So for starters my n/a s4 is suffering from the old bouncy idle syndrome. It bounces around from about 1200 to 1500 rpms. It idled fine for the last three years, then one day on the way to work I was doing my usual 7k pull on a decent straight stretch to keep the carbon out. At the next stop light the issue showed up. The weird thing is it doesn't always do it. Yesterday I did about 60 miles of back roads and the bounce wasn't there. Today I was running errands and it happened before the engine was even fully warmed up.

The details of my setup: mild street port engine with about 20k on it. Rats nest has been removed and the vac system simplified with all remaining vac lines replaced with silicone hose. Still have the Thermowax, Bac, Dashpot, and AWS. The EGR, ACV and Cold Start Injection are blocked off. All unused vac nipples are capped with silicone caps. I still have the mechanical OMP, with new teflon lines. I double checked the TPS while the motor was hot and it was right at 1v. I just replaced my plugs while buttoning the UIM and stuff back on.

I pulled my UIM and TB/Dynamic Chamber off to check the vac lines under the UIM. Resealed everything while I was putting it back together. I'm like 95% there is no vac leak, but haven't smoked it or tried propane'ing it yet.

One thing that is really odd is when the bouncing is happening, if I jumper the Initial Set Connector the idle will settle down to the solid 750 I've had for the last 3 years. But if I jump the Set Connector when the idle is acting normal my idle drops so low that the engine will die.

I'm kind of leaning towards the BAC, since they are often the culprit when the TPS is not the issue. My only concern is that this issue is intermittent, so I'm not sure if that my be the BAC going bad, or perhaps a poor connection or broken wire? I'll also push up on the throttle to make sure it's closing all the way and no binding in the cable.

Any brain storming here would be greatly appreciated.

 

Might want to clean the BAC and perhaps play w/the Variable Resistor.

 

I'm having idle issues too but only when cold, though my car has 0 miles on the engine

Rotary > Pistons

 

satch got called into work today and i probably won't have the time to do any testing until thurday or friday afternoon. i'll update this thread with the results. i also found some plans to build a diy smoker to test for vac leaks. seems like something that'd be handy to have around given how susceptible these motors are to leaks.

wthdidusay82 yeah i came across one or two of your threads while searching. have you replaced that TID yet lol?

 

yea I replaced my tied and like a million other things lol

Rotary > Pistons

 

Ok so here's an update. Ran some errands after work, made sure engine was fully warmed up. idle was bouncing pretty much the whole time. Get home, pop the hood, yank the BAC off. Not very much carbon in it, the plunger looks mostly white with some soot in the folds. Hit it with a little carb cleaner for ***** n giggles. Once it had dried I did the 12v test using two alligator clips, one to the Neg post of the battery, the other being quickly touched and released to the Pos. Can feel the BAC clicking. Next I check the resistance, according to the FSM it should fall between 10.7-12.3Ω, mine reads 12.5Ω so that might be the issue?

Also checked my Air Bypass Solenoid Valve while I was in the area, FSM says 9.2-11.3Ω and mine reads 11.5Ω. Ok so now I got two electrical components that are involved in the idle system and both are out of spec. So I guess it's time to find used parts.

I do have one other question though. When the idle was bouncing before and I jumped the Initial Set Connector things would settle down. Today when I attempt the same thing my idle didn't change at all. What gives?

 

Those readings aren't too far out of spec... perhaps a grounding issue? Shot in the dark, could be completely off base...

 

i'll double check my grounds, but i feel like the issue would be more intermittant if that were the case. i got all the parts to build my $50 intake smoker. i'll get it built tonight and check for leaks tomorrow and report back. hopefully i just missed something when i was checking for leaks last week

 

Did you get this figured out yet? I am having a simular issue and cannot create a post so I am jumping on this one. I have searched the forums and havent really seen the answer. So here is my issue. I drove in this morning and all was good until I dropped the wife off and drove to where I work and actually got on it just a little like 4k maybe? Then as I am pulling into work I see my idle jumping around 1.5k. So I take lunch and it is still doing it. I unplug the TPS and it stops surging. I leave it unplugged and take it for a spin. After 10 min or so the car just wants to die so I go to pull over and it died. I reconnect TPS and crank up the car and the idle is perfect 750 - 800 hunting is all. So Obviously it is something with TPS right? I just did adjustments a few months ago and TPS was right at 1k like it should be. So is there something I can lube up or is the TPS going?

 

fully warm the engine up, and then readjust the TPS. the TPS's move around, so they can need adjusting every 6 months or so.

if it doesn't want to adjust, you might just need a new one.

 

the high idle bounce is usually 1 of 2 things if it pops up out of the blue: a sticky throttle body or a sticky thermowax.

the BAC and TPS usually cannot cause the effect on their own unless the idle was improperly set to begin with(set higher than spec).

i always start by ruling out the thermowax first, pry the thermowax plunger off the throttle cam to allow the throttle to drop to base idle. if there is no change then the thermowax isn't the cause.