Hey everyone,
I just had my Amigo smogged, and it passed. But the numbers jumped up quite a bit since the last time it was smogged 2 years ago (30k miles). HC(ppm) has doubled, CO is up 0.1%, and NO(ppm) has trippled. Before the test, I replaced the spark plugs, distributor cap, and distributor rotor. The O2 sensor is 6 years (55k miles) old. If I remember correctly, it's a Bosch. I've also been running the Amigo with a bad mixture control valve; it backfires between shifts when cold.

Do you think it's just a bad O2 sensor or is there something bigger to be concerned about?

I'm worried that all the backfiring has damaged the catalytic converter. Is there a way to test this?

Below are the smog numbers. My amigo is stock except for the 31s.

2 years ago
test speed | HC(ppm) | CO(%) | NO(ppm)
15 mph | 58 | 0.09 | 227
25 mph | 43 | 0.11 | 226

this year:
test speed | HC(ppm) | CO(%) | NO(ppm)
15 mph | 100 | 0.19 | 634
25 mph | 79 | 0.22 | 574

Its posible it could be something wrong, I'm not sure what would cause all the readings to go up that much though.

It's also possible its just the testing station. Last year the readings on my mitsu went up quite a bit, this year they were back to where they usually are. I didnt even change the plugs, cap, rotor, pcv, or air filter this time either like I normally do. I always change oil before a test though. I used a different testing station this year.

Back fire? That's a classic for burnt valves. High HC indicates poor burn which is usually the result of poor compression, which is generally
the result of leaky valves. High Nox can be a malfunctioning EGR circuit or the catalytic converter is past it's prime. Compression check will answer the HC question and a cleaning of the EGR components may help the Nox.

Thanks!! I know burnt valves are bad, but what exactly are they?

Your motor is built with two large castings. The lower is the cast iron engine block which holds the pistons in their cylinders and the crankshaft. The upper is the cylinder head which is bolted to the top of the engine block and holds the cam shaft and valves. To visualize it the valves are twin corks, the cylinder is the bottle and it takes four strokes of the piston to complete a cycle. First stroke piston goes down, the intake valve opens to let air/fuel get sucked into the cylinder then closes. Next stroke, both valves closed, piston goes up compression/ignition. Next stroke, blast of the ignition drives the piston down. Next stroke exhaust valve opens and the piston goes up to drive the exhaust gas out of the cylinder. To work at maximum efficiency, the movement of the pistons and valves has to be coordinated such that the spark ignites the air/fuel mixture near the maximum compression of the upward stroke of the individual piston. In that position the intake valve and exhaust valve should be shut tight to maintain the compression. That's the timing and that's a mechanical relationship.

The other piece is the air tightness of the individual cylinder. If the piston rings are worn, air/fuel will blow by the rings and you'll get poor compression which will give you a weak detonation, loss of power. What's more common is the valves in the cylinder head get sloppy from wear or maladjustment. When that happens, hot gas from the detonation at high pressure blows by the valve and valve seat. Eventually that gas will erode them both letting even more gas blow by and cause more damage to the point where you will have no compression in the cylinder. You'll feel the loss of power, you'll hear the noise. It's pretty common for the exhaust valve to burn up. It's got nothing to cool it off except the valve seat. Once it loses contact with that seat it will develop a hot spot and the deterioration is pretty quick from there.

It's not a difficult repair to replace a head. Time consuming? Yes.
Expensive? Sort of. Requires great skill? No, but does require careful planning, some strength, labeling/marking everything you take off and a good manual (or mechanically enhanced friend) to guide you. If you are willing to do it the work yourself and tool up to get the job done, it's pretty gratifying to have your buggy restored to full power.

Do your compression check. That will tell you what you need to do.

I'm familiar with the basics of how the engine works. About 30k miles ago, my timing belt broke, and threw a piston into a valve. The valve was bent, and I went in and replaced that one valve. I lapped the other 7 valves and replaced all the rubber seals while I had the head off.

My question is that valves are made out of metal. I didn't think you could "burn" metal. What happens to the valve when it gets to hot? <img src="/forums/images/graemlins/confused.gif" alt="" />

Burning is simply very rapid oxidation. The only difference between wood burning and steel burning is the necessary temperature. I you had ever welded or used a cutting torch, you would be familiar with burning metal.