Why the hell does my car idle?

[My5ABaby]

S4 N/A, mild aux. streetport, no emissions

I've recently run into a problem, my car is idling at a rock solid 750rpm. Normally that would be a good thing, except I'm not sure why. Landers removed the emissions with a rebuild about 8 years ago and ever since then I've had a bouncing idle, the car dies at under 1k, idling high, etc. Basically everything except a good idle.

I recently put on a RB catback and ran into a snag. I cross threaded a bolt so the ypipe sits about 3/4" away from the presilencer, making a super awesome exhaust leak. Anywho, since then I've had the rock steady idle. That I can think of I haven't change anything else. I haven't been under the hood in a while except to change the oil.

Can anyone point me in the right direction as to why?

[Pete_89T2]

IIRC, the S4 NA uses exhaust back pressure to actuate the aux ports, correct? Just a WAG but maybe the change in back pressure has something to do with this. Has the performance across the whole RPM band up to & including redline changed, relative to where it was before?

[My5ABaby]

Yes and no. I believe it's technically exhaust velocity or something, but backpressure is the normally quoted thing that opens the ports.

The performance is terrible at this point. It stutters and takes forever to get to redline in anything but 1st (which still doesn't have the same pull). I think it's more than just the ports not opening (i.e. acting like a clogged cat or something). It's also popping A LOT on decel. Those two make sense to me, but the good idle still doesn't.

[FerociousP]

"IF" he still has that pipe. Maybe a vac leak? Are you using it in any configuration to keep the 6ports? What is still on your LIM. ACV? That little disk valve?

perhaps ur running overall richer now?

I can come look at it Robert this weekend, or you can drive it over, and we'll go over the possibilities. You know what would help you with this? A wideband ;)

[My5ABaby]

I do have the pipe from the presilencer to the aux ports (yay RB presilencer). I have 0 emissions stuff. No ACV, BAC (unfortunately), EGR, etc.

I might be running richer. Apparently my car now pops flames when I redline and let off (hehehehehehehe) which it didn't do before. If my aux ports weren't opening but the engine was putting in fuel assuming they were (does the ecu work that way?), then I could see that.

All that said, still not sure why my idle is good now. I would think a vac leak would cause my previous symptoms, not my current "symptom" of a good idle. :dunno: I just find it all odd that the only change I've made is the exhaust and this happened right as I did that. Seems a bit too conincidental.

A wideband would help... know anyone with one that I can borrow? ;) I can bring it over or you can stop in, either works. We'll figure it out. As a side note, I've been driving it to work which is kind of a nice change. Now I just need A/C..............................................GAH !

[FerociousP]

the ecu just reads gets the air charge from the AFM and temp, so that shouldn't be it.... Hey, if my O2 could be removed easily I'd gladly let you borrow it.

Just bring it over this weekend or next week when I have all my weeknights free

[RETed]

Popping decel is a sure sign that the stock ECU is still injecting fuel on decel.
Normally, the TPS should signal the stock ECU that you've lifted off the throttle and anything over 2kRPM's - this is a guarantee range where the stock ECU should cut all fuel off at the fuel injectors.
With a TPS that's not "zero'd out", the stock ECU still thinks you're (slightly) on the throttle, thus still trying to inject fuel into the engine...

Cycling idle is a sign of an intake leak...
With increased air (due to leak) getting into the engine, it starts to nudge the RPM's higher, since idle is usually on the rich side anyways.
It hits around 1,800RPM where the stock ECU sees that the TPS is supposedly adjusted "closed" and goes WHOA - this is decel fuel-cut territory, so it cuts the fuel off.
Endless cycle...

Now, I realize that these two conditions are mutually exclusive.
At this point, you've eliminated the bottom condition, but you still got the top one - the misadjusted / bad TPS.
Did you test resistance of your TPS yet?
I don't think your condition is exclusive to the exhaust leak...
It could be, but that's a long shot- usually stock ECU doesn't care what happens after the exhaust exits the engine.
(Well, not true if we're talking about the O2 sensor, but the O2 sensor is ignored at idle.)
Stock electronics SUCK donkey balls; that's why I yanked that sh*t years ago and installed my Haltech E8. :D


-Ted

[My5ABaby]

So my misadjusted TPS is letting my car idle properly?

[FRED]

Just backprobe the input wire from the TPS to the ECU and see if the TPS is set right or not. See the Control Unit pages of the free online FSM for the value of the TPS at idle with a fully hot engine. Those pages are in the fuel and emissions section of the FSM.

Pin 2G in the attachment.

[My5ABaby]

That sounds good. 1v at the ECU. :icon_tup: I'll see what mine is. Probably like 12v :rofl: