Running without the fuel pump resistor

[Jesus-Ninja]

Does anyone get overfuelling issues on idle by running 12V to the pump all he time? As I understand it, the ECU should drop the pump rate on idle by running it through a resistor.

Or does it not matter because it's running in closed loop and so the ECU will trim fuel?

 

[stageajay]

I`ve got an 044 running full chat all the time the engine is running and dont have any problems, its probably just there to slow the pump as it doesnt NEED to be running flat out, the fuel pressure will remain the same

 

[melvinmelvin]

On my 1UZ I also have a fuel pump running full speed all the time, haven't noticed any problems,
(its a reasonably high capacity pump for a M Benz).

 

[Jesus-Ninja]

Thanks My main concern would be MOT testing of idle emissions.

 

[George_R]

Then you better worry about cats, sub o2 sensors and valve stem seals

 

[Jesus-Ninja]

Cats and O2 sensors aren't a problem

 

[cribbj]

The resistor does drop the voltage when the engine is at idle and fuel demand is low. The reason is to keep from needlessly pumping (and heating) 300+ HP worth of fuel when the engine is only burning maybe 25 HP?

Toyota replaced the resistor with a PWM type circuit (the "Fuel ECU") in the later models, which is much more efficient.

Fuel heating can be a big issue and its resulting evaporation can put a big load on the Evap system (fuel vent/breather & charcoal canister).

Not only is the evaporation an issue, but hot fuel is less dense than cold fuel and won't make as much power.

 

[Jesus-Ninja]

Thanks, cribbj. So worth keeping, but not for emissions particularly. I've "copied" the stock wiring with regards the Circuit Open and Fuel Pump relays and their triggers from FPR and FC etc, although currently (no pun intended) my "resistor" is a piece of wire, as the original one got binned with the body loom.

Any ideas what wattage resistor it is? I know it's about 0.7 ohms.

 

[George_R]

typical fuel pump takes up to 5 amps, so 20Wt would be safe enough.

 

[cribbj]

I'd be careful sizing that resistor - Toyota's (Denso) fuel pumps draw much more amperage than, say, a Walbro. I had some fuel pumps "dynoed" 10 years ago at Kinsler and they measured over 20 amps @ 14v on a new Supra pump. Granted, it was only about 10 amps @ 9v at the equivalent idle pressure of 40 psi.

Plus these dropping resistors get HOT. I'd probably get several ceramic type resistors and put them in series/parallel to get the ohms needed and the wattage required, but I don't think I'd go less than 50w.

If you burn that resistor up, then every time the car goes to idle mode, and it tries to switch that resistor into the circuit it'll die Ford had this problem with their voltage dropping resistor on several models back in the late 80's & early 90's IIRC.

 

[Jesus-Ninja]

Probably best to use a stock resistor then, I guess. They're about £20 from a breaker. By the time I've bought something new with heatsinks etc, it's not worth trying to fab something. Just annoying that I binned mine!

I won't be using the Toyota pump though. I'll be keeping the MX5 (Miata) tank and running a Walbro GSS342 in-tank.

 

[cribbj]

All depends on what you want to do. Some people run their pumps at a full 13.5 volts all the time and claim it works fine. However there are several good reasons that I mentioned why you might not want to do this.

Another option might be to put an inline fuel cooler in your return circuit. Ferrari did this on my 1999 550 Maranello because they evidently didn't like the idea of a dropping resistor nor a Fuel Pump ECU, so they ran the fuel pumps at full alternator voltage, and they ran the return fuel through a little inline heat exchanger which is in parallel with the main evaporator for the A/C system.

 

[Jesus-Ninja]

Well, it's a track car, the only time it goes on the public highway is getting to and from trackdays, so it's not like it spends a lot of time at idle in it's normal MO. It's either driving steadily on the road or being nailed on the track. I'm not going to be transitioning from being stationary in traffic to driving at ten tenths.

I reckon stock resistor or none.