My 1994 Nissan seems to have developed a severe lack of power. Gas mileage has decreased. It does not run rough or miss. No visible smoke, nor is it burning oil. I have also noticed a change in the sound of the engine, makes kind of a roaring sound like it is laboring. I live and drive at high elevation, 9000 ft, and the problem is even more pronounced at higher elevations. Could it be O2 sensor or fuel mixture? Truck is very reliable but ridiculously slow.
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Good at start up then lack power under load,a plugged exhaust can do that.If there was a misfire that has been ignored the catalytic can only handle for so long. The 1994 Previa uses a vane airflow meter.so a torn intake duct can also cause lack of power under load,so check the air duct at the MAF to the intake and inspect for false air.
Confirm ur fuel pump is working. Ensure ur pickup tube is not blocked. Then ensure u did not leave a rag or parts in the manifold tubes. Not laughing it happens. Is the distributor in the right spot. Etc etc. Did u rehook the alt up if removed . Is tje battery fully charged. And lastly is the fuel flowing and freash. Then recheck your work did u replace all parts right. Any air in the head gasket will make the fuel /air mixture off.
You should try doing a tune up change spark plugs, wires, pcv,and fuel filter.it's reasonable to assume that the rings and valves (inside the engine) have become worn. Worn rings and valves will cause the engine to have low compression and low compression = weak power. You may be curious as to why the engine's lack of power is most evident when driving up a hill. Well, when a vehicle is going up a hill, the engine is under more stress than when driving on a flat road. Therefore, any wear in the rings/valves will tend to be revealed - in the form of compression - when the engine is under stress.
you will need to have a mechanic verify that the rings and/or valves are indeed worn. Incidentally, this generally involves what's known as a "wet compression test". If the mechanic tells you he positively confirmed a worn ring and/or worn valve condition, don't immediately authorize him to do the work. Why? Because, this type of work is very expensive and therefore you'll want to get several opinions before jumping into anything. By the way, even though this vehicle's high mileage strongly points to the above diagnosis, it's possible the cause of this problem is something else. For example, one of the electronic sensors that controls the engine may have gone bad. hope this help have a nice day.