T e c h n o l o g y , L L C
9
DAVCO Technology, LLC
www.davco.com
800-328-2611
1600 Woodland Drive, Saline, MI 48176-1629
DIESEL PRO
®
243
TECHNICAL MANUAL
F1215 REV H
VISUAL DIAGNOSTICS‑AIR VS. 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 (see Figure 1) .
If there is an air leak in the fuel system, air bubbles will be
present in the clear cover of the Diesel Pro . Follow test
procedures outlined in “Diagnostic Procedures for Air Leaks”
for air leak diagnostics . If there are no bubbles present in
the Diesel Pro cover and the engine continues to run rough,
lopes or has a loss of power, there may be an air leak between
the Diesel 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 .
TEST 1:
A quick procedure to determine if the air leak
is between the fuel tank and the Diesel Pro is to remove
the Diesel Pro inlet hose and route a new hose from the
Diesel 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 Diesel Pro because the pressure inside the Diesel 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 increas‑
es . The lower pressure draws the entrained air/vapor out of
the fuel and these bubbles will be visible as the fuel exits the
Diesel Pro (See Figure 2) . 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 .
TEST 2:
An easy way to determine the difference between
vapor and air bubbles is by temporarily removing the filter
element from the Diesel Pro . Fill the cover with clean diesel
fuel, replace the vent cap and re‑run the outlet fitting sight
glass test . If there are no bubbles visible in the sight glass,
then the bubbles that were previously visible were vapor
bubbles . If bubbles are still present then they are air . If air
bubbles still exist, re‑run the test in
TEST 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.
Figure 1
Figure 2
DIESEL PRO
®
243 TECHNICAL MANUAL
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.
FUEL RETURN
ENGINE
LIFT PUMP
FUEL
TANK
LIFT PUMP
FUEL RETURN
ENGINE
LIFT PUMP
FUEL
TANK
LIFT PUMP
AIR BUBBLES
VAPOR BUBBLES
