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E30 Idle Issues (erradic idle, high idle, idle hunting): 90% are caused by a vacuum leak

October 10, 2021

Near most idle issues are derived from unmetered air entering the intake past the AFM (Airflow Meter). Simplified, it means you have a hole in one of your connections, gasket matting surfaces or intake related hoses.

Un-metered air is a fancy way of saying that you have additional air entering your system that the ECU has no way of accounting for, because it relies on the AFM (Air-flow meter) to derive exactly how much is entering. The AFM is at the very ‘front’ of your intake system, just behind the air filter box.

This as much applies to modern cars as it does to the E30 platform, with the main differences being that modern cars have moved to MAF systems (mass air flow). A MAF measures air differently (different type of sensor), but for the sake of a leak, an un-metered entry of air is an un-metered entry of air; it doesn’t matter what sensor type the car is running.

Whats notably different with modern cars, is their ability to self diagnose via the OBD2 protocol (normally as a lean code). On pre-OBD2 cars, its unlikely that you will get any kind of check engine light.

The reason for the erratic idle is due to the O2 sensor identifying on the exhaust side a lean condition and telling the ECU to compensate (called fuel trim). As the ECU tries to compensate, it again references the AFM, thinking its grabbing a valid value; rinse and repeat a cycle of incorrect values.

There is only one real surefire way to identify vacuum leaks on a car, and that’s with a Smoke Machine / Smoke Test. Don’t waste your time trying to feel around, using carb cleaner (to induce a surge), etc.

A smoke machine will be one of the best purchases you’ll ever make and will solve a lifetime of headaches when working on cars. Try to resist making your own, no matter how easy it seems, or using other forms of ‘homemade’ smoke. Spend the money up front and get a proper automotive smoke machine off Amazon (or other supplier).

Professional grade smoke machine that is half the price of competitors

To do the smoke test, its generally best to mimic how the stock system works. First remove the AFM, and use an expandable intake adapter:

Once on, this will identify immediate and glaring vacuum leaks by way of smoke visually flowing out. Remember, in a perfect system, no smoke should be escaping anywhere.

Next, to identify particularly small vacuum leaks, take duct tape and tape off the muffler exhaust tips. Now try to build 2-4 PSI of smoke pressure in the whole system and see if the system holds for 5-10 min. DO NOT GO MORE THEN 6PSI, you dont want to cause a blowout by creating too much pressure. With this approach, not only will you see any remaining small intake leaks but also any exhaust leaks (unrelated to idle stability, but still worthwhile discovering).

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