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Old 10-16-2008, 09:01 AM
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scotty1998 scotty1998 is offline
Master Mechanic
Join Date: Apr 21, 2008
Location: Watkins Glen, NY
Posts: 912
Troubleshooting Technique on Fuel Problems - Late Model Wag

First I want to thank everyone for the great advice on my sudden loss of fuel at the fuel filter this past weekend. I wanted to report my troubleshooting experiences for those of you who may happen upon the same condition.

A little background first...
  • I put in a new mechanical fuel pump about 4,000 miles ago
  • Most (not all) of my rubber fuel lines had been replaced.
  • Symptoms: I was experiencing erratic idle but not all the time, loss of power while going up hill (like I was running out of gas) and finally, while idling at a local store, the thing just up and died.
1. Pull the fuel filter and check for gas. If no gas, give it a good blow on one end. You should be able to exhaust it unrestricted. If not, replace the filter. If it's full of gas, you've got other problems with carb, ignition or the like which I won't discuss here. In my case, it was bone dry.

2. Now, work backward looking for the obvious. There's not much behind the filter that you can point to. Either the pump is bad or there's a potential intake problem like blockage.

3. Check all of your hose fittings for any signs of gas leakage or cracks. Replace those that look suspect. Many people have told me that you don't need to see or smell gas to have an air leak lending to poor suction from the pump.

4. Now remove the inlet hose from the filter and have someone crank over the engine. If you feel pressure on your finger, chances are, the pump is working (but not always!!!) These are mechanical pumps after all and are prone to consistent failure.

5. Remove the rubber hose from the steel line that runs between the pump and the feed line of the tank. Now hook a 3 or 4 foot long rubber hose to it. The longer the better. Remove the gas cap and blow into the rubber hose. You should hear bubbling in the fuel tank with no resistance. If not, your problem is definitely hidden in there. In my case, I heard no bubbling, but experienced the sound of an uninhibited steady stream of air from the tank.

Accessing the fuel sending unit (Only on Late Model FSJ's. Older ones may need to drop the tank?) - Literally took me 5 minutes

6. Tumble the rear seat forward and pull up the carpeting. This will expose the fuel sender access port, a roughly 6" diameter plate that is riveted into the floor.

7. Take an 1/8" drill bit and drill out the rivets. You'll then need to gently pry the plate off the floor because it's held down by sealant. Don't yank it too hard of you'll be hammering it back into shape later!

8. You'll now see the top of the fuel sender and notice your return and feed lines connected to two nipples on top of the unit. Remove these. It would probably be a good idea to replace these rubber hoses while you're in there.

9. Remove the two wires running into the sender. You'll probably want to clean up the leads on the sending unit since they'll probably be pretty corroded. Take a shop vac and vacuum out all of the dirt, dust, and gravel that could fall into the tank!

10. Now. Carefully take a large flat blade screw driver and a hammer and work on the large plastic nut that holds the sender in place. Driving to the left (lefty loosey, righty tighty) since it's plastic, it should come out without a problem. Mine came loose with about 3 or 4 good taps. Try not to bung the nut up too badly so you can reuse it!!

11. Remove the nut and the sender will pull right out of the tank. Careful you don't bend the float arm or you'll make your rig that much more inaccurate on the gauge.

12. Inspect the sender. You'll see two tubes leading out of the unit. One should have a 4" black or tan colored screen on it. This is the screen/strainer/sock.

13. Remove the sock and blow on the end. You should have unencumbered air flow through it. Inspect the sock for rips/tears/breaks. It turned out that this was my system weak point. The junction at the top of the sock where it clamps onto the steel fuel intake was broken. So, when my fuel level got down to about 1/4 tank, I'd start sucking air. These socks are cheap (<$10) and you should be able to mate up a similar one from another car at the parts counter if you can't find one for an FSJ. Don't worry if it's too long either, just don't get a shorter one!


14. Check the o-ring seal for the sender top to make sure that it is still good. If not, go get a new one. Might be worth the extra $1.50 for reassurance anyway. Place the sender back into the tank and nut it down. Replace all of the hoses and wires.

15. Get a tube of clear caulk or your favorite weather sealant and apply a bead to the mating surfaces of the access cover and the floor. Press the cover down and hold in place with four self tapping sheet metal screws.

16. Crank the engine over a few times and hopefully, you'll be back motorin' in no time!

Good luck.
1987 Jeep Grand Wagoneer
Stock Everything
AMC 360, 2-BBL Motorcraft

Last edited by scotty1998 : 10-16-2008 at 09:18 AM.
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