donnoyet

Folks,
I drive a 94' honda accord EX 4 cyclinder.

I failed my smog check with the following numbers

15mph (1791 RPM) HC(MEAS=78, MAX=61) NO(MEAS=142, MAX=490)
25mph (1839 RPM) HC(MEAS=42, MAX=36) NO(MEAS=860, MAX=511)

Any ideas on what could be the problem and an easy fix to get this to pass without spending too much.

slamstrom

I failed my smog (94 Accord EX 4Dr sedan) about 2 months ago. My measurements are:
15mph (1845 RPM) HC (MEAS=74) NO(MEAS=2044) CO(MEAS=.51)
25mph (1864 RPM) HC(MEAS=17) NO(MEAS=677) CO(MEAS=.15)

I had to replace my catalytic converter (replaced with a MagnaFlow 36304, $130) and my passed results are:
15mph (1823 RPM) HC (MEAS=10) NO(MEAS=94) CO(MEAS=.01)
25mph (1901 RPM) HC(MEAS=8) NO(MEAS=81) CO(MEAS=.01)

I'm in California, so I have to install a CARB certified catalytic converter. You may not have to in your state.

MAD_MIKE

Would you happen to have CO measurements?
Usually I change oil and slap a new air filter in and never have a problem. When was the last time you had a tuneup?

donnoyet

15 mph CO (MEAS= 0.18, MAX= 0.51)
25 mph CO (MEAS= 0.01, MAX = 0.49)

Tune up was really long time ago. Not sure what a tune up includes? Do you know?
I changed oil less than 1000 miles ago.

slamstrom

You didn't fail by much. I failed smog miserably. When was the last time you change your spark plugs? If you don't know, please change it. May give you a cleaner burn. Was your engine hot when doing the smog test? It should be hot.

MAD_MIKE

CO seems low, may be running a bit lean. This would cause the higher HCs as well.
Generally a tune up consists of checking paramaters such as idle speed and ignition timing, check resistance of coil and spark plug wires. Parts replaced are spark plugs, spark plug wires, dist cap + rotor, O2 sensor, valve adjustment, air filter along with oil and oil filter.

phootbag

Running lean causes high HC's?

donnoyet

I havent changed spark plugs in 5 yrs.I went into smog center after driving for 10 mins at 25 mph.

MAD_MIKE

Yup. Running lean causes higher HCs than running rich. Not enough heat is generated and what fuel isn't burnt(and doesnt combine with anything) just passes out the tail pipe. HC is greatly affected by CO.
If you were to look at a graph of HC in relation to stoich it would appear as a check mark, it would start somewhat high on the rich side, plummet til stoich and then skyrocket when it leans out.
CO on the other hand starts off high when rich plummets to stoich around .3 and then gently tapers off from stoich to lean.
NOx on the other hand is like an equilateral triangle. From rich to stoich to lean NOx peaks at stoich.

I'm wondering if there is a bit of a stumble or miss or ping. NOx seems high at 25MPH, and a clogged or malfunctioning egr system can cause for a lean burn. The cat can clean up emissions, but if the EGR system is not working properly, it will not be able to compensate for faulty components.
Oh yeah, its time for a tuneup. Replace the spark plugs, spark plug wires, dist cap + rotor, O2 sensor. Most definitely get a new O2 sensor. O2s may not throw a code, but they become lazy with age and is the key component that lets your ECU know what the engine is doing combustion wise. Rather than seeing rich/lean/rich/lean/rich/lean... the ECU will see ...rich.....lean....rich....lean...rich. This is not good for power/mileage and can damage the catalytic converter.

donnoyet

The mechanic did something and just got my smog to pass. He did not replace or change any parts. He just charged me the diagnosis labor and said e had to tweak some stuff to get it working. He noted that the O2 sensor was slow and that i need a new fuel filter.
How much would it typically cost to replace these two on my 94 honda accord ex?

Thanks

donnoyet

The new numbers are as follows:
15mph (1842 RPM) HC(MEAS=37, MAX=61) NO(MEAS=489, MAX=490) CO (MEAS= 0.06, MAX= 0.51)
25mph (1879 RPM) HC(MEAS=21 MAX=36) NO(MEAS=497, MAX=511) CO (MEAS= 0.01, MAX = 0.49)