Emissions fail - high hydrocarbons in exhaust

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.

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?

Quote:

---

Originally Posted by 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?

---

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?

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

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?

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.

A sensor can get contaminated easily. One tankful of gas with any amount of lead in it can do it. Using silicone sealer that is not "sensor safe" can do it too. I asked if the FV was buzzing so that would indicate that the Lambda system was functioning and the duty cycle was at least close if he hears it. A place to start and you don't need any tools to check.

The car's at the shop so I'll have to go by and check out the frequency valve on Mon.

The exhaust IIRC is dual-cat, but was installed by the guy who sold me the car, not actually done at DPI. He said he put a new O2 sensor in, not sure if it came with the exhaust kit or was purchased elsewhere. Are these sensors specific to the car or can they be bought off the shelf?

I did have an issue where the O2 sensor was coming loose last year. Could something have gotten into it or gotten damaged? Although the last time it happened (that I noticed) was around May and it had passed smog since then. It didn't have any sealer on that I noticed.

I generally only buy name-brand, top-tier certified gas, so it shouldn't have had any lead in it, should it? I didn't think you could even get leaded gas in the US anymore (except from airport fuel trucks). The worst gas I remember buying in the last year or so was Shell and Valero. I can't swear with 100% certainty that I never put generic in if I was out in the middle of nowhere and running low but if I did that would have been on my cross-country trip last May (before last year's smog inspection). I always get 91+, too.

You still need to smog your car!!!!! I thought only us Californians were subject to totalitarian smog laws! Being at DCS this month, everybody from the Midwest was shocked that DeLoreans were still emissions tested.

Quote:

---

Originally Posted by Chris 16409

You still need to smog your car!!!!! I thought only us Californians were subject to totalitarian smog laws! Being at DCS this month, everybody from the Midwest was shocked that DeLoreans were still emissions tested.

---

Even the People's Republic of NY exempts cars 25yrs and older from emissions tests. At least where I am not sure about NYC.

I'd definitely start with the oxygen sensor. If it is NOS it may have been installed recently but it's still 36 or more years old.