What is causing high HC on a smog test

[Pyral]

I did my bi-annual smog check for California a few days ago and I passed. But I've noticed a concerning trend in my records, the test at 15 mph for Hydrocarbon is increasing at a rate where I'll fail 2 checks from now.

2 years ago my HC @ 15 mph was 30 ppm, now it's 39 ppm, the max is 52 ppm. The average HC is only 9 ppm.

Even at 25 mph my HC is double average. I attached a photo of the test results.

I've read a bit about this and I think I can eliminate a few causes. Last December I adjusted my valves and cleaned the MAF. 30k miles ago I replaced the plugs. The spark plug wires have 40k miles on them and are aftermarket. I have no misfire or codes.

The only thing I need to do is clean the IAC because I had an intermittent surging idle a few months ago. The fuel filter, injectors, and cat are original.

What could be causing my increasing HC levels?

 

[AuSeeker]

I did my bi-annual smog check for California a few days ago and I passed. But I've noticed a concerning trend in my records, the test at 15 mph for Hydrocarbon is increasing at a rate where I'll fail 2 checks from now.

2 years ago my HC @ 15 mph was 30 ppm, now it's 39 ppm, the max is 52 ppm. The average HC is only 9 ppm.

Even at 25 mph my HC is double average. I attached a photo of the test results.

I've read a bit about this and I think I can eliminate a few causes. Last December I adjusted my valves and cleaned the MAF. 30k miles ago I replaced the plugs. The spark plug wires have 40k miles on them and are aftermarket. I have no misfire or codes.

The only thing I need to do is clean the IAC because I had an intermittent surging idle a few months ago. The fuel filter, injectors, and cat are original.

What could be causing my increasing HC levels?

Bad or going bad Catalytic Converter/s maybe.

 

[pluton]

Did you fully warm up the cat by driving for at least 30 or 40 minutes before the test? Doing that helped my HC readings stay low. (different engine but same idea)

 

[prybar]

Going thru something similar. Got the truck in 2019 and had to smog in 2021. I failed for HCs but luckily I knew the shop owner and he kept running the test til I passed (literally passed by 2 ppm on HCs).

Was nervous about my next smog in 2023, so I hopped on the freeway and drove for about 45 min on the freeway with OD off at high RPMs. Passed smog with flying colors, so that's what I'll do next time as well.

Not sure this really helps address the issue and it's been pretty well documented that this can help you pass but just thought I'd toss in my experience as well.

 

[pluton]

Going thru something similar. Got the truck in 2019 and had to smog in 2021. I failed for HCs but luckily I knew the shop owner and he kept running the test til I passed (literally passed by 2 ppm on HCs).

I wonder if the PO used it exclusively for short trips. Nice to have a sympathetic shop owner.

 

[Pyral]

Did you fully warm up the cat by driving for at least 30 or 40 minutes before the test? Doing that helped my HC readings stay low. (different engine but same idea)

I didn't smog it myself. My mother did because her car had a P0420 on the way to the shop and my car needed to be smogged. The smog shop is a 5 mile drive, so with lights I'd assume it is fully warmed up if it got tested right away.

Edit: the car sat for 10-15 minutes before getting smogged

 

[phattyduck]

I didn't smog it myself. My mother did because her car had a P0420 on the way to the shop and my car needed to be smogged. The smog shop is a 5 mile drive, so with lights I'd assume it is fully warmed up if it got tested right away.

Edit: the car sat for 10-15 minutes before getting smogged

Oh yeah, that'll do it. 5 minutes is wayyyy to short and it should never get shut off when you get to the smog shop until the tests are done.

-Charlie

 

[pluton]

The first time I decided to give mine the big warm up before smogging it, I drove it 50 miles out and back---total 100 miles---went directly to the smog shop, and was told that their testing rig was down and needed to be reprogrammed. Ended up sitting for about 4 hours before they called and said bring it in. I drove it another 6-7 miles back and forth on the local freeway, took it in, and it passed with good numbers. But driving it just 5 miles, starting from cold, isn't enough.