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Isuzu Rodeo running too rich failed emission test

Hi I have a 1995 isuzu rodeo 4wd. My problem is that I took it to a smog check station and they hooked it up everything was fine it passed the low rpm test but then when they did the high rpm test the truck started going way too rich... The guy told me it failed the smog due to too much fuel on high rpm. I had a shop do timing belt and water pump also replace the spark plugs Thank you!
Answer:

A smog check repair station will have to find out why your Rodeo is running rich and possibly producing high CO. The oxygen sensor should be inspected to make sure it is sending the correct voltage signals to the ECU (engine control unit). The ECU should be inspected to determine the oxygen sensor signal is reaching it and it is computing the data properly and sending the correct signals to the fuel injectors. Additional sensors which must be inspected are the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. If the coolant temperature sensor is not operating properly (not sending the correct voltage signal to the ECU) the ECU will not be able to properly calculate how much fuel to deliver to the combustion chambers. When the engine is cold the ECM is programmed to deliver a richer fuel mixture (more fuel). As the engine warms up, the ECM will lean out the mixture. Insuring the ECT is sending the correct voltage to the ECM is important for correct air/fuel ratio (optimum is 14:7). The MAF sensor is used to measure the amount of air entering the intake manifold. This sensor must send accurate information to the ECU as well, in order for the ECU to calculate exactly how much fuel to add to the air entering the combustion chambers. If after an electrical diagnosis if it is determined the fault is mechanical, the locations which need to be inspected are any and all points of fuel entry, most predominantly the Fuel Injectors. It is not uncommon to a stuck "open" or "sticking" fuel injector to cause high CO and a rich fuel mixture. posted by SmogTips Support

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