Why does my Mercedes ML350 sag overnight or the air compressor keep running?

Short answer: On the W164 ML (and its bigger GL sibling), a corner that sags overnight, a faint hiss after parking, or the AIRMATIC compressor (the electric pump that fills the air springs) running for minutes after you shut off, almost always points to an air leak. The usual suspects are an air strut, an air line fitting, or the valve block. Find the leak early, because a compressor that runs too long eventually burns itself out.

Symptom: The rear or one corner lower in the morning, a hissing sound when parked, the compressor running after the engine is off, an AIRMATIC warning, or a harsh ride.

What to check first

1. See which corner drops. The corner that sinks overnight usually has the leak.
2. Scan the AIRMATIC system with a Mercedes-capable tool for faults and ride-height readings.
3. Soapy-water test the strut tops, air line fittings, valve block and compressor lines while the system is full.
4. Make sure the valve block connector is fully clipped in. One owner on MBWorld found the plug wasn't fully seated after a repair.
5. After replacing a valve block or strut, have the system calibrated. The same owner said skipping calibration over-filled and burst an air spring.

Likely causes

1. Leaking air strut (air spring). The rubber air bag in the strut ages and cracks, and that corner drops overnight.
2. Leaking valve block or air line fitting. A leak at the valve block or a fitting lets several corners lose pressure and makes the compressor run more.
3. Electrical fault making the compressor run on. An MBWorld member said a compressor that keeps running after shutdown is unusual and could also be electrical, so scan before buying parts.

What it usually costs

The owners in these threads didn't post repair prices. Get a quote after the leak is found, and ask whether struts are replaced in pairs.

Is it safe to keep driving?

A slow sag is usually drivable once the system raises the car, but don't drive it if a corner is down on the bump stop or a warning says not to drive. A sagging car handles worse and can damage tires and suspension parts. Verify with a mechanic.

DIY or shop?

Skilled DIY or shop. Finding the leak with soapy water is easy. Replacing struts or the valve block, and calibrating the system afterward, needs tools and a Mercedes-capable scanner.

Recalls, bulletins and complaints in plain words

• No recall covers AIRMATIC sagging on the 2007–2012 ML-Class or GL-Class in NHTSA's recall data.
• NHTSA complaints: 74 keyword-matched complaints for Mercedes-Benz ML-Class and GL-Class 2007–2012 (of 1,121 unique complaints queried, NHTSA complaints API by model year, queried 2026-10-19; keywords: airmatic, air suspension, air strut, sag, compressor, ride height).

What owners report

• In W164 ML350 Airmatic Compressor runs on on MBWorld, an owner's compressor ran for two to three minutes after a long drive even though ride height was fine. A member said that isn't normal and to scan for faults and soap-test everything, since AIRMATIC trouble is usually a leak. Another owner's 'running compressor' was solved after he calibrated a new valve block and found its connector hadn't been fully clipped in.
• In ML350 intermittently flat overnight/AirMatic Noise/Hiss on BenzWorld, an owner described the rear sometimes sitting flat overnight with a faint hiss. Replies pointed to a rear air strut or the valve block and said to check which corner drops.

My take

My advice as a former lot tech: I always walk around an ML or GL after it's been parked for a few hours. If one corner is low, I listen near the wheels for a hiss and check for warnings on startup. A sagging corner is a negotiating point, not a reason to panic, as long as you know what you're buying.

Vehicle focus: 2007–2012 Mercedes-Benz ML-Class (W164) and GL-Class (X164) with AIRMATIC air suspension.

Safety / mechanic note

A corner down on its bump stop handles poorly and can damage parts. Verify with a mechanic.