Tools You'll Need:
* Socket set: Various sizes, including those for the valve cover bolts.
* Wrench set: May be needed for stubborn bolts or access issues.
* Ratchet and extensions: For reaching those hard-to-reach bolts.
* New valve cover gasket: Get a genuine Toyota part or a high-quality replacement. Don't skimp here.
* Clean rags or shop towels: Lots of them.
* Valve cover gasket sealant (optional but recommended): A quality RTV silicone sealant specifically designed for engine applications.
* Torque wrench: Crucial for tightening bolts to the correct specification. Incorrect tightening can damage the valve cover or head.
* Pliers: For removing stubborn clips or connectors.
* A drain pan: For catching any spilled oil.
* Funnel: For refilling the engine oil.
* New oil (if necessary): You'll likely lose some oil during the process.
* Shop manual specific to your 1991 Previa: This is essential.
Procedure (General Steps – Consult your Repair Manual!):
1. Preparation: Disconnect the negative battery terminal. This prevents accidental electrical shorts. Warm up the engine slightly to thin the oil – this makes it easier to clean. Let it cool down enough to work on safely. Place a drain pan under the engine.
2. Remove the Valve Cover:
* Locate the valve cover: It's typically the topmost part of the engine, covering the camshafts and valve train.
* Disconnect any components connected to the valve cover: This might include spark plug wires, PCV valve hoses, and other vacuum lines. Carefully photograph or diagram their locations before disconnecting them to ensure correct reassembly.
* Remove the valve cover bolts: Unscrew the bolts evenly, working your way around the perimeter. Don't remove them all at once.
* Carefully lift off the valve cover: It might be slightly stuck due to the gasket. Gently pry it up using plastic tools to avoid scratching it.
3. Clean the Valve Cover and Cylinder Head:
* Remove the old gasket: Carefully peel off the old gasket from both the valve cover and the cylinder head. Use a plastic scraper if needed, but be gentle not to damage the mating surfaces.
* Thoroughly clean both surfaces: Use a clean rag and a degreaser (brake cleaner works well, but be cautious of flammable materials) to remove any old gasket material, oil, or debris. Make sure the surfaces are completely clean and dry.
4. Install the New Gasket:
* Apply sealant (optional but recommended): If using sealant, apply a thin, even bead to the cylinder head mating surface, following the manufacturer's instructions.
* Carefully install the new gasket: Ensure it's properly seated in the groove.
* Position the valve cover: Carefully place the valve cover back onto the cylinder head, ensuring the gasket is correctly positioned.
5. Reinstall the Valve Cover:
* Tighten the bolts evenly: Start by finger-tightening all the bolts. Then, using a torque wrench, tighten them in a crisscross pattern, following the torque specifications in your repair manual. Over-tightening can cause damage.
6. Reconnect Components:
* Reconnect all the components: Reconnect the spark plug wires, PCV valve hoses, and other vacuum lines. Refer to your photographs or diagrams to ensure correct placement.
7. Refill Oil (if necessary): Check the oil level and add oil if needed.
8. Check for Leaks: Run the engine for a short time and then carefully inspect the valve cover for any leaks.
Important Considerations:
* Consult your repair manual: This is crucial. The above is a general outline; the specifics may vary significantly for your 1991 Toyota Previa.
* Safety first: Work in a well-ventilated area. Wear safety glasses and gloves.
* Take your time: Rushing the job can lead to mistakes.
* Photography: Taking pictures of each step before disassembling helps during reassembly.
If you're not comfortable performing this repair yourself, take your Previa to a qualified mechanic. Improper repair can cause serious engine damage.
