Emissions fail - high hydrocarbons in exhaust

[DaraSue]

So my car failed emissions, where AFAIK it passed with flying colors the last two times. It read 200PPM (hydrocarbons) on the 2500 RPM check, 175 on the idle check.

Things we've already tried: Checking the dwell & adjusting the mixture, making sure the O2 sensor wasn't loose, checking for vacuum leaks

Things we're going to try: replacing the spark plug wires

Is there anything else obvious that I'm missing? I feel like I must have screwed something up when I was working on it last fall. I did trip over the mixture unit while it was sitting on the floor, is it possible that I broke something inside it?

The engine was overhauled in winter 2016 at DMCCA and it has a DPI exhaust (new when I bought the car in summer 2016.

[David T]

If the fuel is older than a couple of months use it all up or remove it and get fresh stuff in. Try a can of Techron to clean the injectors. A bad spray pattern can mess up the emissions. Is the frequency valve buzzing?

[DaraSue]

If the fuel is older than a couple of months use it all up or remove it and get fresh stuff in. Try a can of Techron to clean the injectors. A bad spray pattern can mess up the emissions. Is the frequency valve buzzing?

The gas isn't more than a month old and I almost always put Chevron with Techron in it. Not sure about the valve, it should buzz with the mixture is set correctly, is that right?

[David T]

The valve is supposed to buzz whenever the motor is running.

[NckT]

Disconnect the oxygen sensor electrical connection first and see if it buzzes. If it does when the engine is cold but stops when the engine is warm, then the duty cycle may have gone full swing such that the valve is fully open or fully closed.

I'll assume you've not replaced the lambda relay lately, as this needs a double contact single throw type.

Does the DPI "performance" exhaust have catalytic converters in?

Have you done fuel pressures tests?

[bytes311]

I just smogged my car 2 days ago and it failed with higher than normal HC. There were no vacuum leaks, and the dwell was properly set and the FV buzzed loud and continuous. The problem turned out to be a bad or faulty O2 sensor (it was only 3 years old, from DMCH). The shop put in a new Bosch sensor and re-adjusted the duty cycle. The HC was literally cut in half and passed. My car feels like it has so much more power now, too.

[NckT]

Disconnect the oxygen sensor electrical connection first and see if it buzzes. If it does when the engine is cold but stops when the engine is warm, then the duty cycle may have gone full swing such that the valve is fully open or fully closed.

I'll assume you've not replaced the lambda relay lately, as this needs a double contact single throw type.

Does the DPI "performance" exhaust have catalytic converters in?

Have you done fuel pressures tests?

Done any of the above yet?

Disconnecting the electrical connection from the oxygen sensor defaults the fuel computer to operating the frequency (bleed off) valve to 50% duty cycle ie it buzzes. This is when the car is at operating temperature.
The reason I asked if it buzzes when cold is that the lambda switch should default the duty cycle to 60% and you should hear it buzz.
If it buzzes when cold but not when hot then as described above it goes full swing one way.

If you don't hear it buzz at all with the oxygen sensor disconnected then it'll be an electrical issue. This is usually brought on by someone replacing the relaid in the relay compartment without taking note that the lambda relay is a particular type described in the previous post hence the question.
In a small number of cases with a good oxygen sensor confirmed looking at the trace with an oscilloscope and frequency valve not buzzing due to swing of the duty cycle and fuel mixture is correct with oxygen sensor disconnected, then the quad op amp in the fuel ecu has gone bad and needs replacing.

[dn010]

Silver box is the “lambda” ECU. Black box with two plugs is the idle ECU.

[DaraSue]

I'm trying the test with the sensor unplugged, seems like it idles a little choppy but the valve is buzzing. How hot does it need to get? Up to the unmarked white line?

[DaraSue]

https://youtu.be/YElPUoXoAjU