Visual Diagnostics - Air Vs. Vapor Bubbles - Davco fuel pro 382 Technical Manual

T e c h n o l o g y , L L C

VISUAL DIAGNOSTICS - AIR VS. VAPOR BUBBLES

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.
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 7)
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 (page 15) 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 .
ENGINE
LIF PUM T P
AIR
BUBBLES
DAVCO Technology, LLC
FUE RETURN L
FUE PRO L
OR OTHER
PRIMAR Y
FI LTER
Figure 7
P. O. Box 487 Saline, MI 48176
TECHNICAL MANUAL
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 Prois
lower than the atmospheric pressure in the fuel tank . Vapor
bubbles can vary from champagne size up to ¼" in diam-
eter . 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 . (See Figure 8)
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 re-
quired for vapor bubbles. Vapor bubbles do not cause
performance issues and will not be present after the lift
pump.
ENGINE
LIF PUM T P
VAPOR
BUBBLES
Call 800-328-2611
14
FUEL PRO
®
FUE RETURN L
FUE PRO L
OR OTHER
PRIMAR Y
FI LTER
Figure 8
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