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Walbro 255 fuel pump vs. the resistor

22 May 2002
Tucson, AZ, USA
My stock fuel pump bought the farm, and I'm putting a Walbro 255 in today. My question is (on an N/A car with I/H/E/Throttle body/Intake Manifold/ECU) do I have to short out the stock resistor to run this? If so, why, and does anyone have a recommendation for doing this in a clean-looking yet reversible manner?

Thanks for the help!

I cannot see in your post where you have any way of adjusting fuel trim, therfore you should leave the two stage system in place. The resistor is not matched in any way to the pump that would preclude you from using it with a different pump.
All that said, my personal experience is that the factory pump is the most reliable unit available, and unless you have mods I didn't understand from your post, the Walbro is overkill.

If you can adjust fuel trim, such as with an AEM, then the cleanest way to bypass the resistor is to remove it from the firewall stay bracket completely, then take the two wires that used to plug into it and connect them. I would do that by removing the connector (always leave a 1.5 or 2." pigatail in case you have to reattach the connector) and solder the remaining ends.

Thanks for the reply, Mark.

Yes, I do have the AEM FPR, so the fuel trim is not an issue. I chose the Walbro because I wanted to have the capability to do future mods (ie., BBSC :tongue: ) without having to go through the pain in the butt I am going through right now again, and spending more money in the process.

So, with the AEM FPR, you are saying that I should bypass the resistor? Would there be any harm in leaving it as is (connected to the resistor?)

Again, thanks for the info! :)

If you are NA, with OE ECU and OE injectors, there's no need to bypass the resistor. It's no problem to run the Walbro with the voltage variation. You will not have any performance gain by running higher fuel pressure with the stock set up... the ECU will just reduce the fuel delivery to keep it all in balance...

The primary role of the resistor is to improve mileage under cruise conditions and reduce emmisions. The resistor is shunted when you're at full throttle.
It looks like I answered my own question last night after I got the fuel tank installed back into the car. The car runs awesome with the resistor out of the loop, but my car runs like crap with the resistor in the loop. Maybe the Walbro doesn't like the lower voltage or something, but the engine/ECU clearly doesn't like the lower fuel flow. So, I've shorted out the connection to the resistor and zip-tied it out of the way, and pulled the resistor completely out of the engine bay (hey, every ounce counts you know :tongue: ). I suppose I should have listened to Chris at SOS when he told me not to use the resistor, but sometimes I just have to find things out for myself.

Anyways, pulled the clock fuse to reset the ECU, and the car runs awesome now. It idles much smoother than it was before, and a small knocking/ticking sound that I used to occasionally hear is gone. I guess the best way of putting it is that my engine seems a whole lot "happier" now that there is a healthy fuel pump pumping more "blood" (fuel) through it's veins.

The next step is a dyno to see if I've finally fixed my A/F issues.
It is the job of the ECU to manage cruise conditions and emissions. A resistor is a very simple device which is not capable of such management. The primary role of the resistor is to reduce pump noise and flow below 4800 rpm.
Chuck, Sorry I could not answer sooner. Sounds like you have it figured out. If you are tuning / installing critical parts and need help, pick up the phone. I am trying to be better with the mountain of e-mail and PM's I get, but I am always available at the shop. If you can't get past the phone police, (Jane, Bob, or whoever) tell them it is very important and I am expecting your call.
shop- 480 968 8010
cell- 602 312 6877

Good Luck, and

NSXTech said:
(snip)The resistor is not matched in any way to the pump that would preclude you from using it with a different pump.
Mark, that is absolutely untrue. The resistor value is unique to the stock NSX fuel pump. Installing an aftermarket pump that requires more current than the NSX pump will result in more voltage drop across the resistor and less voltage at the pump. In the extreme case (as proven by Viper Driver), the voltage at the pump will be too low and result in low fuel delivery / fuel pressure.
I'm not an authority on this subject, and I'm sure multiple people in this thread have more experience on the subject than myself (MB/Bryan etc). However-- I just installed the same pump in my car about a week and a half ago.

I also have an ems, 550cc injectors, and FI @ around 10psi, with a stock regulator.

I didn't bypass the resistor, even after it was recommended by a few people... not because I didn't trust them, but no one really had a solid reason for doing it and I was curious to find out for myself. I haven't noticed any abnormal fuel pressure spikes or dips. I've played around with it since I put in the pump trying to get the fuel pressure to drop or the AFR's to go lean when in the 'low voltage' mode.. ie; boosting or accelerating under 4800rpm without full-throttle, but have yet to see anything abnormal.

I'm keeping an eye on it, and if i see it do anything weird, even once, I will bypass the resistor... but until then, I'm leaving it...

With the AEM EMS replacing the stock ECU, do you still hear the fuel pump resistor bypass relay click on/off at 4800 RPM under load?
BryanZublin said:
With the AEM EMS replacing the stock ECU, do you still hear the fuel pump resistor bypass relay click on/off at 4800 RPM under load?

To be very honest, at or near 4800rpm (or anything over 2500 really) my car is so god damn loud and generating so much blower/intake noise behind my head I can't hear a damn thing..... (this isn't a bad thing, atleast for me.... ;))

I think I can see the fuel pump voltage through the ems gui, I'll take a peak tonight when I get home..