I'm stumped. Tiny off-idle stumble.
 

This is pissing me off. I've been battling this small stumble right off idle for awhile now, and it's finally time to kill the monster.

There is no stumble until the car gets into closed loop. This lead me to believe an o2 was bad, so I changed them, along with new extensions. The bastard stumble remains. The car ran perfectly with no stumble when I ran without o2s for 2 weeks.

What the hell could cause a stumble that only occurs in open loop besides O2s. I can't imagine what else would affect only open loop operation. I wouldn't think anything to do with intake air could, but my IAC counts when warm/idle/nuetral are about 40-45. My TPS voltage at 0% is .55. The O2s cycle in the scan just like they should.

I keep thinking to play with the idle screw in the TB, but then I remember that can't be my problem since it only occurs once the computer switches to open loop.

Any ideas?

I wouldnt touch the idle screw. id take off your Tb and check for shit in the IAC passage and in the TB itself

Inside of the TB is clean and clear. I'll check out the IAC Thursday when I have time. I still don't think that would have anything to do with a problem that only happens in open loop. If it was intake sensor, TB, or IAC related it should do it all of the time.


EDIT: I meant closed loop. I had it correct in the initial post.

well, when the car is in open loop, it totally ignores the o2s and gets fuel delivery off of the ect, maf, tps, and maybe the iat.

when its in closed, it gets it off of the first o2 sensors, maf, tps, etc.

you have a scanner to read what the o2s are reading?

also, how many miles on the car?

I mistyped in the post above. The problem only happens in closed loop when the car is warm. 85K on the car, and yes I have DataMaster to log. The o2s look normal, and I gave the TPS and IAC information above.

Correct me if i'm wrong, but I was under the assumption that the only thing that changes when the car switches to closed loop is that it starts to use the o2 readings.

Have you cleaned the MAF?

+1 to a maf

No I haven't. What would I wipe it down with? Q-tip and rubbing alcohol? Some kind of electrical cleaner?

I've seen maf cleaner somewhere...can't remember where tho. It was a spray can.

Rubbing alcohol is a terrible idea now that I think about it. I'm finished with class today so I'll just run out for some Lectra-Cleen or whatever the F it's called.

I'll report back in an hour or so.

a plain qtip will work as well

I sprayed down the MAF, and cleaned the throttle body. The TB was sticking slightly, so I found my problem. The stumble is about 75% gone now, so I'm going to pull off the TB sometime and get it spotless.

Drop the TPS voltage down to .51 idle and then reset 0%. That gives a bit quicker throttle response too. Noticable difference w/ my aftermarket TB over .55.

Can you explain how that's done?

Requires rotating the TPS. I would be a little concerned over doing this on a stock TB, I don't see much gain to be had here, but some people like every little possible gain.

If you have a stock TB it will probably not be much noticable over the .55 setting. You have to elongate the TPS sensor holes and rotate it with a DMM connected to the signal lead and ground until it sits at the desired voltage.

It is not a performance gain, but helps with throttle responsiveness and the lag some people get from aftermarket TB's. Made a noticable difference in slow drivability and takeoff with my AS&M 58mm.

Ahh gotcha. I hadn't ever heard of people doing that. I'll keep it in mind if I ever jump to a bigger TB.

at least you found it :)

What you're likely seeing is lack of air velocity. A valvetrain/cam or exhaust change would likely fix the problem better than a band-aid TPS hack. I'm totally not beating up on you here, I've done the same thing myself, just telling you what the problem likely is, from my experience.