,, at cruse speed your IP flows less than 10 oz per min even at full throttle your lift pump flows more than enough fuel for even a 100cc IP

 

Under absolutely NO circumstances should you EVER do that. You'll rupture the mechanical fuel pump's diaphragm and fill your crank case with fuel, and destroy your main bearings in about 10 miles of driving it.

Also... for input pressure affecting timing, just ask one of the guys on OB what happened when he had a Super Duty pump (70psi) supplying his IP with a regulator down to 8psi. It screwed with his timing so bad that he went through 2 brand new IP's from Justin just trying to figure out what his problem was.

 

Overall, I still don't get how outside-of-IP fuel pressure would impact anything. First off, the bleeder on the filter head would alleviate any fuel pressure that would mount in the fuel system. And the return line isn't air tight (once it reenters the fuel tank, that pressure is alleviated). So, I'm not sure how that would impact anything.

 

It doesn't matter if you understand WHY, just that you accept that it DOES. I can't even fully explain it, but it is what it is.

The basics of it is this: pressure does not equal flow. You can have very low pressure and very high flow if the resistance is low enough. Or high pressure and low flow if the resistance is too much.

Also, the return line IS pressurized. It's much lower than the feed, but it is not an "open" system. It's got that little eraser-looking thing on the end of the tube in the tank that creates that back-pressure.

 

I wonder if that may be causing issues on my truck. The return line on mine is just a tube, although it does go to the bottom of the tank to minimize aeration and the tank is in the cab so a bit higher.
Is there a pressure regulator in the return line off the filter head? I know most Bosch systems have the pressure regulator there, but then most of them also run quite a bit more pressure, along the lines of 30psi.

 

That's why I'm not convinced this is a serious issue. I have two fuel systems (the stock diesel one, and my DIY wvo system). They are completely separate systems, except for the selector valve connected to the line running up the IP, and the return selector valve connected on the return line from the injectors.

My wvo system has a duralift pump, heated filter head, 40 plate FPHE, arctic fox fuel pickup and a separate aluminum fuel tank. I don't have anything blocking the pressure on the wvo return line, it just returns to the tank.

Also, I connected the bleeder line from the stock diesel filter head from the IP to the return line of the stock diesel system. If I didn't do that, the wvo tank would fill with diesel, since the mechanical pump (now the e-pump) was always running.

With all that, I never noticed a major issue with my IP, in terms of performance, timing, or smoke (whenever I was flipping between the wvo circuit or the stock circuit.)

Maybe I just lucked out and somehow matched the required pressure in my wvo circuit by using a reduced radius rubber hose (1/4" diameter) for the return line. idk. But part of me just thinks that outside-the-IP fuel pressure isn't that big of a deal.

 

Well I know that feed pressure can make a difference in delivered fuel. I remember a bit of controversy surrounding I believe the Super Moose pumps where they were actually flow tested at higher input pressure than stock, skewing numbers. And the Bosch M/MW pumps used on the Mercedes diesels are sensitive to input pressure changes as well. The P pump on the 5.9 Cummins is similar as well.
I know the DB2 is different in that it's a rotary rather than inline, and the advance mechanism is different, but there must be an effect on fuel metering as well, right?

 

Not completely true, but mostly...


The inlet pressure effects transfer differential, which effects absolute differential, which does effect power, It was one of my first big lessons in IDI tuning.... Not so much N/A, but to much inlet on a Turbo IDI will effect spool and total power. I gained 10whp going from 9psi inlet to 6.5psi inlet, that being the only change, and it blew my mind.


The IP is calibrated at 5psi +/- .5psi, and anything other than that will effect calibration... Less fuel pressure will retard timing, more will advance it, and the only way to change that is on the stand...