Low fuel pressure......ideas?

[ChuckMC8]

MC8, 8V71TA, fuel pressure gauge at secondary filter reads 10 psi with engine running. Da book sez 60 to 70 lbs normally.
In attempting to remedy the situation, I have: Changed fuel pump. still 10psi, changed filters, 10psi, put original pump back on, still 10, removed and checked the check valve in the return line, put it back on, still 10.
    10psi is also what this gauge read with the previous engine. So, I changed the gauge. Reads 10psi also.
The DD manual sez to make sure that the restrictor is in the return line. Where in the return line would that be located? I scoured the firetruck (donor) and cant find a restrictor. I dont know what the fuel pressure was when this engine was in the truck, it didnt have a gauge.
  Solutions? Ideas? thanks- Chuck

[gumpy]

Ummm.....

I just put a pressure gauge on my output side of the pump when I removed my original filter setup last week. It reads only a bit more than that when idling. When I rev up the engine, though, it goes to about 60.

Did you try reving the engine?

[n4rsn]

Hello Chuck:
I see no one has answered you yet, so I will throw my 2 cents in.
I think the restrictor fittings on a 8v-71 is the fitting at the end of the cylinder head, on the return side.
It has been a while, but, I seem to remember them being there.
Thamks
Steve

[DrivingMissLazy]

Mine was the same as Gumpys. 10 at idle, 60 at road speed.
Richard

Ummm.....

I just put a pressure gauge on my output side of the pump when I removed my original filter setup last week. It reads only a bit more than that when idling. When I rev up the engine, though, it goes to about 60.

Did you try reving the engine?

[ChuckMC8]

I started the engine again, and the pressure does not rise with the engine revs. It has to be something in the plumbing...I'll check my DD parts book and see if I can find the restrictor in the head. I plumbed the return line in the same place it was on the truck...but I may have changed a fitting to get a working fuel hose combination. Keep the ideas coming please.....chuck

I checked the fitting that I removed and dont see any type of restrictor. In the book, it sez theres a restrictor tee where the return line comes off the head. That tee is on the engine as it came from the factory.........argghhhhh

[Utahclaimjumper]

Chuck, I just went through this with my 8V71 in a 4106. mine was consitantly low, found the restrictor in a brass fitting in the return line from theb head. The manual said the orifice should be .080, mine was .095, so I silver soldered it up and re-drilled to .065. Now mine holds 65lbs. at anything over 1000 RPM.>>>Dan

[Len Silva]

It's been a lot of years for me but as I remember, the restrictor was just inserted it the return line at the head.  It would be very easy to have it fall out when you had a fitting apart and you not even notice it.

Len

[Ncbob]

As best I can recall from DD school, way back when, the restricted fitting is in the fuel return line at the rear of the cylinder head.  In the case of the 8V71 there should be one in the fuel gallery at the rear of both heads.

To have one in the supply line would defeat the fuel volume supply.  I believe the Field Service Data Manual gives a volume of fuel return at idle for all the series of DD engines.

Simple...start the engine and measure the volume of fuel being returned to the tank. ...x per minute.

That should lead you to the solution.

FWIW

NCbob

[dooneb4]

low fuel pressure i had a 671 with the same problem went thru everything rang detroit ,had 3 experts look at it .
ended up being a hole in the intake line inside the tank. i hope not for your sake because it was not fun sorting it out ,but if all else fails?

[Dallas]

To narrow down the problem, try running a hose from a 5 gallon can of fuel instead of from the tank.

If the pressure picks up, it will have been a hole in the fuel line or pickup.

If it doesn't, try removing the return fuel line and putting a restriction in it and running it into the fuel can.

If the pressure picks up, it may need a new restrictor.

If it it doesn't, you may have a fuel pump problem, or you may have a bad fuel line. R

emove the fuel lines and check to see if they are clear. Fuel lines are known to deteriorate over time, so it's completely possible one has collapsed or gotten a piece of junk in it.

Another thought, make sure your filters are plumbed in correctly. I had one where the owner replaced the original cartridge filters with spin on filters, and plumbed the secondary in after the restrictor, causing the same problem.

I may be off track on this, but, it sounds like a fuel restriction/pressure problem before the primary filter.

IHTH,

Dallas

[Sammy]

Chuck, 10 psi at idle, 60psi at full fuel is good fuel pressure for a non-DDEC engine.
The restricted fitting is found on the return side of the fuel system. It's at the last point in the fuel system as it leaves the head to return to the tank. Usually mounted near the right bank head. I have seen them as a  brass 90 degree fiting, with a stamped mark on the top - R80, etc. Send e-mail if you need more help. Good luck.
Sammy 

[ChuckMC8]

ok, I think this one is resolved.......thanks for the great ideas.
What I determined is that I didnt have a restrictor in the return line. I used Dan's (Utahclaimjumper) suggestion and removed the brass fitting at the head where the return hose connects and brazed it up..then I drilled an .080 hole (actualy drill bit closest I could buy)
now has 10-20 at idle and 60 when it reves up.
Wow, what a difference in the way it runs!  throttle response is much better and runs lots smoother, esp at idle. Thanks again for the help. Chuck

[Dallas]

Glad to hear it worked out for the best Chuck!

Dallas