Questions and Answers

Volkswagen Cabriolet No Cold Fast Idle Q. Dear Vincent, I own, or am owned by, an 1986 Volkswagen Cabriolet with a 1780cc fuel injected engine and a 5 speed manual transmission. It has 151,000 miles on it but generally runs well. The problem is with cold starts. It doesn't like to start until it's been cranked over several times and then it idles rough and below 1,000 rpm.

When you try to rev the engine, it cuts out before you can get it to 2,000 rpm and wants to stall. After about ten minutes at idle, when the engine is fully warm and the oil temperature reaches about 85°C, it abruptly starts to run smoothly, like the turn of a switch.

After this, it continues to run perfectly until the engine is turned off and allowed to cool. It only takes it a couple minutes to cool enough to where the process of warming must start all over again. Is this maybe a thermostat problem? Oxygen sensor?

Does this car have a MAF sensor and if so, would that effect anything? I am a tinkerer but not a mechanic. Is this something I could do myself? I would be thankful for any information you might supply on this problem, thanks for the great resources your guide provides, kudos to you and this fine site.

T.C.

A. You have a CIS injection system. It's a mechanical fuel injection system and electronic controls are limited.

The Auxiliary Air Regulator (AAR) is what controls the cold idle speed, like the choke on a carburetor. When the engine is cold, the gate valve is open. The heating coil is connected to the fuel pump and the control pressure regulator circuit so that as the engine warms up, the bi-metallic strip is heated. The deflection of the bi-metallic strip rotates the gate valve, gradually cutting off the passage for additional air as the engine warms up.

When the engine is cold, remove the bypass hoses from the AAR and look inside. You should see the gate is open. You will be able to see right through it. If the gate is closed, then the AAR is bad and needs to be replaced.

As a quick test, with needle nosed pliers clamp the hose from the air valve to the air intake boot closed. If the rpm drops substantially, the air valve is working. Clamping the hose when the engine is warm should have little or no effect.

Another possibility is a bad Thermo Time Switch, they often go bad.

You do have an Air Flow Meter (AFM), and it could be sticking. There is a metal plate inside that is deflected by the incoming air. The amount of deflection tells the computer how much air is entering. If the plate sticks inside the housing, it could cause the symptom you describe. A good cleaning will fix the sticking.

These are the three most likely causes of the symptoms you describe, hopefully one of them will fix it for you.