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[Lovesthedubs]

So my GTI started messing with me today......

It started as stalling after driving. It started and idled fine, but after driving a little ways and depressing the clutch pedal the car would sputter and stall. It started back up fine and would idle fin until i would drive around......Th en again i would depress the pedal to stop and the RPMs would drop and struggle and the car would die.

Hoping that the idle screw had just loosened i adjusted it to 900RPMs and starting tonight it starts and idles fine. Until i drive and stop now its dropping to 500RPM then revving itself to 2500-3000RPMs and dropping back to 500..... It repeats this the entire time im at a stop and the entire revving cycle takes about 5 seconds..... But it just keeps revving at stop lights......

I dunno where to even start looking any ideas?

[.:Reaper]

What year?

[Smokestack]

ISV is bad or you have a vacuum leak. Try those first. Could also be a bad coolant temperature sensor.

[Nate]

spark plugs?

[scurvy_bandit]

could it be a failing fuel pump?

[Southcross]

I assume this car has a Frequency Valve... if it does, does it buzz at the same rhythm as the engine reving up and down?  if so, try unpluging your O2 sensor

[Smokestack]

I believe it's a 91 8v.

[Lovesthedubs]

GTI is a 1992

[.:Reaper]

ISV is bad or you have a vacuum leak. Try those first. Could also be a bad coolant temperature sensor.

in that case... X2 on smokes comment...lope s blows asses

[Lovesthedubs]

Im sure i will catch hell but what is the ISV? i had hoped that it was something easy but it seems to be getting worse

[Smokestack]

The ISV is the Idle Stabilization Valve. It allows the computer to adjust the idle for warm-up and AC use on the older mk2's. The ECU uses it on newer engines (90 and up 16v and 93 and up ABA) for 100% idle control. It will look like one of these and mounts just above and behind the valve cover in front of the intake manifold (second pic).



The first thing you need to do is go over all intake hoses and vacuum hoses and check for leaks. If you have a spare ISV or can get ahold of one you can try changing it, but I'd try changing your coolant temp sensor (CTS). They're relatively cheap (if memory serves me correct) and are known to cause a lot of headaches in the Digi2 8 valve motors. It is possible your O2 sensor has gone whack too. A bad O2 sensor caused me to switch to a stand alone engine management.

[Lovesthedubs]

Would the best way to test for a vacuum test be a smoke test? I think these engines are mostly vacuum driven so that could be an endless search for a leak

[Smokestack]

Take a stethoscope that doctors use, unplug the "mic" so that it's just an open hose (a vacuum line with one end by your ear can work too), and point the hose at anything that might leak (intake boots, injectors, vacuum hoses, etc.).  If you hear a jet engine (or a whistle) as you move the hose past something, you've found your leak. You can also spray starter fluid around anything that might leak and listen for the idle to raise (the leak draws in the ether), but that only works ok for finding large leaks. You can pay a shop for a smoke test, but that gets pricey.

[Lovesthedubs]

Thank you so much guys for the input. I am gonna get on all this stuff on friday i gotta finish the work week

[Southcross]

for giggles anyways... on a warmed engine, try unplugging the O2... Smokestack can agree with me that a bad O2 can cause bad/weird things to happen

[scurvy_bandit]

you can ohm out or check the DC voltage on the O2...

[Southcross]

you can ohm out or check the DC voltage on the O2...

you can't "ohm" an O2 sensor

and voltage checks are only so accurate.  What can happen with an Oxygen sensor when it gets old, it gets slow to react to changes in A/F.... so if it detects lean, the ECU richens, the O2 may not detect the change untl the ECU passes the stoich to *very rich*, so the ECU starts to lean out the mix and the O2 doesn't detect it until its *very lean*.... causes the mixture, and the idle, to bounce up and down drastically.  Its actually very easy to spot this occurance on a Mk1 with a frequency valve as you hear the buzzing in rhythm with the engine.

[scurvy_bandit]

you can "ohm" anything... if it is open then it is bad.

[Lovesthedubs]

Okay so I took off the ISV and doused it in carb cleaner. I had read that they are notorious for sticking. after i put it back on the idle issue seemed to be fixed. Now im hearing an interesting sound from the engine bay at about 3k rpms. It sounds like either valves are being smashed or the AC bearings are going bad.... Im not losing power driving around, but while idling the AC compressor is shaking rather bad. I dont hear the sound at low rpm only over 3k....... I love my mk2 but jesus its always 1 thing after another.... Any thoughts?

[Southcross]

you can "ohm" anything... if it is open then it is bad.

an O2 sensor doesn't work that way

edit... the only thing you can Ohm on an O2 is the "self-heating" wiring, not all O2 sensors have heating elements

[Scott]

you can "ohm" anything... if it is open then it is bad.

yeah but that is almost NEVER the case with automotive electronics...

[scurvy_bandit]

thats how I found my bad MAF on my old MK4... the bentley tells you what the resistance is and it was waaaaaaayyyy off.

Either way, the problem was fixed!

[Southcross]

MAF on my old MK4... the bentley tells you what the resistance is and it was waaaaaaayyyy off.

Either way, the problem was fixed!

an O2 sensor isn't the same as a MAF