Join Date: Jul 2003
Location: Mt. Airy, MD, USA
Drives: 94 Stealth TwinTurbo
Re: Hot-wire overruns FPR - any easy fixes?
Originally Posted by Bruhaba
I work as an engineer in automotive and frequently work with OEMs in the powertrain field. They are all cost concerned- always. I know many people have hotwired without problems, but you can guarantee mitsubishi did not add logic into the ECU and an extra resistor and relay for nothing($).
Could noise reduction have been the sole justification? The impact on longevity is debatable.
Originally Posted by Highboosted
Too many variables to start guessing on fuel pump life, but pumps are rated at battery voltage. They don't use stepped systems to increase life, they use them to decrease sound. That being said, decreasing voltage MAY increase the life of the pump, and increasing the voltage (beyond normal, ala boost-a-pump) MAY decrease life. There is not enough statistical evidence to make a conclusion, but it is generally known that running a fuel pump beyond 16v (boost-a-pump'd) will decrease it's life somewhat, but how much is unknown because of manufactoring tolorances.
Is the Whinebro as unstable as the Denso through voltage transition? How do the pressure measurements compare?
Originally Posted by ErikGross
MKIV Toyota Supra Fuel Pump Upgrade
One note, however, is that the Supra pump's increased flow and voltage sensitivity cause mild fluctuations in fuel pressure in the low-load range and around the voltage transition point. With the RDR Fuel Pump Rewire, I observed the following. At idle and low load, fuel pressure stays at 3.2kg. As load increases, fuel pressure will drop to 3.1kg as load approaches the pump voltage transition. If load increases enough to cause the ECU to run the pump at full voltage and the load stays very close to that point, fuel pressure will increase to 3.4kg. As load increases further beyond the pump voltage transition point, fuel pressure gradually falls back down to the target of 3.2kg. Pressure then stays at 3.2kg for all loads above this point. I have not definitively determined if this pressure fluctuation causes problems, but I suspect that it does. I noticed frequent low-mid level knock around throttle transitions at light to moderate loads. My suspicion is that this fuel pressure fluctuation interferes with the ECU's ability to properly set the low- and mid- fuel trims.
My current course of action (December, 2004) is to rewire the Supra pump to receive full voltage all the time and install a new fuel pressure regulator that can handle the increased flow at low loads. So far, with just the "hotwire" wiring change (i.e. running the pump at 13V all the time), knock levels at light loads and throttle transitions seems to be greatly reduced, but not eliminated. I suspect that the limitations of the ECU's 3 discreet fuel trim ranges prevent it from correctly accounting for the extra fuel pressure at low loads due to the full-voltage Supra pump and OEM fuel pressure regulator. Once I install the new fuel pressure regulator, that will validate or disprove my theory.
Originally Posted by ErikGross
AEM Adjustable Fuel Pressure Regulator Upgrade
If the fuel pump is wired according to the RDR Fuel Pump Rewire Method, I see a slight (1-2psi) drop in fuel pressure as load increases toward the fuel pump voltage transition. I believe this is a result of the Supra pump's inability to flow enough fuel at this load with the reduced (8-9V) voltage. At the fuel pump voltage transition, fuel pressure spikes about 3-4psi over normal (total change 4-6psi). My suspicion, based on datalogs, is that this fluctuation in fuel pressure causes the ECU to be unable to set the fuel trim values to ensure proper fueling under light to moderate loads.
If the fuel pump is hotwired (~13V constant), then the OEM fuel pressure regulator is unable to lower the fuel pressure to normal (43-45psi) under low-load conditions. This results in a fuel pressure of around 51psi at idle, and this pressure decreases to 45psi as load increases. Once the fuel pressure decreases to 45psi, it stays there as long as fuel demand (IDC) remains constant or increases. Again, my suspicion, based on datalogs, is that this fluctuation in fuel pressure causes the ECU to be unable to set the fuel trim values to ensure proper fueling under light to moderate loads.
Some may argue that this fluctuation in fuel pressure (~6psi) is not significant enough to be a problem, but based on my datalogs and findings after installing this AEM FPR, I contend that it is a problem. I cannot say for sure whether this is unique to my particular car or typical of all 3000GTs and Stealths, as I have not tested other cars. However, based on what I've read of others' experiences, I believe a significant number of other owners have experienced these problems. The main symptoms I noticed after installing my Supra fuel pump were throttle-transition knock (knock when you abruptly open the throttle plate) and low-RPM/part-throttle/high-load knock (such as when you're at 40% throttle in 5th gear on the highway and you build reasonable boost). I tried having the Supra pump installed with the RDR Rewire method and with it simply hotwired to the battery. The hotwire setup actually made things *worse* in my car, so I encourage all owners with hotwired Supra pumps and the OEM regulator to datalog somlow-RPM/part-throttle/high-loade "regular" driving and look for knock at throttle transitions or low-RPM/part-throttle/high-load conditions. Note that during all of the aforementioned experimentation, I rarely saw significant knock at sustained WOT (wide-open-throttle), even up to 15psi of boost.
- Mt. Airy, MD, USA
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