I went in for my emission test and was surprised I failed. The car drives fines and there wasn't any trouble light on. The results showed HC and CO is fine, but NOx exceeded the limits by about 25%. That pointed to the EGR system.
I found a leaky vacuum hose between the EVR and the EGR valve. I replaced the hose and went in for the second test a few days later. Again, it failed. To my surprise, the HC and CO is fine, but NOx numbers were higher than the first test.
I dug through the manuals and searched the internet. There are conflicting information about my Explorer. Apparently, my 1997 5.0L V8 doesn't have a DPFE in the EGR system. Instead, it has a EGR position sensor that sits atop of the EGR valve. The EGR valve is pulsed by the EVR with signal from the PCM. The EVR pulls on the EGR valve with vacuum from the intake manifold. That's it. (The 1997.25 and later has the DPFE in the EGR system.) I ohm tested the EVR coil and it measured 33 ohms. That's with in spec. I applied a vacuum to the EGR valve and it didn't hold too well, so I removed the EGR valve from the manifold. (Let me tell you, that was a b*tch to get off. I had to first removed some hoses and several brackets in order to get access to the mounting bolts. I had to climb on top of the engine block to gain access. My neighbors were more than curious.) I found the valve was all covered in carbon. I measured the EGR position sensor output and it was within spec. The pintle didn't slide very smoothly after the clean up, so I only ordered a new EGR valve from Ford. When the new valve came in and after almost $400 later, I replaced the valve along with a new gasket. I put everything back together, reset the PCM, drove around for a few days. I scanned for any trouble codes and nothing came back. Took it in for the third test and once again, failed the NOx only portion of the test. This time the NOx numbers were better than the second test.
I am at a lost. As a test, I disconnected the vacuum line and connected it directly to the EGR valve while the car was idling. The engine stumbled like it should. the vacuum is good enough to lift the EGR pintle to cause the engine to stumble at idle. I have not measured the live signal from the EGR position sensor. The resistance test was fine, so I hope the live signals to the PCM should be fine too.
Could the excess NOx output be from residue carbon buildup? If it is, is there an easy way to remove this carbon? Someone told me about a product called Seafoam. Will that clean it up for me?
Any assistance is much appreciated. Thanks.