been trying to figure out an intermittent erratic idle for a few weeks with no success. here are the details

car is stock 997tt (stock exhaust, stock DV no issues at all.

then on a friday night cel comes on about 2 minutes after starting the car on a warm night. first time i had seen the cel since owning the car. wasent able to get over to garage to get code read that weekend so decided to clear the code using apr code clearing feature via the cruise stalk. CE light is gone, doesnt come back immediately - so i figured from my vag days it was likely an o2 sensor or something of the like out of spec intermittently.

a few days later notice an erratic idle. NEVER immediately on warm up never consistent with wet weather no patterns noted. when i would get to a stop, noticed the rpms wouldnt drop to idle speed as before. it would hover at 1100 for about 5-7 seconds then settle to normal idle and settle there.

went to reputable local indy garage - basically were busy and said not to sweat it and booked in an appt for the following week. sure enough, that week on friday cel came on - drove it straight over and it gave a code "lean condition bank 1" if i remember correctly.

they smoke test the car and there was a vac hose that came off which was reattached. i need to dig up the service invoice to recall which hose but i was told it was near the back and difficult to get to and had to let the car cool down in order to gain access.

vac line secured, re smoked - no leaks. get the car back and after a few hours of driving the idle issue is still there! slowly getting more frequent and to the point now where it no longer hovers at 1100 rpm before settling but rather is fighting to stay alive and not stall the motor wavering between 600-800 rpm.

at this point i should note that the car never bucks on acceleration and drives smoothly but i have definitely noticed that in chipped mode, the ecu seems to be pulling back boost. it hits 1 bar then pulls back to about 0.6 750 rpm later. i know limp mode and this didnt feel like it (maybe soft limo mode?). plus it wasent pulling as strong as when i first got chipped.

returned to the garage - pressure test was normal. try to read mafs - data wasent able to be read in chipped mode ( i wonder if this is apr protecting its software with some type of lock on this data) in stock mode MAFs are showing way too much flow as per the tech. swapped over mafs from another 997tt that was working fine - still has intermittent weird idle and mafs reading are way off.

also developed this weird intermittent airbag light that durametrics kept finding fault codes for different sensors on passengers side only - weird

call apr - they suggest to go to dealer and get ecu remapped with stock software. and then drive for for a few weeks to ensure issue wasent due to software as they thought perhaps the controllers in the ICU may have been fighting with each other (their words not mine )

so today - got the ecu flashed to stock - car holds boost in sports chrono mode - feels stronger. airbag light was caused by faulty sensor behind front bumper.

car idles perfectly all day....

i get it home and thought i heard the idle fluctuate a bit.

NOW this is where it gets weird - all along i have been able to reproduce the erratic idle by creating vaccum thru pumping the brake pedal off and on. after about 4-5 pumps - erratic idle. the indy mechanic actually noticed this and first pointed this out to me.

so i try the brake pump test and sure enough it works ...erratic idle.
and it reproduces the symptoms regardless of wheter the ebrake is also on or not (not sure if it matters)

i should also add that i have on previous cars felt what a bad brake master cylinder feels like and i have never felt give or spongyness in the brake pedal modulation

as of this point between the indy and dealer i have
1) smoke tested
2) pressure tested
3) remapped stock ecu
4) checked line to fpr
5) found one vac line that was apparently reconnected


anyone have any thoughts as to what to do next?
repeat boost leak test?

 

check to see if your brake vaccum booster is leaking

 

Thanks will do. I should also add that the erratic idle isnt limited to being on the brakes. Before i had the car ecu reflashed to stock it would have an erratic idle with ebrakes on and no foot on brake pedal.

Wondering if there are two problems - ecu fighting itself which now seems better, AND something to do with brake vacuum.

Btw can the brake vacuum boost leak be detected via smoker test?

 

I'm thinking repeat boost/vacuum leak.

 

thx - car goes back to get pressure tested again monday ...keeping fingers crossed