as the title says...lately my em1's been running rich when low on gas(e85). when gas gauge is at E...no light yet, it'll idle @12.2-13.0 afr. when at low speed/gear at 40mph or less, afr is 11.8-12.8

above 40mph, afr is normal at 14.0-15.2
when filled up, afr back to normal. any clues what's going on.

walbro 255, aem fpr, rail, 6an feed from rail to filter(aem stock replacement), stock lines, delphi 880 cc

going to topmount from inlinepro, how long of feed n return line should i get?

 

I have dual walbro 255s and I can hear the pumps start to suck air below half a tank. I never let it get below 1/4 tank and I never boost on a low tank. Upgrading the fuel line wont fix this.

 

so, it's a common thing, huh.

 

Usually it runs lean when the pump sucks in air, or if the fuel overheats due to being low on fuel (if the setup has multiple pumps).

If you are running ID750's/ID1000 or similar injectors, check your static fuel pressure when your car is low on fuel and compare it to full tank. Depending on what your fuel pressure is set at, sometimes lower fuel pressure in relation to injector response times can cause the car to actually idle and cruise richer, but at higher pulsewidths it would still be leaner due to lower pressure.

It's probably not a problem because you shouldn't be ripping it in WOT anyway if your fuel pressure is actually lower with near empty tank. If you really want to drive your car down to near empty, you should consider a surge tank setup.

 

car was running fine last year and until now. i'll just have to filled it up at 1/4 tank.
880 cc injectors on stock base pressure(43 psi @idle).

low afr is rich, high afr is lean...right? sometimes, i confused myself.

 

when you're low on gas the tank is usually capable of building much higher pressures, which raises the actual fuel pressure in the rail, which richens it up.
also when you're low on gas, the gas itself heats up because there's less of it to cycle through the full system, and it heats up as it passes through the engine bay and back to the tank. so the tank and all the gas in the tank get hotter. higher fuel temps cause the fuel to evaporate faster and atomize much better once injected into the engine, richening it up.

 

It shouldn't build enough pressure inside the tank to affect rail pressures like that, unless the EVAP system or charcoal canister is completely blocked off for whatever reason. It will take almost equal pressure inside the tank to create resistance for the fuel return line. I am guessing, 2-3 PSI inside the fuel tank to actually affect the return line and affect fuel pressure as a result, but most EVAP systems or charcoal canisters are built to maintain only up to 0.5 PSI then it vents.

I've never seen a car get more fuel pressure at idle when the car is low on gas. You're actually the first person who said this, and I find it very odd.

Hot fuel requires more injector duty to maintain AFR.. The hot post start enrichment is to compensate for hot fuel temperatures. OEM cars like Subarus or Evo's with fuel temperature sensors would have increase fuel compensation for rising fuel temps. It's exactly opposite of what you're stating.

 

well tony, I assume most people remove their evap systems when going the boosted route, but most people also probably don't plug the vent line. I always plug the vent line in my cars because I'm not a fan of having it just open venting to atmosphere, especially considering how much fuel can be lost by simply driving down bumpy roads.

most of the evap systems I've researched allow a 1psi pressure buildup in the tank and release after that. I've also measured tank pressures in my own car while having the vent line capped, and I've seen up to 3.5psi on a really hot day when below 1/4 tank of gas right after driving down a super bumpy road. colder temps, and anything over 1/4 tank though and I never see more than 1.5psi. usually only 0.5-1.0psi