FUEL PRO
®
485
TECHNICAL MANUAL
®
F1485 REV B
T e c h n o l o g y , L L C
8
DAVCO Technology, LLC
www.davco.com
800-328-2611
1600 Woodland Drive, Saline, MI 48176-1629
There are two kinds of bubbles that may be visible at the fuel pump inlet of a diesel fuel system. The bubbles can be
characterized as either air bubbles or vapor bubbles.
VISUAL DIAGNOSTICS - AIR VS. VAPOR BUBBLES
AIR BUBBLES
VAPOR BUBBLES
Air Bubbles
Air bubbles are caused by any air leak on the vacuum (suc-
tion) side of the fuel system from the fuel tank pick-up to, and
including, the lift pump .
If there is an air leak in the fuel system, air bubbles will be
present in the clear cover of the Fuel Pro follow Diagnostic
Procedures for air leak diagnostics . If there are no bubbles
present in the Fuel Pro cover and the engine continues to
run rough, lopes or has a loss of power, there may be an air
leak between the Fuel Pro outlet port and lift pump inlet . This
type of air bubble can be seen if a sight tube is installed at
the lift pump inlet . Air bubbles may also be visible in the fuel
return (spill) hose out of the fuel gallery . These leaks are easily
eliminated by checking and torquing the fuel fittings in the
area of the leak .
NOTE 1: A quick procedure to determine if the air leak is
between the fuel tank and the Fuel Pro is to remove the Fuel
Pro inlet hose and route a new hose from the Fuel Pro inlet
into a container of fuel or the fuel tank fill cap opening . Start
the engine and check for bubbles .
If there are no air leak symptoms, but bubbles are present in a
sight tube at the fuel lift pump inlet, they are most likely vapor
bubbles .
Vapor Bubbles
All diesel fuel has some level of entrained air caused by the
natural splashing that occurs in the fuel tank during normal
vehicle or equipment operation . Vapor bubbles develop in the
Fuel Pro because the pressure inside the Fuel Pro is lower
than the atmospheric pressure in the fuel tank . Vapor bubbles
can vary from champagne size up to ¼" in diameter . They
may increase in size or volume as engine rpm increases . The
lower pressure draws the entrained air/vapor out of the fuel
and these bubbles will be visible as the fuel exits the Fuel Pro .
As the fuel enters the lift pump, it is pressurized and the
bubbles are compressed back into the fuel . There will be no
bubbles on the fuel return side of the system . These vapor
bubbles will not affect the performance of the engine .
NOTE 2: An easy way to determine the difference between
vapor and air bubbles is by temporarily removing the filter
element from the Fuel Pro . Fill the cover with clean diesel
fuel, replace the vent cap . Tighten the vent cap by hand until
it clicks . Re-run the outlet fitting sight glass test . If there are
no bubbles present in the sight glass then they were vapor .
If bubbles are still present then they are air . If air bubbles
still exist, re-run the test in NOTE 1 to eliminate the chassis
plumbing as a variable .
There is no troubleshooting or repair procedure required
for vapor bubbles. Vapor bubbles do not cause perfor-
mance issues and will not be present after the lift pump.
