91 idle surge and FIAV question.

[n0va91TSI]

Ive read a million threads and articles concerning idle surge and I think that it my surge is FIAV related.


These are my symptoms:
Untill the car is at normal running temp (needle straight up) anytime I go from in gear to clutch depressed or neutral the idle will rise from normal idle to about 1500-2000 rpms a few times before settling back down to normal idle. Also when I first start my car the idle is about 2000-2100 rpms and slowly falls to normal idle after about 30-50 seconds. If the car hasnt been run for a few hours and has cooled off completely to room temp, the idle even in gear will be high (around 2000 rpms) for a few minutes but is steady and not surging like it does in neutral.
All of the above occur in all temperture weather.


The thing I dont understand is what is goes wrong with the FIAV that makes it surge. Isnt it just a wax pellet operated open or shut path for intake bypassing the throttle plate? Isnt it just doing what its supposed to do? Increase the idle untill the engine is all warmed up. How can it cause my rpms to rise and fall so quickly? If someone can explain how this thing works I can fix it. Im sick of people asking me why Im reving my car like that.

help!

 

[me612]

you're pretty much correct on how the fiav works. Coolant is running through the throttle body. Once the engine is warmed up, it will melt that wax pellet and cause the valve to close. Well like all things, it gets old and eventually might not seal as well, causing an ever so slight vacuum leak. That vacuum leak can cause idle surge. That is how the FIAV can cause idle surge. Many other things can be the cause of it. Have you absolutely narrowed this down to the fiav yet, or are you just guessing it is the cause?

 

[n0va91TSI]

after reading enough threads I think it has to be the FIAV because the problem goes away after the car has warmed up

 

[me612]

have you tested anything else? the ISC is relly simple to test and a good idea to do that before you go and spend money on a new FIAV. If you really think it is then go for it. If it were me, I'd troubleshoot it like I troubleshoot our radars. Get rid of all other possiblities before putting a new part on order. That way you don't spend money out of your budget on something that wasn't even necessary.