i have dual fuel tanks on my truck. i'm only getting 15psi of fuel pressure while the truck is running. (if it does run that is..... ) plus when i stop the truck i'm not sure how fast the pressure is supposed to drop, but i can actually see it drop right before my eyes.

this is no where near the expected 40psi.

i'm wondering which pumps (front or rear in-tank low pressure or the frame mounted high pressure pump) goes bad the most.

this would explain this code:
41 HO2S (sensor) System Lean Right Side

 

If the pressure drops that quickly (it should hold for 30 sec. or more), it may not be the pump. It could also be the fuel pressure regulator, a bad injector or a bad return line shuttle valve.

Do you get low pressure on both front and rear tanks?
Does the pressure drop instantly on both tanks when you turn the ignition off?

If they both behave the same, the problem is not in either in-tank fuel module. If one of them is "normal" then the problem is with the other.

Pull the vacuum line off your regulator. Is there fuel present in the line? If not the FPR is OK. Pull your plugs one at a time. Are any of them sooty black while others are tan or grey? If you have a sooty (or wet with gas) black plug you have a leaky injector.

That leaves the shuttle valve and the inline pump. I'm sorry i don't know the exact diagnostics for the shuttle on the older models, but if you're not transferring fuel from tank to tank, the shuttle works OK and is not likely to be causing the problem. That leaves the in-line pump.

You don't just throw high-probability parts at it, that's way too expensive. You have to think your way through it and try to figure out which parts to throw at it.

 

i haven't checked for both tanks. it was only ran on the front tank when i checked it. and yes, the pressure drops instantly. i'll check on the rear tank tomorrow.

the vacuum line to the FPR is good as i did not smell any fuel nor did i see any at all. the FPR holds vacuum when i tested it with my vacuum pump.

i also checked on the cylinder compression and here are the results:

cyl # in psi
1 = 171
2 = 174
3 = 175
4 = 170
5 = 160 dry/179*
6 = 174
7 = 179
8 = 179

*with oil
#5 is showing outside of the rest, although it's still within 75% of the highest reading dry.

here are the pictures of the spark plugs in numerical order (left to right):



as you can see #5 is of concern. i retested #5 with some oil and it tested fine. i unplugged the fuel pump relay and locked the TB plates open when i tested the compression. i also removed all of the spark plugs prior to testing as per service manual.

now i know i have some blowby in #5. but a stuck injector too? low compression really has nothing to do with low fuel pressures.........unless there's something i don't know.?

 

Yes and no. There's no direct connection but a leaking injector will wash down the cylinder walls causing a loss of lubrication and increased ring wear. Is there any gasoline in your crankcase oil? If so you've found a problem, possibly THE problem. If not, the problem is elsewhere.

I think you're probably on track by suspecting a pump; and at 15 PSI I suspect it's the HP framerail pump that's FUBAR but you need to eliminate everything else before you pop the $$$ for a new pump. Run the front tank/rear tank pressure test and you'll have most of your answers. When you turn the key on does the framerail pump make any noise at all or does it vibrate like the motor's turning? Crawl under the truck and listen/feel while someone else turns the key to on.

 

long story short i think i was just out of gas in the rear tank.:o ran to the gas station bought 5 gallons and dropped it into the rear tank and now the truck stays running.

i was like wtf? i grabbed the code scanner and now i don't have the lean running code.

i'll probably still check on the pressure later on. i can't now cuz the o-rings in the fuel gauge got messed up with the exposure to gasoline.

now onto the next problem.......