Diagnosing No Start Condition
First let’s start with how to diagnose whether the fuel pump is the problem. The symptom would be that your Yukon does not start but your starter cranks. Of course, if the Yukon does not start and the starter does not crank, then you need to diagnose a problem with your starter.
This condition can be caused by either a fuel or spark related problem. The first thing to check is whether your Yukon has a spark in every cylinder. This can be easily done using either an in-line spark tester (such as this one sold on Amazon: Amazon In Line Spark Tester), or a simple touch type spark tester that I used (see an example: Touch Spark Tester on Amazon). These are very useful and inexpensive tools. If either tool shows good spark in every of the 8 cylinders, then your start problem is most likely related to fuel delivery.
Fuel problem can be either related to the fuel pump (most likely) or to the fuel pressure regulator. To diagnose this problem properly, I suggest buying a simple fuel pressure gauge. There are many different fuel pressure test gauges sold on-line. I bought the cheapest one at the Harbor Freight Tools store for $20 that did the job (http://www.harborfreight.com/fuel-injection-pump-tester-92699.html). You need to remove the engine cover and connect the fuel pressure gauge to the fuel pressure check connector. The fuel pressure connector is number 24 in the diagram of the Yukon fuel system below.
This condition can be caused by either a fuel or spark related problem. The first thing to check is whether your Yukon has a spark in every cylinder. This can be easily done using either an in-line spark tester (such as this one sold on Amazon: Amazon In Line Spark Tester), or a simple touch type spark tester that I used (see an example: Touch Spark Tester on Amazon). These are very useful and inexpensive tools. If either tool shows good spark in every of the 8 cylinders, then your start problem is most likely related to fuel delivery.
Fuel problem can be either related to the fuel pump (most likely) or to the fuel pressure regulator. To diagnose this problem properly, I suggest buying a simple fuel pressure gauge. There are many different fuel pressure test gauges sold on-line. I bought the cheapest one at the Harbor Freight Tools store for $20 that did the job (http://www.harborfreight.com/fuel-injection-pump-tester-92699.html). You need to remove the engine cover and connect the fuel pressure gauge to the fuel pressure check connector. The fuel pressure connector is number 24 in the diagram of the Yukon fuel system below.
After connecting the fuel pressure gauge, turn the key one click to ON (do not crank the engine). The fuel pump should activate and raise the pressure to at least 55 PSI. If you have zero fuel pressure, then the problem is with the fuel pump. If the fuel pressure is between 55 and 62 PSI, then have someone crank the engine and monitor the fuel pressure. The fuel pressure may drop but then should stabilize. If the pressure drops by more than 5 PSI after it stabilizes, then the problem may be with the fuel pressure regulator or another fuel injection component. You will need to do a diagnosis of your fuel system -- I suggest checking this page for a full GM diagnosis procedure: http://www.justanswer.com/gmc/79k9g-gmc-yukon-denali-2001-gmc-yukon-xl-denali-4wd-6-0l-truck.html.
If you have no fuel pressure, then you need to make sure that the fuel pump is getting electricity. Check all ignition and fuel pump related fuses in the fuse box and then check the fuel pump relay. You can check the relay by pulling it out and replacing it with a similar relay in the fuse box. If all the relay and all the fuses check out, then it is almost certain that your fuel pump broke and needs replacement.
Replacing the Fuel Pressure Pump
The challenge with replacing the fuel pump is that you need to drop the fuel tank. GM’s service manual standard time for this repair is 3.2 hours. It requires to lift the car and to have another lift to support the heavy 32 gallons fuel tank in order to drop it. I was able to replace the fuel pump by making an opening in the Yukon floor as described below. I bought a cheap fuel pump on Amazon for $77 (instead of paying $400 to the dealer!). This pump had a one year warranty, and if it fails, I can easily replace it again using the opening in the floor.Step-by-step instructions of making the opening and replacing the pump are as follows:
1. Disconnect the positive terminal (red terminal)
from the battery to avoid the risk of spark when you remove the fuel pump.
2. Remove the long rear bench. There are four bolts that bolt the bench to
the floor, but also note the two bolts that connect the long bench to the
remaining one seat bench as shown in the picture below.
3. Remove the carpet and insulation and mark the
opening with a marker on the floor.
Please refer to the photo below for recommended opening location (which
is a little different than the opening I made).
I recommend cutting along the solid red line shown in this photo, and
then making a light surface cut of less than a millimeter deep along the dotted
line to allow for bending of the metal. Please
note the spot welds above the dotted line indicated by the white arrows. There is a box welded to the floor below. Make sure that your bent along the red dashed line
is about ¼ inches before the spot welds.
The following photo shows the suggested
opening location as opposed to the one I made.
4. I cut the floor using a simple Dremel tool with
406 15/16” cut-off wheels. I used about
a dozen wheels to complete the cut (they break easily), but they come in a 36
pack. You can use any metal cutting
wheel that is not larger than 2 inches in diameter (there is a little over an
inch between the floor and the fuel tank in the narrowest gap location on the
left side of the opening).
5. After you cut the three sides of the floor and
make a surface cut of the fourth side, you can bend the metal to expose the
fuel pump below as shown in the following photo (note that I recommend to bend the metal in a different location as discussed in the previous step).
6. Removing the fuel pump is straight forward. You need to remove the cover first,
disconnect the fuel lines (be careful not to break the fuel line connectors)
and wires, and then turn the ring that holds the fuel pump. The only tricky part was to disconnect one
of the wire harnesses. You need to insert
a flat screw driver to disconnect the clip as indicated in the photo
below. The new fuel pump typically comes
with a spare harness – you can see how this harness connects to the fuel pump.
7. After inserting the new fuel pump and connecting
the fuel lines and wire harnesses, I suggest reconnecting the battery and
testing the new fuel pump before proceeding.
8. If the new pump works, reinstall the fuel pump
cover/cap. Now you are ready to close
the opening and complete the installation.
I sprayed paint on the opening edges to prevent rust and taped the edges
of the bent portion with Duct tape to prevent rust and make sure that there is
no metal-on-metal rubbing once the metal is bent back into place.
I then bent the metal back and taped it with Duct tape as shown in the photo below.
I hope that this blog helped you to deal with your Yukon fuel pump problem.
