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Diagnosing Honda EGR SystemsPosted 11/7/2001By Wayne Pasicnyck
With Honda's PGM-FI fuel injection, it is now possible to monitor EGR valve lifts. If the ECM does not see the EGR lift during expected EGR operation, it will turn on the “Check Engine” light for code 12 on pre-OBD-II vehicles. The lift values are programmed into the ECM for the different engine loads and drive cycles. When diagnosing a Honda EGR system it is always good to remember the basics - so let's look at the basics of code 12. A lift sensor on top of the EGR valve works just like a TPS sensor. There are three wires: a signal wire (yellow or white/blue) that goes back to the ECM, a 5-volt reference wire and a ground wire. With the EGR valve closed, the voltage should be about 1.2 volts. Normally anything more than 1.4 volts will set the light because the ECM thinks the EGR valve is open when it should not be. The voltage will increase as the valve opens. The ECM watches the voltage and then compares it to the predetermined values set in the programming of the ECM. The EGR is lifted by engine vacuum and the ECM has a solenoid it grounds when it commands EGR operation. The ECM will ground this solenoid when it sees the correct engine temperature, correct MAP sensor voltage, and throttle position sensor voltage. With the solenoid, the constant vacuum control (CVC) valve controls a constant vacuum for EGR operation. This valve takes the manifold vacuum signal and puts it to a constant 8 to 10 inches. Without a constant vacuum signal, EGR operation would be erratic. The EGR solenoid and the CVC valve location varies by model, but are often found in the black emission boxes under the hood on the fire wall. To locate, use a component locator or just follow the vacuum line. Testing for a code 12 is not difficult if you read what the EGR lift sensor is doing in relation to engine operation. The most important thing to remember is that when the engine control unit grounds the solenoid, the valve should have its full vacuum signal within one second. A slow or delayed vacuum signal or no signal will set a code 12. If it is a slow or low vacuum signal, it will not lift the valve fast enough or far enough and the code will set. If the solenoid and the system check out at the time of testing, the next step is to manually ground the solenoid up to 20 times to see if it fails once during this test. As it heats up with operation, it may fail and will not allow the vacuum signal to go to the EGR valve. The ECM grounds the solenoid for EGR operation by inputs from the MAP sensor, the sensors in the distributor, the throttle position sensor, and the coolant sensor. If the solenoid is not being turned on at all, check the ground wire from the solenoid all the way to the ECM. It is also important to verify lift sensor voltage all the way back to the ECM. Note: When there is an EGR problem and the ECM senses that problem, it stops grounding the solenoid and stops EGR operation. Make sure the code is cleared from the ECM so it will try to operate the EGR solenoid. When diagnosing Honda driveability problems, always check the EGR system. Block off the vacuum to see if the problem goes away. Usually the valve is not the problem - often it is a lean run condition or an EGR distribution problem with EGR going to one or two cylinders, rather than all four. Certain 1990-93 Accords experience a misfire condition or hesitation warm at 1500 to 2000 rpm. When vacuum is removed from the EGR valve, the misfire or hesitation goes away. This is usually caused by plugged EGR ports in the intake manifold. To correct, remove the brass plugs in the intake manifold and clean out the carbon. Honda offers a kit with replacement plugs so removal or replacement of the intake manifold will not be required.
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