1991 VW Cabriolet runs rough during idle

Hello,
to me its sounds as though you have dirt ,gunk & grime ,water, in the main inline factory fuel filter, ,it will be a steel canister type, between the feul tank and the intake manifold, change filter,( u may need a mechanic, or someone handy with a spanner, once off, tip contents into and look for laquer peel, some tanks are seal with this, and it will some times peel ( especially if tank not kept full). if that is the case, have your tank taken out & cleaned, also, check airfilter, car will run better n cheaper to run, Hope this Helps. Kipper

Have you checked for air leaks in the intake after the air box

1st run scans for fault codes.

You are equipt with a idle speed control motor.There is no way cleaning is going to clear it,When the RPM jumps it is either working and trying to compensate for a vacuum leak.Have the vehicle running as it jumps up and down feel the EGR is it hot to touch? Becuase that can cause the jumping RPM,Check the PCV valve with the car running plug it does the RPM stabalize? If it does replace PCV. You are searching for a vacuum leak

Idle Air Control value is most likely the problem.

CHeck fuel pump. Check oxygen sensor

Check for vacuum leaks. Also try cleaning the ISV.


If that doesnt work go to vwvortex.com and post your issue there

Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

The ISV (Idle Stablizer Valve) could be clogged or just not working. It's a cylinder with 2 vacuum lines and electrical connector going to it. Just above the valve cover and below the intake manifold. Best check for that is turn the key on engine off and the ISV should buzz. If it is buzzing remove it and clean it with some throttle body cleaner or brake clean. Reinstall and check for idle concerns. Have to be quick to see if it buzzes though. Better to have someone turn the key for ya while you check the valve. If it doesnt buzz replace it. Also check the switched on the throttle body. See if any of the wires are broken.

sounds to me you have a emissions problem check your EGR valve and MAP senser