The Chevy Blazer was General Motor's first real sport utility vehicle (SUV) that was based on the C/K truck platform. It proved to be very popular due to its ease of repair and cheap parts since C/K based truck components could be interchanged with the Chevy Blazer. However the Chevy Blazer could develop a number of problems over time that could be responsible for abnormal RPMs. A faulty engine control unit (ECU) or an overheating engine would be the most likely culprit.
Pop the hood open on the Chevy Blazer by pulling the hood release latch in the cabin and then propping it up with a hood prop stick. Inspect the radiator first for any damage or debris on the heat sink. Remove any debris on the radiator that could be causing the engine to overheat and make the RPMs fluctuate. Consult your maintenance records for the last time the radiator fluid was changed and put in new fluid if it is time or if there has been a leak.
Inspect the battery for any damage or if it is dead. A malfunctioning battery could cause the engine to have a lower RPM if the spark plugs are not receiving enough current to supply a charge. Also inspect the alternator for any damage or corroded contacts that could also be supplying too little charge.
Turn the Chevy Blazer on and just let it idle. If the idle RPM is above 2000 RPM then the engine may be getting too much fuel and air in the carburetor. This could be from a stuck carburetor valve or a malfunctioning engine control unit (ECU). In either case it is best to take the Chevy Blazer into a dealership for repair. The ECU cannot be reflashed without dealership codes in some circumstances.
Shift the Chevy Blazer into Drive and give it a little gas. If the RPM suddenly surges then the throttle cables may be too tight and the engine "thinks" you are asking for more gas than you really are. If the RPM fails to go up then the cabling may be too loose and the inverse occurs.
