1992 5.0 failed smog test!

[Kyle700]

Hey everyone! Maybe someone else knows or has been through this. I have a 92' 5.0 Tbird with 70,000 miles, just failed smog test. The CO measured 40 at 15mph (with a max allowed of 530) which is good, but at 25mph it measured 569 (with a max allowed of 487).

I have never had a check engine light come on, I did replace both oxygen sensors a few months back due to very bad acceleration and chugging, ran like a champ after those were replaced.

I don't have an older OBD tester just the OBDII which wasn't used til 1995(?)

Any ideas where I should start?

Thanks

 

[fordrodsteven]

high CO readings mean that it is running rich. There could be many reasons. You really need to do some investigating to get it right instead of just throwing lots on money at it. Many possibilities. You already changed the oxygen sensors. could be as simple as worn spark plugs or could be much more serious. bad coil, Bad CO2 sensor, Defective MAP sensor, defective TPS, bad engine coolant sensor. Like a stated earlier you need to get together more information before you can fix/replace the correct part. Hope you get it figured out easily / quickly.

 

[Kyle700]

I apologize, the CO was ok. The numbers that are in the original post are correct, but they are for NO (PPM). Sorry about that!

 

[fordrodsteven]

High NO readings are the opposite. It means your car is running lean rather than rich. Again you need a lot more information to proceed. Lean fuel can be caused by many things. Vacuum leak, defective air flow meter, engine coolant level (overheating), EGR valve, Catalytic converter, even carbon buildup in the engine due to high mileage. You need to know more about what is going on with your engine. Did the smog technician try to point you in any direction as to what he thought was wrong?

 

[fordrodsteven]

Has the car had a tune-up? Just changing the air filter and plugs/wires can make a car run better and not produce as much carbon in the exhaust (which raises the NO readings) I suspect that a very dirty air filter will cause exactly what you are seeing with NO increasing as the engine revs go up.

 

[Kyle700]

70,000 miles on the car, so its not high mileage. I have not done a tune up on it myself, but the car was meticulously maintained by by wifes grandfather before we got the car. I will get the oil changed, air filter replaced and probably the spark plugs. The car has been on and around dirt roads for the last 3 years...
Smog tech suggested looking at the EGR system. But as a retired Ford mechanic pointed out to me on the results page, the tech didn't appear to know what he was doing after looking at variable he put in the computer such as;
EGR Functionable... he put, "Not Applicable"
Ignition Timing..."Not Adjustable"

 

[fordrodsteven]

I wouldn't get too thrown by the technician banging in N/A. He might have been trying to just run it through quickly because he didn't want to wait for the machine to go through the checklist. I would start with making sure it's breathing clean air and has good plugs in it. Did the car sit for a long time? After air filter and plugs I would go after the EGR system. The problem is that it ends up being you throwing money at it because you don't know enough of all the peripheral information to make a good troubleshooting guess. It could be a vacuum leak in the EGR system or the EGR valve ($75 at Rock Auto) or the EGR solenoid ($25 at Rock Auto)

 

[Kyle700]

Thanks for all the advice so far. The car used to sit years ago (driven about 3,000 miles per year), but it has been a daily driver for the last 3 years. I am trying not to throw money at it, that's why I am here. Also I have a retired Ford technician asking his emission/smog friends for ideas. I'll will post the fix when it happens. Thanks

 

[Kyle700]

On the contrary fordrosteven, it does matter if he "banging in N/A." That says he did not verify the timing is set at 10 degrees BTDC.