Questions and Answers

VW Golf Idle Speed Fluctuation Q. Hi Vincent, I have a 1986 Volkswagen Golf, manual 5-speed, no A/C, with GX engine code. The idle fluctuates like crazy (warm or cold) with or without any electrical components on, but especially when components are on. The idle speed boost valve opens when the RPM's go below 750 and cuts off at 1050 RPM's, but the engine never stabilizes.

I tested all of the following to make sure the are operating correctly, receiving voltage, and that there are no shorts or opens in wiring:

Idle speed boost valve(solenoid)
Auxiliary air valve
Control Pressure Regulator
Cold-start valve
Oxygen sensor (new)
Frequency valve
All relevant relays and fuses
Alternator (new)
Battery (new)
Coil
Distributor and wires (new)
Spark plugs (new)

The Bentley and Chilton book stated that the resistance across the terminals of the control pressure regulator should be approximately 22 ohms, which it wasn't, it is at 12 ohms. But, I bought a new one and it had the same ohm reading across the terminals - 12 ohms. I am now stumped. What could it be?

Thanks,
Tom

A. I think the problem is with the Idle Speed Boost Valve signal from the Electronic Control Unit (ECU). Here is the procedure to test it:

The idle speed boost valve is a solenoid valve controlled by an ECU on the fuse/relay panel that turns the valve ON when the idle speed drops below 750 rpm, and OFF when the idle speed gets above 1050 rpm. The valve is always grounded, and is opened and closed by supplying and cutting voltage. A second boost valve is fitted to cars with A/C, and is switched ON whenever current is supplied to the compressor clutch.

1. Disconnect the harness plug from the boost valve and check for voltage. Battery voltage should be available when the engine speed drops below 750 rpm. If voltage is correct, replace the valve.
2. If no voltage is available, check the signals to and from the control unit.
3. Using the terminal numbers on the back of the control unit on the fuse box as a guide, test the fuse box sockets.
   1. There should be no resistance between socket for terminal 31 and ground.
   2. Battery voltage should be available at socket for terminal 15 with the ignition ON.
   3. Battery voltage should be available at the socket for terminal 1 with the engine running.

4. Check the wiring between the fuse/relay box and the boost valve harness plug for breaks, or shorts, and repair as necessary.
5. If the valve(s), wiring, and circuit tests are all okay, the control unit is faulty and should be replaced.