1999 Toyota Tacoma 4WD 6 cylinder did not pass smog second time.

There are several reasons your Toyota Tacoma could have failed the NO portion of the smog check. The most common is the malfunctioning EGR system. The EGR valve is responsible for re-circulating a small amount of exhaust gas back into the combustion chamber. This is done to cool down the chamber temperature by producing an inert gas along with the regular fuel mixture. Nitrous Oxides emissions are formed when engine chamber temperatures rise above 2500* F. Keeping the chamber cool reduces NOx emission. The EGR system is prone to collecting carbon build-up. Since the EGR valve pulls exhaust gasses from your exhaust and introduces them back into your engine's intake air over time it will become clogged or stuck open/closed from the carbon deposits. It is possible to clean the EGR valve if you remove it from your vehicle and either spray it out or let it sit in regular gasoline. Other reason which could have caused your 1999 Toyota Tacoma to produce high NO emissions are: Bad engine cooling, lean fuel condition and high mileage may cause NOx failures as well. Bad Engine Cooling - If your Tacoma's engine cooling system is not working efficiently, chances are there is an excessive amount of NOx being created. Remember NOx (Nitrous Oxides) is created only when an engine's combustion chamber temperatures reach over 2500F. A bad cooling system will create NOx. For this reason you want to make sure your vehicle's temperature gauge is always normal and that your cooling system is working properly. If your vehicle has an overheating problem, be sure to perform the necessary repairs. Lean Fuel Mixtures - Lean fuel mixtures cause high NOx also. A lean fuel mixture is when the engine receives less fuel then is necessary to obtain efficient combustion. Lean fuel mixtures are often due to vacuum leaks. These leaks will allow the suction of additional and un-metered air into the combustion mixture or exhaust (depending on where the vacuum leak is located) disturbing pre/post fuel combustion and increasing NOx emissions.