1974 1.8 Stock fuel injection needs troubleshooting

[tshih]

Hi,
Got a stock 1.8 FI 914 which has trouble maintaining a steady idle at 900 rpm. Idle hunts up and down incessantly after warm-up. Initial cold idle is 2000 rpm. What's the most likely culprit of all the sensors and hoses in the system? Any source of step by step troubleshooting procedure posted here in the archives or links?

Tom

[Dave at Pelican Parts]

The most likely cause is a vacuum leak. Anywhere from the air flow meter to the cylinder head, including all of the vacuum hoses...

--DD

[Will98D]

Mine did the exact same thing until I blocked off the line to the auxiliary air valve. Pinch one line at a time and you will probably come upon the offender pretty quickly.

[tshih]

I pinched the 13 mm hose from the oil filler cap side location to the top of the engine near the throttle valve and the idle stabilized what does this hose connect and what is it's function?

[Dave at Pelican Parts]

This hose helps get rid of crankcase oil vapors, and keeps the case from building up air pressure inside of it.

I would replace the cork gasket under the oil filler/breather box, and the O-ring and seal under the oil cap. These need to be (reasonably) air-tight or you will get an air leak into the intake through them. I believe there may also be an O-ring between the oil filler funnel and the filler/breather box. Check that for leaks as well.

--DD

[Will98D]

It functions as Dave said, known as PCV (positive crankcase ventilation). If none of that stuff works try a new cap, I think it has a valve built into it.