soldave
13-08-2009, 04:41 PM
Found this out very recently and thought it was pretty interesting information and might be relevant to one or two of you. Have been doing some problem solving, trying to find out why my idle was a little rich. I worked my way back to my Sard fuel pressure regulator and checked fuel pressure. It was at 45psi so I went to take it down to stock 43.5psi. But no matter what I did I couldn't get it to go any further. I even had the top elbow unscrew completely but still pressure stayed there.
So I thought "screw it" and thought I'd live with 45psi, but then when I put the vacuum line back on the top nipple the fuel pressure didn't change at all. I know it should drop a few psi so I'm thinking my FPR has possibly died on me. But then I did a little research and found that the 2G AWD DSMs suffer from the exact same problem: fuel pressure regulator overrun.
What usually happens in this situation is that the fuel return line isn't wide enough to take back enough fuel, and so the pressure stays high. But this wasn't the case in the situation of the Evo I. What I think is causing it is the design of the fuel tank itself. Because of the driveshaft, the fuel tank is basically in 2 separate parts, with a siphon between them that draws fuel from one side of the tank to the other. This is the restriction in the Evo I system and not the fuel pressure regulator. I might be wrong, but I am pretty sure the Evo II and III have a different design of fuel tank (Mitsubishi have it listed with a different part number anyway) with a bigger siphon.
Now there are a number of solutions, the main one being to enlarge the siphon between the fuel tanks, but there is an easier one you can do with your tuning software and is the one that I adopted. I upped the fuel pressure at the FPR to 55psi, and then told my tuning software that my injectors were 840cc and not 750cc ones. Problem is then solved!
Hope this is helpful to someone out there, and is something to think about if you're idling rich. Note that this problem does not occur when at full throttle; only at idle and light cruising.
So I thought "screw it" and thought I'd live with 45psi, but then when I put the vacuum line back on the top nipple the fuel pressure didn't change at all. I know it should drop a few psi so I'm thinking my FPR has possibly died on me. But then I did a little research and found that the 2G AWD DSMs suffer from the exact same problem: fuel pressure regulator overrun.
What usually happens in this situation is that the fuel return line isn't wide enough to take back enough fuel, and so the pressure stays high. But this wasn't the case in the situation of the Evo I. What I think is causing it is the design of the fuel tank itself. Because of the driveshaft, the fuel tank is basically in 2 separate parts, with a siphon between them that draws fuel from one side of the tank to the other. This is the restriction in the Evo I system and not the fuel pressure regulator. I might be wrong, but I am pretty sure the Evo II and III have a different design of fuel tank (Mitsubishi have it listed with a different part number anyway) with a bigger siphon.
Now there are a number of solutions, the main one being to enlarge the siphon between the fuel tanks, but there is an easier one you can do with your tuning software and is the one that I adopted. I upped the fuel pressure at the FPR to 55psi, and then told my tuning software that my injectors were 840cc and not 750cc ones. Problem is then solved!
Hope this is helpful to someone out there, and is something to think about if you're idling rich. Note that this problem does not occur when at full throttle; only at idle and light cruising.