Why this matters (the theory, simply)
- Combustion leaks past the piston rings into the crankcase — that mixture of air, fuel vapors and combustion gases is called "blow-by." If you just sealed the crankcase, pressure and contaminants build up and cause oil leaks, sludge, seal failure and poor running.
- The PCV/blow-by system vents that crankcase gas and routes it back into the intake to be burned again. Think of it like a house ventilation fan that pulls stale air out and routes it into the furnace to be cleaned and reused.
- If the hose, grommet or PCV valve fails you get a vacuum leak and/or oil spraying/leaking where the hose routes. Symptoms: rough idle, oil on the firewall or back of engine, oil in intake, excessive oil consumption, check-engine light, or a whistling/hissing vacuum leak.

Main components (what each one is and does)
- Valve cover vent port (on top or rear of valve cover): the physical hole where crankcase gases exit the engine. Often fitted with a rubber grommet or adapter. If the port or grommet is cracked, it leaks oil/vapors.
- PCV valve (positive crankcase ventilation valve): a one-way check valve that meters flow from the crankcase to the intake. It prevents boost/backflow into the crankcase and meters vacuum at idle. Some Toyota engines have a simple plastic spring valve; others use a diaphragmless insert. If it sticks open/closed it causes vacuum leak or pressure buildup.
- Blow-by/PCV hose (rubber or hard plastic tube): routes the gases from valve cover/PCV to the intake manifold/throttle body. These harden and crack with heat/age.
- Intake manifold fitting / throttle-body connection: where the hose connects to intake vacuum. Oil can collect here if the hose is failing. If the mating seat is damaged, it leaks.
- Grommets / O-rings / clamps / brackets: small parts that seal and secure the hose at both ends. Worn grommet -> leak.
- Oil separator or breather box (if equipped): traps oil droplets before gases go to intake. If clogged it increases crankcase pressure.
- Vacuum lines around the area (adjacent hoses): often get disturbed when replacing the blow-by hose; check them.

Tools and parts you’ll need (basic)
- Replacement blow-by/PCV hose for your 4Runner (OEM-replacement is best), replacement PCV valve and grommet if applicable
- Small flat screwdriver or pry tool, pliers
- Needle-nose pliers or hose clamp pliers (for any clamps)
- Clean rags and brake cleaner or intake cleaner
- Gloves, safety glasses
- Optional: small mirror or flashlight, new clamps if factory clamps are brittle
- Time: 30–90 minutes depending on access and model

Basic safety and prep
- Work with a cool engine. Hot engine = burned hands and brittle hose breakage.
- Park on level ground, set parking brake.
- Disconnect negative battery only if you plan to remove electrical connectors or worry about idle issues — not strictly required for simple hose swap but safe.
- Keep rags handy to catch oil. Use gloves.

Step-by-step replacement (beginner-friendly)
1. Locate the system
- Find the valve cover vent. On most 3.4L/4.0L 4Runners the valve cover has a nipple that a hose runs from the rear/side of the valve cover forward to the intake or throttle body. Follow that hose and note attachment points and clamps.
2. Inspect before you pull
- Look for cracks in the hose, oil spots, wet oil near the grommet, or a collapsed/squishy hose. Wiggle the PCV valve — it should be snug. Note routing so you can reinstall correctly.
3. Remove the old hose
- Use pliers or screwdriver to loosen any clamps (many are push/pinch clamps). Slide clamps back, then pull the hose off the fittings. If it’s stuck, twist back-and-forth while pulling. If the hose breaks, pry the remainder out with a flat screwdriver carefully to avoid damaging the plastic fitting.
- Remove the PCV valve if it’s separate (pull straight out from the valve cover grommet). Some PCV valves are inside the hose assembly—replace it too.
4. Inspect mating parts
- Check the valve cover port and grommet. If the grommet is cracked/loose, replace. If the intake fitting is oily or clogged, wipe with a rag and spray intake cleaner; wipe until dry.
- Check the PCV valve by shaking and blowing through it: it should allow air in one direction only and have spring resistance.
5. Install new grommet/PCV valve (if replacing)
- Push the new grommet into the valve cover port until seated flush. Push the PCV valve into the grommet with firm even pressure until it clicks/seats. Confirm orientation: the PCV is one-way; the side meant for the intake must face intake (air flow direction usually marked or by design).
6. Route and attach the new hose
- Route the hose the same path as original to avoid kinks and contact with hot/exhaust parts. Push hose onto the PCV valve and the intake fitting. Use OEM-style clamps or new OEM hose clamps; tighten snugly but don’t over-torque.
7. Final checks
- Ensure clamps are secure, hose clear of moving parts, and grommet seated. Start engine and watch for oil leaks or hissing vacuum leaks. Let idle stabilize and listen — a sudden change in idle could mean a vacuum leak or wrong routing.
8. Clean up
- Wipe any spilled oil, inspect around the firewall/intake for oil that may have been thrown previously and clean if needed to spot future leaks.

How to test the repair
- Visual: no oil spray or wet seals where the hose connects.
- Idle: should smooth out if the old hose was leaking. If still rough, check other vacuum lines.
- Manual vacuum test: with engine idling, you can temporarily block the PCV port (not recommended for long) and feel increased crankcase pressure or changes in idle; correct function draws vacuum at idle.
- Shake/test PCV valve: replace if stuck or clogged.

Common things that go wrong and how to handle them
- Hose cracks or clamps fail soon after install: wrong hose, poor routing (contact with exhaust), or reused brittle clamps. Use OEM hose length/routing and new clamps.
- PCV valve stuck closed: causes pressure build-up -> oil leaks, pushed seals, blown gaskets. Replace PCV valve and check for oil leaks elsewhere.
- PCV valve stuck open: causes vacuum leak -> rough idle, lean codes. Replace.
- Grommet pulled out or torn: causes oil to leak out around valve cover. Replace grommet and clean sealing surface.
- Hard-to-remove remnants: use a pick to carefully remove old rubber from plastic nipples. Breakers sometimes work; if nipple damaged, the valve cover or intake adapter may need replacement.
- If oil is present in intake manifold or throttle body: clean the intake, replace air filter (if contaminated), and check the oil separator (if installed). Persistent oil in intake often means prolonged blow-by and possibly worn rings.
- Vacuum lines routed incorrectly: can lead to boost being fed to crankcase (on turbo models) or reversed flow; always match original routing and PCV valve orientation.

Tips and analogies
- Think of the PCV hose like the household dryer vent. If it gets crushed or torn, moisture and debris spray back inside — like oil sprays back into the engine bay or intake.
- When reconnecting, don’t make the hose taut. Allow a little slack so vibration doesn’t rip it off.
- If you’re replacing just the hose and not the PCV valve, at minimum inspect the valve for one-way action. Replace both for a reliable fix.
- Keep the old hose as a template if you buy a universal hose; cut to exact length and use the same bends.

When to seek a shop
- If you find excessive oil in intake or very high crankcase pressure, or if symptoms persist after replacing hose/PCV—these can indicate worn rings, valve guide seals, or a clogged oil separator and may need professional diagnosis.

Estimated time and cost
- Time: 30–90 minutes for the DIY job (shorter with good access).
- Parts: PCV valve + hose + grommet/clamps ~–60 depending on OEM vs aftermarket.

Done.
rteeqp73

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