Fixing Tracker EGR Code Can Be a Real NightmarePosted 2/10/2005
By Dave Martin
Anyone who has worked on a Chevrolet Tracker knows that fixing an exhaust gas recirculation (EGR) code P0400 can be a real nightmare. This was the case recently when I took a call on a 1997 Tracker with a 1.6 liter engine. It was evident the technician had years of experience, good equipment and was determined to find out why this code kept returning. The technician's frustration was dwarfed by the owner's - a trail of shops had been unable to fix his problem, so the heat was on.
An EGR system can be monitored by several methods, including checking the temperature of the exhaust gas when the EGR valve is opened, sending a pintle position feedback voltage to the power control module (PCM), or monitoring the change in O2 sensor voltage or intake manifold pressure when the EGR valve is opened. Measuring the change in manifold pressure is the method used by this Tracker. The P0400 code is set when the EGR valve is commanded wide open and the expected change in manifold pressure measured by the MAP sensor does not occur.
This EGR system uses several components. The EGR vacuum solenoid is controlled by the ECM and is turned on to allow vacuum to the pressure transducer - it basically turns the system on or off. The pressure transducer regulates the amount of vacuum to the EGR valve depending on backpressure in the exhaust (engine load).
Unless at wide-open throttle, the more the load, the more the EGR valve opens. The last part of the system is the EGR bypass solenoid, which is activated by the ECM during a system test to apply full manifold vacuum to the EGR valve. When this occurs, the maximum amount of exhaust gas flows into the intake manifold, decreasing manifold vacuum. This event causes an increase in voltage output from the MAP sensor, monitored by the ECM. A code P0400 will be recorded if the voltage change is not within a specific "window."
This engine had a new EGR valve, new pressure transducer, new EGR solenoid vacuum valve and a new EGR bypass valve. All vacuum hoses were connected correctly. The MAP was working properly. The ECM was operating the solenoids as intended, but the P0400 code continued to set. It was at this point that the technician called our hot line.
After discussing the problem, it was evident the system was working correctly, so the problem had to be related to the amount of exhaust gas entering the intake manifold. This becomes more believable when looking at the route the exhaust gas takes on its way to the intake. The gas is brought into the system from the No. 4 cylinder exhaust manifold. From there, it winds through the back of the cylinder head into the intake manifold and to the EGR valve. Then it goes back into the intake manifold and up an EGR transfer tube. From the transfer tube, it takes a turn and gets vented into the intake air stream behind the throttle plate. It's amazing the system works at all!
The technician had lifted the EGR valve at idle in an earlier test. This caused the engine to almost stall so he assumed the passageways were not plugged. He was right; they were not plugged, but they were restricted, and that slight restriction caused the code to reset repeatedly. As it turned out, the EGR port behind the throttle plate was coated with carbon. This is a common problem when hot exhaust gas makes contact with cool air flowing though the intake manifold. The rapid change in temperature causes the carbon to condense at the port outlet.
How could one small restriction affect the entire system? Think in terms of partially restricting a 50-foot section of garden hose with a needle nose vise grip. Although the pinched-off section is small, the flow through the entire hose is reduced. That same principle applies to this problem. The fix was to clean out the port behind the throttle plate with a No. 3 Phillips screwdriver. This increased the flow of exhaust gas enough to allow the EGR system to pass the self-test. The troublesome P0400 never set again.
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