Toyota Repair: 1992 Toyota truck failed smog check, downey offroad, santa fe springs

Question
QUESTION: My 1992 Toyota pickup will not pass smog. It is a 3.0 liter V6, SR5 4X4. In 2001 my truck blew a head gasket and I hydro-locked the engine. The Toyota dealership replaced the top end of the engine and covered all related repairs due to the V06 factory recall. At the same time I had the dealership install aftermarket headers made by Downey Offroad in Santa Fe Springs CA.The EGR valve was attached to the new headers The stock  exhaust system was replaced a few years prior to relieve pressure were the exhaust manifolds dumped into one pipe.It was replaced with 2 1/2 inch pipe all the way back including a new aftermarket Cat.  I gave all this history because ever since I have had problems passing smog. In 2003 I failed smog. I had a mechanic diagnose the problem and he recommended to replace the Cat because it may have been damaged by coolant when my head gasket blew. I replaced the Cat and it still didn't pass. The mechanic did more tests and must have rigged something to get it to pass. It passed and I was good for another two years. In 2005 it failed again. So I took the truck to the Toyota dealership where I had the previous work done. They replaced the VSV valve # 90910-12074 and the EGR valve assymbly # 25870-65010. They also recommended that I replace the Cat and the oxygen sensor. So I did and it passed. I was good for another 2 years. I was expecting the same problems in 2007, but it barely passed. Now we come to the present 2009. The first test I had done it failed miserably. So, I replaced the Cat again and it failed the second time just barely. I did some research on the internet and decided to check and clean the EGR Valve itself. I took it a part and cleaned it. I replaced it and tested it with a vacuum and it made the truck stall like it is supposed to. I think it works fine. I also replaced the cap, rotor, plug wires and plugs all with Toyota parts,and a fresh oil change just for good measure. Well today my truck failed for the third time and I'm about to go crazy. most of the failed inspection were due to high NOx. The Cat helped a lot but still no go.  Here are the NOx #'s at 15mph/25mph for the present tests. Test #1 1920/1432, Test #2 1160/652 and Test #3 1170/525. The Max to pass is 1031/891. I would appreciate any and all the help you can give me! Thank you!

ANSWER: High NOx readings can be caused by excessive combustion temperatures if the engine is running hotter than it should, it can also be caused by higher than normal combustion chamber pressures caused by carbon built up in the engine, these concerns should be addressed next.

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QUESTION: Are these concerns that I need a mechanic to look at or can I do some tests myself. As far as I can tell my truck does not run hot. The temperature gauge always reads in the middle, when the truck is warmed up, and has not changed for as long as I can remember. There may be carbon build up in the combustion chambers (my truck has 174,000 miles on it). How would I check and what should I do to clean them. Also, does the EGR valve have a lot of affect on NOx. How is the best way to make sure it is working properly. Thank you again for your help!

Answer
Have the engine compression tested, it should be about 180-200 psi max, anything over that would indicate a lot of carbon in the combustion chamber, the only way that can be corrected would be to remove the cylinder heads. The EGR system's only purpose is to reduce NOx emissions, it does so by cooling some of the exhaust gas and reintroducing it into the combustion chambers through the intake system, this cools down the combustion process temperature and thereby reducing NOx emissions.