Mrspencer

Ok boys now I'm really lost,sent both my maf and my LH unit out to be calibrated and serviced due to my erratic idle, so get updates today maf was out of calibration, my The LH unit got a clean bill of health no issues at all, injection labs did my Maf and Electronik repair did my LH, so if these things checked out what am I missing,also smoke checked the system no leaks at all. The car is an 85 s3 5spd.

FredR

I know you have written about this previously but did you replace the ISV along the way?

Mrspencer

yes 3 times 2 used and one new,the new unit was not a Bosch unit.

FredR

Given you have replaced or checked the more obvious candidates this usually leaves one looking for the less obvious. I have an after market ISV and it works fine but I do have an irritating surge that causes the idle speed to fluctuate by about 200 rpm either side of the set point of 675 rpm for my S4 motor. I could not find any leaks and have taken a number of measures to help prevent such to no avail. I suspect the problem reduces to one of a small leak around the bearings of either the throttle butterfly or the flappy in the inlet manifold [unique to S4/GTS]. I do not know if your model year is prone to such leaks in the inlet tract or indeed whether smoke testing would reveal such.

GregBBRD

One of the overlooked issues with the '85/'86 cars is the charcoal canister purge system, which commonly is leaking vacuum....sometime huge amounts....and it doesn't show up on a "smoke test". The system has two valves to control the purge vacuum....they added a second "redundant" valve as an extra measure of safety....and it is actually uncommon to find the primary valve still holding vacuum. If the secondary valve also fails, you will have a vacuum leak, which varies in size according to how badly the two valves leak. If the charcoal canister "Y" is broken or if any of the hoses are bad, the result will be outside air being drawn into the intake. If there are no leaks, the gas tank will be sucked flat.

Note that this is a very common cause of cracked tanks (almost 100%), because the tank is continuously being sucked in and then returns to normal when the car is shut off.

Mrmerlin

a good thing to do is put a vacuum gauge on the small check valve port see what your idle vacuum is SB close to 20 in.
Also be on the lookout for a holed brake booster,
this can be tested by pulling the big check valve out of the booster,
and plugging it,
see what your vacuum reading is.
To test the purge valves crimp the line with needle nose pliers, then read the vacuum

JBGold07

I ran my 86 with the TPS unplugged, caused idle surge. Maybe unplug it and see if there is change. You replaced it but might be worth checking again.
I think I read back you mentioned about mouse nest when you first bought it. Semi chewed wire somewhere?
Sorry to hear about this ongoing battle. Good luck it'll be worth it

Mrspencer

thanks for the info so I pulled my passenger side wheel off and then the fender liner and sure enough the y tube is snapped in 2 places , hopefully that's it for the idle issues.

Mrspencer

So the y tube to the charcoal canister was broken going to both valves.

FredR

This is a universal failure point- there are replacement options in metal. It will be interesting to see if rectification of this cures your problem- do keep us informed of the result.

Mrspencer

so replaced the y tube and still the same surging idle, intake coming apart again this Sunday.

Mrspencer

So after all is said and done, I began to pull the intake apart again, and while checking the isv I find a small crack in the 90 degree hose coming off the idle control valve. Thanks to all .