Failed Emissions Inspection- Please Help

[JohnnyG]

OK, I have a 1991 GT with a bone stock motor with the timing advanced to 14*. My MAC H-Pipe is about 5 years old and came with Hi-Flow Cats. I just replaced the O2 Sensors, cleaned the air filter, and I did a complete tune-up about three months ago.

I failed the Maryland emissions inspection in the areas of Hydrocarbons (HC) and Nitrogen Oxides (NOx) by a very small margin.

The guy at the inspection station told me that it could be due to a number of things... running too rich or too lean... a faulty EGR valve... bad timing... a bad catalytic converter...????

The car seems to run fine... no check engine lights... and plenty of power on tap. Any ideas of what to try first? I only get one free re-inspection and want to nail it on the first try without having to pay somebody to diagnose the problem. Thanks in advance.

John

[Old Guy with 87 GT]

my first thuoght would be ,what temp was moter running at .....if it's cold it'll run rich .....is your thermostat stock replacment or is it a low temp one (180 or 160).....the stock temp range is like 190 to 195 (i think ).....it's pretty popular to put low temp ones in but it makes the moter run richer

i'd also check your temp sencors (air and coolant) make sure there operating ok and check for vacuum leaks as well

[gtsr515]

exactly, go to a Ford dealer, buy a 192-198 degree Motorcraft thermostat, this is what came in your car from the factory, this will make a difference, trust me. Run timing at 10 degrees, stock headers would help too, if you still have those. Is this a stock MAF and injector car?, if not definetly change back to those. There are products you can dump in your tank, go out and run the bagels out of the car and then go to your test, some say these work, I'm not much for magic potions and such.

Unfortunetly if your cats or cat is bad, none of this will work.

Hope this helps.

[PKRWUD]

Originally posted by gtsr515
exactly, go to a Ford dealer, buy a 192-198 degree Motorcraft thermostat, this is what came in your car from the factory, this will make a difference, trust me. Run timing at 10 degrees, stock headers would help too, if you still have those. Is this a stock MAF and injector car?, if not definetly change back to those. There are products you can dump in your tank, go out and run the bagels out of the car and then go to your test, some say these work, I'm not much for magic potions and such.

Unfortunetly if your cats or cat is bad, none of this will work.

Very well put.

Reset your timing to 10 degrees, verify it has a 192 degree t-stat, and get it as hot as possible just prior to the test.

Take care,
-Chris

[JohnnyG]

Thanks guys, I'll give it a shot... I think my thermostat is due to be replaced anyway... it's been stuck open for about a week now

Take care

John