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more than likely you have a valve cover leak its common you will need to re move the cover
1 take out the bolts 6 i believe 7/16 socket remove the small bolts
2 pry the valve cover off
3 clean the old gaskets off the valve cover and motor
4 clean the valve cover and paint the valve cover light blue so you can see any leaks
that may happen // i dout if you will have any
5 put a small amount fo permatex (BLACK) it works well with oil and it will hold the gasket in place
once you put the gasket on the on the valve cover put a small amount on the bottom of the
gasget/// you will have permatex on both sides of the gasket
6 now you are ready to in stall tha valve cover back on the motor and put in all the bolts finger
tight once all the bolts are in you want to snug the bolts don't try to tighten the bolts to much
or you will cut the new gasket
7 now that every thing is back in check your oil level and run the engine for ten min this will set up
the new gasket and give the permatex a chance to cure
most likely it is the valve cover gasket. it covers the valve train where there is alot of oil to lube the top of the motor. if the head gasket was leaking it may have more signs of running poorly. also having coolant leaks also. so i will have to say valve cover gasket. now usally you may have a oil burning smell with it depending on the area of the lek but not always.
the valve cover can easily be seen when the hood is open. it is right on the top and where it meets the clyinder head is where the gasket is and the leak can be seen.
Can you tell us more? How many miles on the car? Which side of the engine area? Drips after you park etc? Oil ever low? If so how often do you ad oil? Any visible oil on engine and bay components under hood?
likely areas for oil leaks may include depending on model and make: Oil pan gasket seal. Oil pressure sending unit. Oil filter installed dry or over tightened at your last oil change. Oil drain plug loose or plug gasket needs replacing. Valve cover gasket needs replacing. Crank shaft seals need replacing. Distributor seals need replacing. RTV gasket sealer beyond expected life and need replacing for other engine components, inspection plates or covers.
The toyota previa uses a formed silicone gasket for the valve cover and the cover is held in place by metric shoulder bolts ( 10mm). The valve gasket leaks because you can't get enough preload on the bolts to re-seat the old gasket when it gets old and shrinks. The shoulder bolts only allow you to tighten the gasket enough to seat the shoulder on the bolt. If you continue to torque down the suspected bolts, you will snap the bolt off ( don't ask me how I know this ). You can either 1.)replace the old gasket with a new one or 2.) remove the existing bolts around the leak area and put in new metric bolts that do not have a shoulder. You may want to put in a small sleeve into the existing bolt hole but shorter than the thickness of the valve cover to account for the smaller diameter bolt. You can now torque down the bolt and get enough preload to seal the valve cover without removing the old gasket. Make sure you use a torque wrench ( in-lbs) or be very careful not to snap these small valve cover bolts.
If you're certain it isn't your oil filter, then it can only be three other things. The first (and easiest) thing to check is a small diamond-shaped plate on the side of the block, under the intake manifold, towards the alternator. It has a bolt and a stud in it. Verify that both of those are tight. If they are tight, check the four bolts that secure the "push rod cover" to the side of the engine. The push rod cover runs the length of the engine and is located directly under the intake manifold. This was the culprit of my oil leak, and I imagine it will be yours also. However, if you find that those bolts are tight and no oil can be found leaking from that area, check your valve cover. The Fiero valve covers were stamped from very pliable steel and it is difficult, at best, to get them to seal correctly. Problems usually occur when people torque the bolts too tight and bend the valve cover. I recommend a product called "Indianhead" gasket shellac. Remove the valve cover and ensure all gasket surfaces are clean. Ensure the valve cover's gasket mating surface is straight - straighten with a hammer and block of wood if needed. Follow the directions on the bottle to the letter, making sure to coat your valve cover gasket surface, both sides of your gasket, and the gasket surface on the head.