Horrible mpg, low / variable fuel pressure... Motos + twin walbros
07-30-2005, 01:16 PM
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Horrible mpg, low / variable fuel pressure... Motos + twin walbros I have moto 60s (IFR = 69.x lb/hr @ 58psi) and twin walbro GSS430s (one only active above 4psi).
My fuel pressure gauge reads 48psi on average at idle (whereas the mechanical gauge read 57psi when the motor was installed in Feb). The weird thing is that the idle fp ranges from 54psi through 46psi. Mostly the pressure is pretty consistent from start up (i.e. if it reads 48psi on start it is mostly 48psi at idle), but I will occasionally see weird events like a dip to 45psi every 25 seconds, and then a rebound to 48, etc. When the second pump kicks, the pressure goes up, so I'm pretty sure it isn't a plugged line or something, but am at wits end.
I need to somehow verify the voltage to the fuel pump, and I need to rule out a vacuum problem (have a few vac devices teed into the vac regulator line), but other than that am stumped.
No misfires, no overly rich running. Any ideas?
Forgot to mention -- nothing I do in the tune (including running lean / rich, open loop /closed loop, lean cruise, DFCO, etc) has ANY impact in the mpg I see. I did highway test today in open loop mode with 12.1 AFR set (doing some SD logging) and got 9 mpg. HIGHWAY!! Normally, I see 8 +/- .5 mpg -- regardles of tune, how heavy I'm in it, etc. Is it possible to have an injector stick half open / leak??
My fuel pressure gauge reads 48psi on average at idle (whereas the mechanical gauge read 57psi when the motor was installed in Feb). The weird thing is that the idle fp ranges from 54psi through 46psi. Mostly the pressure is pretty consistent from start up (i.e. if it reads 48psi on start it is mostly 48psi at idle), but I will occasionally see weird events like a dip to 45psi every 25 seconds, and then a rebound to 48, etc. When the second pump kicks, the pressure goes up, so I'm pretty sure it isn't a plugged line or something, but am at wits end.
I need to somehow verify the voltage to the fuel pump, and I need to rule out a vacuum problem (have a few vac devices teed into the vac regulator line), but other than that am stumped.
No misfires, no overly rich running. Any ideas?
Forgot to mention -- nothing I do in the tune (including running lean / rich, open loop /closed loop, lean cruise, DFCO, etc) has ANY impact in the mpg I see. I did highway test today in open loop mode with 12.1 AFR set (doing some SD logging) and got 9 mpg. HIGHWAY!! Normally, I see 8 +/- .5 mpg -- regardles of tune, how heavy I'm in it, etc. Is it possible to have an injector stick half open / leak??
Last edited by TurboBerserker; 07-30-2005 at 09:37 PM.
08-12-2005, 10:45 AM
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I have the Walbro 255 and I'm seeing the same issues in testing that I didn't witness with the oem pump. Fuel pressure is everywhere from a baseline of 53 psi in vacuum, it will drop to 46 in vacuum without reason. I think that the Walbros have inconsistent flow.
08-17-2005, 07:27 PM
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Ok, I finally verified it after some datalogging, the usual battery voltage at idle on my Honda S2000 test mule is 14.1v, that is when the alternator is giving juice. Then when the big fans kick in the voltage drops to 12v because of the current draw. Fuel pressure drops an average of 5-7 psi, due to the lower voltage to the Walbro, and then goes back up to where it was when voltage reaches 14.1v again.
I verified this effect both with my Synchronic regulator and another manufacturer's FMU. You might want to watch your battery voltage.
I verified this effect both with my Synchronic regulator and another manufacturer's FMU. You might want to watch your battery voltage.