What to look for when fuel pressure gauge shows decrease?
 

I bought my truck with a fp gauge on it & was told to watch it for pressure drops & to shut it down it down or risk more spendy damage if it does drop...

Are ANY pressure drops normal and please give specifics on what kind of a drop should justify a shutdown? FWIW, I'm not seeing any drops currently, but I did notice a decrease a few weeks ago while navigating around a grocery store parking lot, but nothing unusual when I came back out from the store & started it back up...

Thanks!

Dirty fuel filter will drop it over time. It shouldn't instantly but it did on my '98 Chevy after I added a fuel additive. I did not expect the additive to work (a spur of the moment purchase) and within that tank my pressure dropped to zero with a completely clogged filter.

drops are normal. you will be fine as long as there is pressure, 5psi or greater at WOT seems to be the accepted norm, some say greater than 8psi...

When I bought my truck it used to barely stay above 3psi and would drop to zero, I had no idea how long the truck had been like that. the previous owner said that nothing had been changed on the truck, when I bought it in May it had 297,000Kms now 5 months later I have 333,000KMS on it, and my VP has not crapped out....yet although I expect it to at any time (BTW I installed a campaign LP from Cummins shortly after buying the truck)

This is from Cummins: http://i14.photobucket.com/albums/a329/DrEvilx/FP2.jpg


there really is no hard and fast rule here - but the whole point is to provide enough fuel to the VP-44 so that it can stay cool and lubricated. Especially important with the dry ULSD.

Wow, is the chart saying that a Cummins should be shut down if the fp gauge is showing less than 10psi?

No.,..that is/was their spec.

Thanks for the chart Doc. Made a copy of it to put in my truck info book.
I am still able to get LSD fuel, but run Stanadyne additive. Pressure, lift pump running when cold 11-12 PSI. Idle, 9-11 PSI, down from 12-13 PSI.
Cruse down the freeway at 65MPH runs about 8-9 PSI

Dealer runs flow test at fuel filter change, and says I have a good volume, about at the top of the specs.

When is the test port located, and what pressure gage can be used to double check the pressure?

The bad thing about the dealer flow test is it may be the way they've been told to check it, but flow is not the only test to verify any pump is good. It's got to be able to maintain a certain pressure. Even a bad pump will move fluids when there's no obstruction, or something to pump against. Once a load is put onto that free flowing pump, things can and will be diffrent. That's why I'll be the one keeping an eye on my LP, not a dealer.

is that chart for a 12v ????

No its for a 24V. A 12V should be seeing between 23-40 psi or so.

The way I see it, is if you have a completely stock truck and you are seeing less than 5 psi or less (post filter) its time to do something soon. Whether thats simply a new lift pump or fuel system upgrade, is up to you. But a stock LP is just a band aid fix at best.

What are your pressures at idle, cruise, and WOT? That will give us something to work with. If your pressures dip to 8 psi or below, then it is time for a new lift pump. A FASS, I might add.

Mine is typically at 12--I haven't noticed any consistent variation from that for cruising or idling (and I'm drawing a blank on what WOT means), but I'm an attention deficient kinda guy...
There are also the aforementioned incidents where it falls into low numbers like 5 or less, but they don't seem to ever last.

My original post was intended more as a question as to what to look for in the future, especially to know when to shut down to avoid damage to other parts of the system, rather than any big worries about what's going on currently.

Wide Open Throttle....take it on the highway, stomp your foot on the go pedal and see what the pressure is. Thats really the only pressure reading that is an indication of lift pump health.

In the winter when the fuel is cold the pressure may fall. Also in the winter when you grid heaters are cycling it may fall. See sticky in 24 Valve Engine and Drivetrain section. While accelerating it may fall. Dirty filter will cause it to fall. If it starts staying below 5 psi. theres a problem