Rough idle 7A 20v

[AudiRS]

I`ve got a little problem with my Coupe Q with a 7A 20v.

The engine got a nice idle when it`s cold, but when it gets warm it starts to jump up an down between 1000 and 1400 revs. Sometimes it even stays at 1400.
The car perfom very good, revs steady and does`nt use abnormaly with fuel (in what way a five cylinder engine from late 80`s got a good fuelconsumption )

- Have checked vacumhoses with no good luck.
- Have cleaned the Idle Stabalizer Valve but no difference.
- There where no errors in the diagnose.

Anyone knows what errors at the 7A-engine that can cause this problem?

[hermanio]

fault codes ?

[Kiku]

Try different ISV.

[AudiRS]

fault codes ?

No fault-codes!

[AudiRS]

Try different ISV.

I can try if I can get a hold of one.
But the ISV is cleaned, it ticks when the engine runs and when I pull the plug at it the engine stalls, so i presume it is doing its job.

[mrt105]

Mine is doing something similar at the moment, cold it idles well, then when warm the idle will rise up to 2k! changing through the gears can be interesting when pottering about as when i dip the clutch the revs rise each time.

I've cleaned and changed the isv, swapped the maf and checked all the hoses etc, the next thing i'm going to try is the throttle body, i have a spare so i'll see if that makes a difference!

[Error404]

check the idle switch on the throttle body

[hermanio]

thats a common problem, because of the heat from the exhaust manifold the underside of the idle switch will crack.

[Error404]

even with the heatshield they get pretty roasted!

[AudiRS]

I`m going to take a better look at that idle-switch! Luckily I`ve got one extra.
The engine goes down to idle som what kind of slow

[mcandmar]

Idle switch can be tested with a multimeter, its just a simple switch. You should be able to hear it click when you open/close the throttle.

They do melt with time, especially if you run without a heat shield. I have a few that look like this. Idle switch works on them but the throttle position sensor went faulty.

Also with 7A's first thing to check for air leaks is the crank case breather, there is a large rubber 90 degree hose that connects to the block, they always rot and fall apart with age.

[AudiRS]

What is the typical symptoms of a faulty idle-switch?

[Error404]

, will idle often at 1100rpm or more, and return to idle vvvvvvvvvveeeeerrrrrryyyyyyy slllllllooooowwwwlllllyyyyy

[AudiRS]

I`ve now changed this idleswitch, but whit no luck. It still got a bad idle.
But i noticed something.
When the engine is cold and running, and I take of the plug for the idle stabilizer valve the engine goes a bit rough - this is normal.
But when I do this same presedure when the eninge is hot (and idles roughly) the engine revs down to aprox. 8-900rpm, the rough idle stops and it is acting like everything is normal!

Do we think that the problem layes within the ISV?

[simon20vq]

def sounds like isv issue as it works for not just when the engine is cold but also hot. Did you clean it with brake cleaner? If not try that squirt in both holes and cover them with your fingers shake it and then poor the crap out. you may need to open the valve with a fine tip screw driver. Has your mpg changed recently? Check the o2 sensor is connected correctly and not come loose as well.