- Aug 3, 2006
Autoblog Maintenance 101: In-tank fuel pump R&R
If you own enough vehicles with an electric fuel pump, eventually it's likely that your luck will run out and you'll find yourself with a vehicle that refuses to operate properly due to a lack of go-juice. Most modern vehicles place the pump in the tank where it can be kept cool with a constant bath of fuel, but this location isn't so ideal when it comes time to replace that troublesome component. Fortunately, the task usually isn't quite as daunting as it may first appear, and tackling this project yourself can save several hundred dollars.
Fuel pumps can either fail outright, in which case the vehicle will simply refuse to run, or they will lose pressure over time, causing a slow and sometimes not easily noticed drop-off in performance and drivability. Increased fuel pump noise (it's the whine coming from the area of the tank) or longer cranking times are often signs that your fuel pump may not have much longer to go.
Originally, this article was going to be written using our '96 Buick Roadmaster as the subject (its fuel pump is dying a slow death), but then our '91 Chevrolet Caprice wagon decided to stop running. Yep - we have two vehicles down at the same time, with the same problem. Hopefully that emphasizes the frequency of this particular problem. Fortunately, the fix is straightforward, and the wagon died in our own driveway and with a nearly empty tank of fuel.
Since we're working with a highly flammable liquid, hopefully it's clear that extreme caution must be taken to avoid an unintentional fire. Also keep in mind that gasoline is hazardous if it comes in contact with the eyes or skin, so don the appropriate safety gear. The toxicity of gasoline also means that it should not be allowed to soak into the ground, so be prepared to soak up any spills with kitty litter or rags.
After proceeding past the first step in the fuel pump diagnosis (Will the damn thing start/run?), we then moved to energizing the fuel pump by using the test jumper. This connects +12V directly to the pump, and bypasses the relay. If the pump doesn't run at this point - and ours didn't - then it's either the wiring or the pump. And the odds are very good that it's the pump, and not the wiring, so it's time to drop the fuel tank.
If the pump does run, connect a fuel pressure gauge to the test port and compare the results with the recommended operating range in the service manual. In the case of our Roadmaster, the gauge read about 32 PSI at idle, and 18-20 PSI under load (we taped the gauge to the windshield wiper and went for a quick drive; not a recommended technique, but an effective one). The idle fuel pressure wasn't far off from the expected 32-37 PSI, but the WOT fuel pressure should be at least 42 PSI. This likely explains the vehicle's severe hesitation, surging, and spark knock upon tip-in, and continued operation with such low fuel pressure could cause engine damage.
Anyways, back to the wagon project...
We're not always as good as we should be with regards to disconnecting the negative battery cable before starting any work, but gasoline and sparks make for a poor combination, and so it's a step that we don't skip when playing with an electric fuel pump.
Lift the vehicle as high as practical, as you'll need enough room for yourself and the fuel tank. As always, don't depend on a hydraulic jack to keep the vehicle in the air, so drag out a set of appropriately rated jack stands and place them under the vehicle.
With the vehicle in the air, we now see the plastic fuel tank, and the metal straps that hold it in place. The rear two bolts only serve as a pivot for the straps, and do not need to be removed.
Remove any heat shields that may interfere with lowering the tank or accessing the fasteners, fuel lines, or wiring harnesses.
Disconnect the fuel tank filler and vent lines.
Take this opportunity to disconnect the wiring harness, because it's easier to do it now than when the fuel tank is resting on your chest (er, we cheated and moved this photo up two spots to show the way that it should be done, rather than how it was done).
Remove the bolts that hold the straps in place. Be sure to support the fuel tank during this step; we simply braced it up using any appendages that were free at the time.
Gently tug down on the retaining straps and pivot them out of the way, while still holding the tank in place under the car.
At this point, we perform a small dance number under the car, as we attempt to squirm out from under the tank while trying to avoid the retaining straps. The "empty" tank had about seven gallons in it, and when that gets sloshing around, it feels far heavier than its nominal weight of 45 lbs would suggest.
Set the tank on the ground, making sure that none of the fuel lines are being stretched.
Pinch the retaining clips and remove the fuel lines from the tank (high-pressure EFI systems may require an inexpensive disconnect tool, available at most parts stores).
Drag the tank out from under the vehicle while attempting not to spill any fuel from the now-disconnected filler and vent lines.
Remove the ring that retains the pump and sender assembly. While studs and nuts are used here, other vehicles may use a twist-lock retaining ring. A bit of force may be required to remove those rings, but go slowly so that the tank is not damaged (it's better to hurt the retaining ring than it is to deform or crack the tank).
Gently remove the sender/pump assembly, being especially careful of the sending unit's delicate float arm. If this is damaged, a new sending unit may need to be installed, which can greatly add to the expense of the project.
Here we see the reason for the fuel pump failure. The bottom of the tank was filled with crud, and the in-tank filter (commonly called a "sock") was obviously dirty. Since this is a plastic tank, the contamination likely came from an outside source. This much dirt probably wasn't the result of a single "bad tank of gas", but rather the result of repeated refills from a contaminated source. One of the vent lines was also disconnected, the apparent result of prior work by an inattentive mechanic, and this also could have been an easy route for debris to enter the tank.
We procured a new sock, and spent about an hour emptying the tank of the contaminated fuel and cleaning out its internals (both tasks were made somewhat more difficult by the tank's internal baffles).
Installing the new pump on the existing sending unit is as easy as loosening a hose clamp and unplugging the power connector. This is a good time to inspect the wiring harness, as the additional current draw of a failing pump can result in thermal damage. If the wiring or connectors appear to be burned, replace the sending unit.
While it's usually a good idea to simply install a direct replacement for the original pump, it's not necessarily a bad idea to consider a high-flow replacement if engine modifications are likely in the future (there are a large number of different Walbro-style in-tank pumps available). In this case, we replaced the stock low-pressure pump with a high-pressure unit, as eventually we'll be swapping out the low-pressure Throttle Body Injection (TBI) unit for a higher-performance Tuned Port Injection (TPI) unit that requires greater fuel pressure (approximately 45 PSI, compared to the stock 12 PSI). Since we've left the stock regulator intact for now, the fuel pressure will remain at the lower stock level until those future modifications are performed.
If in doubt, just get a stock replacement, and don't be afraid to spend a bit extra for a pump with a longer warranty.
Reinstall the gasket at the top of the tank.
Drop the pump/sending unit assembly into place, once again being careful not to damage the float arm.
Torque the fasteners to specification, or until snug.
We replaced the plastic clips at the end of the fuel lines, since they got a bit chewed up during removal.
Prepare the retaining strap bolts by applying a coat of anti-sieze compound.
Slide the tank back under the car.
Reconnect the fuel lines. They should make an audible click when fully seated, and it's not a bad idea to give them a gentle tug just to make sure.
Lift the now-empty (and much lighter) tank back up into position, and install the retaining strap bolts.
Hook up the filler neck, vent line, and electrical connections, and reinstall any heat shields that were removed. Turn the key to the On position, and listen for the pump. Assuming that it runs (it may operate for only a second or two), fire up the engine and make a careful and thorough examination for leaks.