So, I've been experiencing a higher than normal idle for the past few months when stopped at a light. So far, I have not been able to tie the issue to outside temp., not letting the car warm up, highway vs. city driving, or anything else. I have seen a few threads regarding this, but they are typically titled/labeled "car shakes/vibrates when stopped", which spawn into a what motor mounts do you have discussion (FYI, I have stock mounts). I want to figure out why it feels it needs to increase the idle randomly.
Here's some more info:
Normal idle engine speed is typically 600-700 rpm
Stopped a light, after 10-15 seconds, rpm jumps to 1000-1200rpm
Stays at higher rpm for 15-20 seconds, then goes back down, eventually to "normal" rpm
When in high idle "mode", putting in park, R, N, D, S, M does not make a difference
A/C on or off
Outside temp temp 50-80 F (only been happening since May or so, so haven't tested in cold)
Does not happen always, will happen at first long stop after startup, and also after 2+ hours of highway driving, or not at all
If it happens once, I can usually bet it will happen again at the next stop
Letting the car warmup fully seems to help, but it will still happen
Not letting the car warmup seems to reproduce the issue more consistently, but not always
Why this bothers me:
Even worse MPGees (not statistically proven by me yet)
Car does shake rather badly (motor mounts may help, or may make it worse), I'm sick of passengers asking what's wrong with my car
Depending on rpm when proceeding from stop, car lurches worse than normal for DSG
I've attempted to have the shop diagnose the issue, but so far is non-reproducible. I'm hoping I can have it happen again on my next visit, but don't want to pay for technician test-drives each appointment.
Also, I'm stock except for flapper mod (vacuum tube removed and plugged w/ screw). Thanks in advance for any suggestions or advice.