Could be the head is cracked.
do you have smoke or steam coming out the exhaust?
how is the coolant level?
this engine WILL NOT tolerate running with out coolant.
SOURCE: rough idle
check you throttle position sensor first then fuel pressure regulator, then fuel pump. i have a 92 and had to recently do a massive overhaul. my rough idle was caused by a malfunctioning tps with a small exaust leak causing it to race and an intake manifold gasket. also check your idle speed adjustment settings.
SOURCE: 1992 Toyota Previa van,Oil dripping onto exhaust manifold, replace head gasket? need a free manual!
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.
SOURCE: manuales de toyota previa 1992
primero que nada , revisa filtro de aire y filtro de gasolina ,que no esten sucios , segundo , las bujias que no esten desgastadas
SOURCE: Does a 1992 Toyota Previa need to have its timing
Toyota uses a self adjusting timing chain in this vehicle, so if you are having problems with the timing then the chain should be changed.
SOURCE: 1994 toyota previa idles rough and cuts off only
Change fuel filter, air filter and if that doesnt help check fuel pump pressure.
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