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if it is a 3ltr diesel follow the top hose to the engine and that the housing for the thermostate
With the thermostat.
The thermostat on a 1998 Isuzu Trooper is located on the front of the motor. It is in the top radiator hose where it connects to the motor. To access the thermostat, the hose needs to be disconnected from the motor.
On top in front of engine
The thermostat on a 2000 Isuzu Trooper is located on top of the engine, below the upper radiator hose. It controls the flow of coolant to the radiator and throughout the vehicle.
Its under the intake
Under the intake mannifold (you have to remove it to get to it).
The thermostat for the 3.5 Liter is under a flange bolted to the top of the block (in the valley), connected to it is a black pipe that connects to the lower radiator hose. This flange has four bolts that are accessed only after removing the intake manifold. Removing the housing that connects to the upper radiator housing optional.
Isuzu Trooper was created in 1981.
I would start by checking the thermostat, it may not be opening.
1999 Isuzu trooper
You can find the PCV valve, on the outside of the air cleaner housing. The PCV valve will be attached to the back of the housing.