My Porsche 924 idles high and hisses, and the idle screw does nothing. Where's the leak?

Short answer: A hissing high idle that the idle screw can't bring down means extra air is getting in. Do a careful smoke test with the engine off, check that the throttle plate truly closes and how the throttle stop screw is set, then look at the throttle shaft, AAV hoses and injector O-rings. Owners say new 924 throttle bodies are scarce, so rebuilding or a good used one is common.

Symptom: Warm idle sits around 1,200-1,400 rpm, it may hang there after you blip the throttle, and you hear a hiss or whistle. Turning the idle screw does nothing.

What to check first

1. Confirm the throttle cable isn't holding the plate open.
2. Smoke-test the intake with a good smoke machine.
3. Spray a little carb cleaner around the throttle shaft and hoses while idling and listen for the rpm to change. Keep it away from hot exhaust.
4. Check the AAV and its hoses, the brake booster line, and the injector O-rings.
5. Check how the throttle stop screw is set against the factory procedure.

Likely causes

1. Worn throttle body shaft letting air in. Air can leak past the throttle shaft where it goes through the body. Owners disagree on whether these have seals or just bronze bushings, but a leak there makes the idle screw useless.
2. Throttle stop screw out of adjustment. If the throttle plate is held open slightly, the idle stays high no matter what the idle screw does.
3. Vacuum hoses, auxiliary air valve or injector O-rings. Any unmetered air raises the idle. The auxiliary air valve (AAV) adds air when cold and should close when warm.

What it usually costs

Hoses and O-rings are cheap. A used or rebuilt throttle body costs more, and owners say parts for the 924 are getting hard to find.

Is it safe to keep driving?

A high idle isn't dangerous by itself, but it makes the car creep and can push it forward when you start it in gear. Fix it, and watch for any fuel smell.

DIY or shop?

Good DIY job for a careful owner with a smoke tester. Throttle body rebuilds take patience.

Recalls, bulletins and complaints in plain words

• Related recalls: NHTSA lists these recalls touching this area of the Porsche 924: 90V061000 (1987-1988 model years, fuel system, gasoline:fuel injection system:fuel rail). Check your VIN at nhtsa.gov/recalls. If your car is part of this recall, the dealer fixes it for free.
• Plain words: The complaint count below shows how often owners reported problems that mention these words to NHTSA. A count is not proof of a defect.
• NHTSA complaints: 4 keyword-matched complaints for Porsche 924 1977–1988 (of 6 unique complaints queried, NHTSA complaints API by model year, queried 2026-11-01; keywords: coolant, leak, overheat, idle, vacuum, stall) (NHTSA returned no data for 61 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In [[924] 1400 rpm idle with idle screw fully closed — can’t find the air leak](https://www.reddit.com/r/Porsche924/comments/1umr0pi/924_1400_rpm_idle_with_idle_screw_fully_closed/) on Reddit r/Porsche924, an owner's 924 idled at 1,200-1,400 rpm warm with the idle screw fully closed, with a loud hiss near the airbox. A smoke test and spray test didn't find it. Replies suggested injector O-rings, checking that the throttle fully closes, plugging the brake booster line to test, and that the throttle stop screw is another way to set the idle.
• In Vacuum leak in throttle body? on Reddit r/Porsche924, an owner found a small leak at the top of the throttle body and asked whether to replace or repair it. Owners said new throttle bodies are hard to find for the 924. Some said to clean it and replace the seals, while others said theirs had only bronze bushings and no seals. Another owner said their idle screw also made no difference.

Quick tip

Before anyone sells you a throttle body, make sure the plate actually closes. Many of these turn out to be a throttle stop or cable issue, not a broken part.

Vehicle focus: Porsche 924, mainly the 2.0L cars, 1977-1988.

Safety / mechanic note

Carb cleaner is flammable. Keep it away from the exhaust and ignition. Verify with a mechanic.