16valver

I know when you unplug the O2 sensor puts the car in open loop. It bypasses a number of measuring elements and runs pre programmed fuel maps based off of engine RPM alone. Does it bypass the fuel quality switch as well?

I recently did a short trip south of portland to visit a fellow 944'er, and decided just for experimentation sake, to run the car in open loop at -3.1% on the FQS.

I had leaned out the fuel mixture before when the O2 was plugged in and noticed the car running hotter, slightly less power and a slightly more rough idle. When I did this with the O2 unplugged there was absolutely no difference from when I had the car plugged in w/ stock mapping. I actually think I got worse gas mileage than my usual 13mpg... I burned through a quarter tank in not very much time.

I'm wondering now if I might have a bad DME. The O2 sensor, relay, FPR, injectors etc. have all been gone through. I'm sort of left with the DME and the AFM. I feel like if the AFM was going bad it would have a more noticeable effect.

Eric_Oz_S2

The FQS is never bypassed. 13mpg is terrible mileage.

16valver

Ok thats what I thought.

HA! you're telling me!! Its ridiculous. I DD this thing too... I think my plan of action will be to snag a known good AFM and if that doesn't work I might have a DME issue. Might seafoam it one more time just for stupidity sake.

Arominus

Just a random question.. Are you sure the odometer is accurate? How many miles a day are you driving?

16valver

You know...thats a damn good question. The mileage I drive per day varies. Today I'll have driven 0 miles. Wednesdays I drive ~16 miles, most other days it won't be more than 8 miles.

I suppose it could be turning too slow or something. I'm running slightly larger rear tires on 16's than what you would normally find, but don't feel like it would make a huge difference.

On the stock setup @80mph the tach would be at exactly 4k rpms. Now @80mph the tach displays about 3800rpms. On the other hand, whenever I drive by one of those neighborhood speed signs that tells you what your speed is (and usually tells me to slow down ), it displays a lower speed than what the speedo says. So if the speedo says 40 the magical sign would say ~37.5.

Oooooooohhhhhh man...

Gawernator

Try this out and report back http://www.miata.net/garage/tirecalc.html

krystar

something else is off then. my early 8v has o2 unplugged and i still get high 20's mpg. i have a wideband o2 in place of the original o2 and i'm still reading ~14ish AFR.

could be your AFM is bad. an incorrect airflow reading would cause the DME to dump fuel. also possible one of your injectors is not closing and it's just dumping fuel. what do your spark plugs say? probably wet and black i'm going to guess.

16valver

Yeah I did some rough calculations like that before putting the new setup on

Heres what that one came up with (which is probably more accurate than my back of the napkin calcs)

16valver

Injectors were witchhunter'd recently and all tested out great so I'm hoping its not that. I would guess you are right about the plugs haha. I replaced those a little while back as well. I also have an immense amount of carbon build up on my tail pipe that didn't used to be there.

My money is on the AFM now. I was holding out for Rogue to finish his 16v creation but it looks like I'm going to have to bite the bullet. It sucks because I have an AFM laying around from a 2.7...

Anyone have a spare S AFM??

Gawernator

So that would indicate running very rich, which also explains your TERRIBLE gas mileage (I get in the upper 20's)

16valver

Yep already knew I was running very rich. All the signs have been there for quite a while. Carbon deposits on tail pipe, cat pulling excessive heat, etc etc...

I just hope the AFM is the reason why. I'll go ahead and try buying a "good" one and go from there.

Gawernator

Open it up and see if the little contact blade can be adjusted to a new spot

16valver

Ok, I might as well do that. My theory is that the needle valve has been severely messed with. Cap is missing, I re-adjusted it to the 1.5 turns clockwise from closed to get the ~stock position but it obviously hasn't done much...

Eric_Oz_S2

Disconnecting O2 makes the car in richer. This will also happen if the O2 sensor is faulty. Have you checked the O2 and wiring?

The sensor can also be tested by adjusting the plug wires on the variant switch plug. This will switch to non O2 mode and it will ignore the O2 sensor (different from unplugging the sensor).

Your best bet is to check the afm using a voltmeter. Use test leads between the afm plug and socket so you can measure voltage on the output terminal while you move the vane by hand.

16valver

Well I replaced the O2 sensor as you can see from my avatar but haven't ever checked the wiring.

When I get a chance I'll go out and do that to my AFM. I'm curious to see what happens.

ALSO, I think my reference sensor is starting to act up. I've always had the intermittent starting issue that happens when the car has cooled off most of the way but is still slightly warm (mentioned in other threads). I've been keeping a close eye on the tach for tach bounce. Now I'm noticing that sometimes I don't get tach bounce when I'm supposed to. I'll keep watching to see what it does and order a new sensor from pelican.