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  Fuel Pump Upgrade



The heart of the fuel delivery system is the fuel pump.  If it is not up to the task at hand nothing else you do will get the needed fuel into the combustion chamber.  Early on I discovered that the stock fuel pump is not sufficient to supply enough fuel to meet the added demands of the TRD Supercharger.  I have come up with a couple of solutions to this problem.



I loved the added performance of my supercharger, but I noticed a dramatic power loss as the speed passed 90 MPH.  If I would let off of the throttle just a tad there would be more power.  To me that seemed to indicate a fuel starvation problem.

When I installed an electronic fuel pressure gauge from Auto Meter, my first test drive the problem became crystal clear.  With the pressure increasing FMU the fuel pressure would rise under boost and would max out at around 65 PSI at full boost.  That all seemed to be working fine until the engine pass 4000 RPM.  At that point I would see the fuel pressure start to drop off and drop all the way down to 50 PSI.  This was the heart of my fuel starvation problem.  TRD later deleted the pressure rising FMU from the kits so now they just run lean all the time.

When I started my dyno testing I had the dyno shop temporarily install their exhaust gas temperature (EGT) gauge.  This showed that when the fuel pressure would drop off the EGT would spike to very high temperatures and this is where my power loss was coming from. Later I installed my own EGT gauge and think it is a must have instrument to have with the supercharger.

Click here to download a video that shows what the hell I am talking about here.  This was filmed in my 97 4Runner with a manual transmission.  It is equipped with 305cc injectors.  The left gauge is the EGT gauge, the center is the fuel pressure gauge, and the right one is the oil pressure gauge.

What I did is to get it into third gear at 2000 RPM and then open the throttle all the way.  When the throttle goes full open you will see the fuel pressure jump up to about 50 PSI with the Free-FMU and as the RPMs build you will see the fuel pressure drop off and if you look closely at the EGT gauge you will see the EGT spike into the red line.  Then as the engine hits the rev limiter and starts to shut down the injectors you will see the fuel pressure jump back up.  My 97 does not drop off as bad as my 96 did, but it also only has 16,000 miles on the pump which may have a lot to do with it.  This drop off would have been a lot worse if I was still using that TRD pressure increasing FMU.  Watching the pressure nose dive around 20 PSI is something that falls into the Aw-Shit category.  



Clearly I needed to increase the pump output so that I could supply the fuel needed for the supercharger.  There seemed to be a couple of ways to go.  One is to install a larger fuel pump which involves figuring out what size pump was needed and then dropping the tank to install it, or using a new gadget that increases the output of the stock pump.  This gadget called a Boost-A-Pump (BAP) splices into the fuel pump wiring and increases the voltage to the pump. This increases the speed of it impeller so it will flow more fuel.  Initially I went with the BAP because I had no idea what size the stock pump was and what size I needed.

The BAP is pretty simple to install.  It gets spliced into the wire going to the pump, has a control to set the amount up boost to the pump,  a pressure switch that is mounted in the manifold to activate the BAP under boost, and a small control box. 

My dyno tests with the BAP showed that a setting of 35% on the control now prevented the fuel pressure loss, eliminated the noticeable power loss and stabilized the EGT.


Pump Replacement:   

I later figured out that installing the proper size fuel injectors and removing the FMU would be a better way to go because I still was not getting enough gas into the engine.  I was hoping that with the increased injector size I could reduce the fuel pressure back to normal levels and that the fuel pump could flow enough fuel at the lower pressure to supply the engine.  When the fuel pressure goes up, pump flow goes down and I was hoping that as I reduced pressure the pump flow would go up and be sufficient.  Well unfortunately that did not work and I still needed to boost the pump, but not as much.   

The Supra pump is on the left and the stock one is on the right.

I still wanted to find a replacement fuel pump that would flow enough fuel but without knowing what the stock pump was rated at it was hard to guess what to move up to.  Since I was using a Supra MAF and Supra fuel injectors I thought that maybe a Supra fuel pump was in order.

I ordered a Supra fuel pump through Conicelli Toyota.  When it arrived I sent it out to RC Engineering to have it flow tested.  That is one hell of a fuel pump.  At 45 PSI it flowed 285 liters per hour (LPH) and would support 753 HP, but would draw 19.6 amps while doing it.  It did seem to be at least enough for the supercharged 3.4 so I pulled the tank and installed it.

This is the Supra pump installed in the tank unit.

On the first test drive it popped the 15 amp fuse supplying voltage to the pump which I expected.  What I did not expect was that it over ran the fuel pressure regulator and the fuel pressure was 5-10 PSI higher at all RPMs and manifold pressure then it was before. 

I checked with the folks on Supra Forums and learned that the Supra that this pump belongs to has an interesting way to deal with this high output pump.  The ECU can reduce the voltage to the pump to reduce its output to reasonable levels until more gas is needed.  Well that is something that the 4Runner canít do and I was not in the mood to installed one of the aftermarket gadgets that does the same thing.  I needed to find the right size pump.

I sent the stock pump to RC Engineering to have it flow tested.  I had them really work it up with many different voltages.  Using the 35% increase in voltage from the BAP to the pump would be around 17 volts to the pump so I used that chart from RC Engineering and at 50 PSI that worked out to be 133 LPH from the pump.  I figured that to be the bare minimum and know that the that flow needed should be around 80% of the pumps capacity.  I figured I needed at least 175-180 LPH from the pump at 50 PSI and started my search for the right sized pump.   

The left pump is the Walbro 190 LPH pump and the right one is the stock pump.

I found a 190 LPH pump that is an after market upgrade pump for a Mustang.  I bought it and sent it off to RC Engineering to be flow tested.  The flow test showed at 14 volts (regular system voltage) and 50 PSI (the pressure from the Free-FMU) the pump was flowing 179.81 LPH and was drawing only 7.3 amps.  Well that looked really good so I yanked the tank and installed the pump. 

The test drive showed all my math was right and it was the perfect pump.  It was not over running my fuel pressure regulator and maintained fuel pressure at full throttle all the way to red line just fine.  This is the pump that is needed for the TRD Supercharger when used with the proper sized injectors.  

If you are planning on staying with the stock injectors and use a pressure rising FMU do not plan to use this pump  above 75 PSI.  RC Engineering noted that the pressure bleeds off at 75 PSI. 



You guess it, pulling the tank is a pain in the ass to do.  It would be best done using a lift and a transmission jack so you donít break your back.  It can be done at home using some jack stands and a floor jack. I jacked the truck up and supported the rear axle with jack stands and then unbolted the skid plate. With the engine running I unplugged the connector at the top of the tank to kill the pump and depressurize the system.  Then I disconnected the fuel lines, the filler hose and the vent hose.  When disconnecting the fuel lines be sure to mark them so you get them right when reinstalled.  One is a vent for the EVAP canister and the other is a fuel return line from the fuel pressure regulator.  I am sure you can see that if you get them backwards you could have a serious problem. 

The vent hose going back up to the filler neck, is the hardest to do as it uses those spring type clamps and they are hard to get at and seem to be at the wrong angle.   There is a second bundle of wires that goes out through the floor plate with the fuel tank bundle.  There is a plug tucked under the seat that can be disconnected and then you can toss the plug through the access port in the floor.  This wire is connected to the top of the tank and then goes up to the filler neck.  As the tank is lowered you will have to unhook this wire from the tank so you do not rip it out of the filler neck.  I placed the floor jack under the tank and unbolted it and slowly lowered it. 

This is the tank unit with the stock pump.

The tank unit is the part that has all the pipes that go into the tank and holds the pump, fuel level float, and low fuel level warning light switch.  It has eight small bolts around the plate.  Unbolt these and pull the tank unit out of the tank.

There is a plug on the pump that unplugs and then two clamps on the hose connecting the pump to the hard line.  Once those clamps are removed the pump can be pulled from the bracket.  There is a foot on the bottom of the bracket that holds the bottom of the stock pump.  That needs to be cut off so the new pump will fit.   

This is the Walbro pump installed in the tank unit.

I took some fuel line and split it long ways and slipped it over the edges of the bracket to cushion the pump and held it in place with a large hose clamp.  

The stock wire needs to be cut and the new pump wires spliced in.  If you look at the stock pump you will see that connection at the top of the pump is labeled with a + and Ė signs.  The red wire gets connected to the stock wire that went to the positive lead on the stock pump.  On mine it was a blue wire.  The black wire gets connected to the other wire. 

I used the new worm gear clamps that came with the new pump to connect the hose to the pump and the hard line.  Then the tank unit gets reinstalled into the tank and the bolts get torqued to 30 inch pounds and the tank is reinstalled.

That is all there is to it.  Just make sure you donít have a full tank of gas when you do this.  Gas weighs 6 pounds a gallon.  The less in the tank when you get started the better.




After much testing I have finally found the perfect replacement fuel pump.  It is a version of the 190 LPH fuel pump that is designed to be a direct drop in our trucks.  You can get one from UNDERDOG RACING DEVELOPMENT for $83.