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Learned something new about the SR20DET (why it was running rich)


jovial_cynic

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I've been scouring the web to try to figure out why my SR20DET has been running rich. Ever since I installed it into my car, the thing has ran annoyingly rich at idle, blowing around 12.2 on average on the A/F meter. I installed a fuel pressure gauge to see if there was a pressure issue, and it looks like I was pushing about 10PSi higher than normal into the fuel rail. At idle, you want to see around 37, and I was blowing just under 50PSI. So... I thought maybe my FPR was bad; increased fuel pressure could certainly cause the engine to run rich.

 

But then... I stumbled upon some Nissan forum post that talked about something called the Fuel Pump Control Module (FPCM). This is fed by PIN 105 on the Redtop S13 ECU. When I first wired up my engine, I was told that this was unnecessary, so I ignored it. The FPCM wasn't in a box of parts that came with engine, so I figured it wasn't important.

 

Well, it turns out that the FPCM does something quite important: at idle, it run the power to the fuel pump through a dropping resistor, slowing down the pump whenever ECU pin 105 signals idle. Once it's off idle, the FPCM bypasses the dropping resistor and gives the pump full power.

 

For my particular setup, I have a couple of boxes of 25-watt white-brick-style resistors sitting around, and after messing around with different combinations, I found that my particular fuel pump makes my engine happy at idle when there's 0.46ohms of resistance in the way. I *think* the stock setup is 0.67ohms, but since I have a non-stock fuel pump, I have a different need. Once I step on the gas, I can run the signal from the ECU to a relay and send the fuel pump down a direct path with no resistance, and when I back off the gas, it reroutes the fuel pump down the path of 0.46ohm resistance and prevents unnecessary backfire and bad fuel mileage.

 

Anyhow, just thought I'd share! I have successfully solved the SR20DET-runs-rich problem!

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