sudden onset bad gas mileage :ideas?

[SondraS2k]

arrived in Topeka Tues, 26+\- mpg the whole way, no issues. Check engine light appeared Wed afternoon; obd indicated lean condition.verified with tuner that we actually set it a hair rich in March to be safe, so we reset the computer and it hasn't reappeared. Had about 3 gal of 91 octane in it and added 1 gal of 100 oct at the track, completed autocross runs. Experienced bogging twice on course under WOT after sharp lefthand turns. Afterward, filled the tank with 91 octane and left Topeka this morning. Mileage has been terrible since I left Topeka, averaging 18 mpg. No visible fuel leaks.

Ideas welcomed! 467 more miles at 18 mpg is going to be pricey!

[My_yella_s2k]

sounds like a faulty front o2 sensor to me.

[Goku]

the bogging is from lack of fuel. When racing you need about half a tank to avoid the pump sucking in air. the pick up is on the left side of the tank so any high G left turns will cause this to happen. As for your fuel, since the ecu has been reset try driving very consertive. The ecu still maybe running to rich because of some mods you have. Also you could have just gotten some bad fuel, and are over reacting due to the problem in topeka.

[SondraS2k]

I drove it around town that night after resetting the CEL; I thought I did so enough to remap it, I suppose time will tell.

[repiv]

A good wide band a/f gauge would be most useful here. It would confirm whether or not the OEM primary O2 sensor is behaving correctly. You may be running very rich, yet the ECU can still think you're running lean due to a bad sensor and try to dump more fuel. Are you seeing any large quantities of black smoke coming out the back when you "punch it"? Can you smell any gasoline on the dipstick when you pull it out? After running hot, shut it down and open up the oil fill cap. Can you smell any gas there? I'm thinking that perhaps if the injectors are leaking, you could be pushing raw gas past rings and valve guide seals.
There's a chance that you could have a small leak in the fuel delivery system that you would not detect as a visible leak. Small leaks in the engine bay can quickly be evaporated away (not to mention dangerous). Can you smell fuel if you open up the hood after running to operating temps?
Bogging could also happen if the MAP clamp is not hiding boost completely. Extremely rich can cause bogging, too.

[stantaur]

Agree with all above. Need at least 1/2 tank of gas, esp. if you're running R compounds and hard left turns. I fuel-starved away my best run at Nat Tour in '04 because of that. That could be part of the bog.

As for the fuel MPG, might be time for investing in a wideband O2 and learn more about the cause/effect of various tweaks you have in your setup. Give a person a fish, feed them for a day vs. teaching them to fish & feeding for a lifetime.

This is an unlikely scenario to ask, but did you do any quick alignment changes (such as adding a ton of toe-in to cure tail-happiness)? If so, that might have some impact on your MPG too via the increased drag of extreme toe-in. Ask the Honda Insight guys about optimizing their toe & camber for uber MPG. That won't explain all of the delta btwn 26 MPG vs. 18, but it could be a part of it.

[karno]

[QUOTE=stantaur;387506]Agree with all above. Need at least 1/2 tank of gas, esp. if you're running R compounds and hard left turns. I fuel-starved away my best run at Nat Tour in '04 because of that. That could be part of the bog.

As for the fuel MPG, might be time for investing in a wideband O2 and learn more about the cause/effect of various tweaks you have in your setup. Give a person a fish, feed them for a day vs. teaching them to fish & feeding for a lifetime.

[QUOTE]

Well, I have a same problem recently twice and that is a week apart. My Fuel ratio meter indicated that I experienced rich durring bogging, even when it is at low RPM ??? I think it has to do with the car being FI and the fuel rail pressure regulator and return fuel regulator are neither vacuum or pressure driven sensories. May be it is time to change these item or perhaps insulate them from the heat of the engine. I say this because it only happened when the engine is up to temperature or more in my cases.
I had the ECU fuel remap real-time twice already. But after reset the ECU things seem to get back to normal for awhile, and then after a few days of driving it would happened again. :doh: