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Finding and Fixing Vacuum Leaks on the 280ZX

Standard Disclaimer: Modification of your car involves risks and may void your warranty. I cannot be held responsible for the modifications you consciously decide to undertake nor for the results of doing so.

The 280ZX's fuel injection measures the air coming into the engine with the air flow meter (AFM) and adds fuel to match. Any air that gets in after the AFM is air the computer doesn't know about, so the engine runs lean. That "unmetered" air can come from a cracked hose, a bad seal, or even an oil dipstick that isn't pushed all the way in.

Signs of a vacuum leak:

  • Rough or "hunting" idle (the RPM rises and falls)
  • Idle speed higher than it should be
  • Stalling, hesitation or stumbling, especially when cold
  • A hissing sound under the hood
  • A hard brake pedal, if the leak is in the brake booster or its hose
  • Heater, defrost or A/C air coming out of the wrong vents, if the leak is under the dash

These cars are now over 40 years old, and their original rubber is the main culprit. As Astro Bob put it on zcar.com, "everything, seemingly, can cause vacuum leaks on a Z car."

Where to Look

  • The rubber boots between the AFM and the throttle body. These are notorious for cracking. Flex them and check the folds and the ends.
  • Old vacuum hoses. Decades of heat make the small hoses crack, split or slip off their fittings.
  • The large breather hose to the valve cover, and the PCV valve connection under the intake manifold.
  • The oil dipstick and oil filler cap. Both have rubber seals, and the crankcase is connected to the intake through the PCV system. Astro Bob "fixed" a friend's ZX just by pushing the dipstick all the way in. The trouble had started after a quick-lube oil change, and the owner never connected the two.
  • The brake booster hose and its check valve, and the brake booster itself, whose diaphragm can leak.
  • Under the dash. The heater and A/C controls run on vacuum. A leak there adds to the engine's leak and also stops the air from going to the right vents.
  • Fuel injector O-rings and seals. They get hard and crack, letting air in around the injectors.
  • The intake manifold gasket, between the manifold and the head.
  • The distributor vacuum advance unit, on distributors that have one. Its diaphragm cracks with age. One member in the same zcar.com thread called it a big leak that's "commonly overlooked".
  • Emissions and other add-ons: thermal vacuum valves, the cold start valve, the air regulator and its hoses, the EGR system and the charcoal canister purge lines.

How to Find Them

  1. Check the easy ones first. Make sure the dipstick is fully seated and the oil filler cap is tight with a good seal. Look over the AFM-to-throttle-body boots.
  2. Listen. With the engine idling, listen around the intake manifold, the injectors and the vacuum lines for a hiss. A short piece of hose held to your ear works as a stethoscope to pin down where it's coming from.
  3. Spray test. With the engine idling, spray small bursts of carb cleaner or starting fluid around the suspect spots: hose ends, injector bases, the manifold gasket and the boots. If the idle speed rises or changes tone, you've found a leak.
    Caution: both sprays are very flammable. Keep a fire extinguisher handy, use short bursts, and keep away from the hot exhaust manifold and ignition parts.
  4. Pinch test. Carefully pinch off flexible rubber lines, such as the brake booster hose or PCV hose, one at a time with pliers (padded so you don't damage the hose). If the idle suddenly smooths out, the line or part you just closed off is leaking.
  5. Vacuum gauge. Connect a vacuum gauge to a direct manifold vacuum source. A healthy engine at a warm idle should show a steady reading in the high teens (inches of mercury). A low reading points to a vacuum leak, though low compression or retarded timing can also cause it, and a needle that won't stay still tells you as much as the number.
  6. Smoke test. A shop smoke machine fills the intake with visible smoke, which comes out wherever there's a leak. It's the quickest way to find leaks you can't hear or reach.

How to Fix Them

  • Replace all the rubber hoses at once. On a car this old, fixing one hose usually just leaves the next one ready to fail. Astro Bob's advice: rubber vacuum hose is cheap, so there's no sense in not replacing every hose at the same time. He bought about three meters of every small size the parts store had, which gave him room to route the hoses without running a couple of inches short. Use the hose routing diagrams in the factory service manuals.
  • Reseal the injectors. Carefully remove the fuel rail, take out the old hardened injector O-rings and seals, and install a new injector seal kit.
  • Replace the intake manifold gasket if the leak is at the manifold-to-head joint. Clean the old gasket off both surfaces before installing the new one. See Installing a New Intake Manifold and Gaskets.
  • Test the vacuum advance. Disconnect its hose and apply vacuum with a hand pump. If it won't hold vacuum, replace the vacuum advance unit or the distributor.
  • Simplify the hoses. Many owners plug unused vacuum ports and remove lines to emissions equipment they no longer use, which cuts down on future leak points. Check your local emissions rules first. See the ZDriver guide below.

Videos

Vacuum testing an L-series engine, from Restoration For Beginners' Datsun L-series testing and tuning series:

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