Hi all,

I need advice with passing California smog. Car is a 1993 Honda Civic EX, D16z6 engine, 291k miles. It’s all stock, only factory-spec parts used. I’ve owned it for 15 years now, mostly doing my own maintenance. Car drives fine, no noticeable engine problems besides oil leaks.

Back in January I tried to smog the car; I only drove the car 15 mins beforehand, and failed for high NOx with the following results:




So, I spent the next few weeks fixing/testing various things:

• Installed new: Denso oxygen sensor, air filter, NGK spark plug wires, oil/filter change
• Did piston soak with Berryman B12; there was quite a lot of carbon on the piston tops; boroscope camera after soak showed it mostly clean
• Fixed distributor inner/outer seal oil leaks (did note some wire’s insulation inside seemed to be crumbling, so I coated the bare wire areas with liquid electrical tape)
• Cleaned out the IACV and MAP sensor with throttle body cleaner
• Ran 2 cans of B12 cleaner thru the tank
• Verified: base timing is at 15 BTDC with timing light. Fuel pressure is within spec (service manual page 11-104). Tested for vacuum leaks by spraying carb cleaner around vacuum lines while idling, but didn’t notice any idle surges.

So here we are in February; based on advice gathered from the internet and friends, I pumped tires to 40 psi, then added 0.75 gal E85 to tank and filled the rest w/ regular 87 gas E10 (calculated ethanol concentration at test time should be ~E15). I drove it in D3 for ~40 mins at 40-60 mph before I got to the smog shop, to get the cat hot.

I figured that since I barely failed last time, then at least either the E85, carbon cleaning or hotter cat would lower the NOx just enough to pass. There was no line and they tested straight away; BUT it still failed, this time with high HC at 25 mph:



WTAF. My NOx has gone down, but HC and CO both went up. I’m not sure where to go from here; most of the ignition components are near-new.

Smog-related maintenance history, besides the stuff above:

• In 2020 @ 250k miles, car did have a head gasket leak. I replaced the HG and sent the head to a shop; they also milled it and put new valve seals. At this time I also did new timing belt/water pump, coolant flush, new coolant temp sensor, and a valve adjustment.
• I think the car does burn oil, though a lot of it also leaks externally. Still working on the leaks so I can verify the true oil burn rate; but the combined burn/leak rate is about 1qt per 1100 miles.
• In mid 2025 the ignition coil went out; at this time I put in a new NGK coil, and an Autozone rotor and distributor cap, and new OEM NGK 5574 plugs.
• There was occasional pinging at low load/low RPMs, not very often, though I think this was resolved with the piston soak last month.

Some thoughts:

• What can I infer from the 2 smog test numbers? i.e. is the cat actually working or not? Did it go from running lean to rich?
• From the fixes I did, I can’t figure out how/why the HC/CO2 went up. The only thing I can think of is that the piston soak fouled the plugs, and I didn’t clean them enough before reinstalling. They’re only 9 months old.
• Does the P28 OBD1 ECU need time to ‘re-learn’ before smog? Maybe fuel trims? I did disconnect the battery the night before the 2nd test, but drove it for more than an hour over 2-3 trips after reconnecting it.
• I did note that after the HG job in 2020, my 2022 and 2024 emission numbers seem to have shot up (comparing with 2020-prior), though they still passed in those years. Could a bad valve adjustment in 2020 cause higher emissions?
• Any other testing/instrumentation I can do? Long term I do want to get the car actually fixed, rather than simply passing the test. The E85 trick was just a bandaid solution and I’m not even sure it helped.
• With failing smog, I can’t get new tags. Does anyone have experience with CA legal workarounds to this situation? I vaguely recall some program where the BAR will grant a smog exemption for 2 years under certain circumstances. Money is tight and it’s costing me $120 each time just to test, so it’s incredibly expensive and frustrating to fail.
• If anyone knows of a smog shop in the SF Bay Area, or Los Angeles area (I’m occasionally down there) that can do a pre-test, i.e. trial run before the actual test, let me know…

Current plan:

• New spark plugs
• Valve adjustment (I need to fix the valve cover gasket leak anyway)
• Maybe replace the PCV valve, and/or clean out the breather box below it. (The damn PCV won't come out easily, it's stuck in the hardened grommet)
• Buy a thermometer to see if the cat outlet is hotter than inlet
• Maybe buy a Hondash dongle for better monitoring, but it'll take weeks to arrive

Any ideas or advice is appreciated, thanks in advance.

 

Smog test cost $50 with certificate so you need to find a shop that isn't screwing you over first.
Why would you use e85 on a stock engine/ecu? Makes no sense.
Base timing should be 16*
You'll need to address the burning oil. I barely passed with a y8 that was burning about a qt every 1000 miles.

 

Unfortunately where I live (SF Bay area) alot of smog shops won't even do pre 1996 sniffer tests anymore. The few shops who do all quoted between $120 and $190, so $120 was actually the cheapest one. The same place only charged $80 2 years ago... The $50 prices can only be found for OBD2 cars. I agree it's a ripoff.

The E85 is because ethanol burns cleaner (but has lower power output). Some forums suggested it to help older cars that are borderline failing. This guy on youtube even went all the way to 1:1 E85 to gas ratio on his 280z and got a significant reduction in emissions, though I didn't want to go that far especially with a stock tune. My thinking was that adding 0.75 gallon of E85 (the other 11.2 gallons was regular gas) might just be enough to nudge the nox down. The car seems to run fine on the E15 mix, no misfires. Still debating if I would do it again for the next test.

Re: the oil burning, assuming its the rings, I might try the new Valvoline Restore & Protect OCI regimen to see if it frees up the rings; but short term, at least for the sake of smog, would a switch to 0w-40 oil help? If burning oil were the culprit behind the high HC, I would've expected test #1 to have equal to or higher HC than test #2, since the piston soak after test 1 should've helped reduce oil burning and lowered the HC for the second test.

 

I did some more tests. The car has only been driven a few miles since the failed second test, nothing changed on it, so tests should be representative of the state of the car at test time.

I pulled the spark plugs and they look ok IMO, what do you guys think? Maybe a little whitish ash on them. They're only 9 months old.



Piston tops have some carbon on them (a little more than I’d have hoped for, since I did do a piston soak 2 weeks ago) but nothing crazy.

I did compression test on cold engine, first dry, then wet. I used a rented Autozone tester and cranked each cylinder 10 times.
Wet tests were done with 1 tsp 0w-40 down each spark plug hole, followed by hand cranking a few revs to spread the oil.

• Cold and dry: 176, 176, 178, 190
• Cold and wet: 202, 192, 210, 212

Did a second compression test on warm engine:

• Warm and dry: 177, 177, 180, 188
• Warm and wet: 195, 196, 189, 204

I assume the warm + wet psi’s are lower than the cold + wet psi’s due to the oil being thinner once warm.
Unfortunately across all tests I forgot to hold the throttle at WOT, but if anything the numbers should go up with WOT.

Checked catalytic converter temps at inlet and outlet with infrared thermometer.

• After short 10 min in-town drive: 296F inlet, 393F outlet
• Then did a spirited 10 min freeway drive back home: 465F inlet, 554F outlet

From these tests I'm drawing these conclusions, please correct me if I'm wrong:

• The spark plugs seem ok and unlikely to be the source of the high HCs
• Engine appears to be holding decent compression.
  • Comparing the warm dry vs wet test, compression increased anywhere from 9 to 19 psi. Is that delta enough to indicate the piston rings are bad?
• The catalytic temperature suggests the cat is working ok (or at least, doesn't have some kind of catastrophic internal failure)

But, now I'm not sure where else to look for the high HC problem.

 

I do not think catalytic converters have an infinite life. In multiple different subaru and honda vehicles that have been well cared for, around 200-250,000mi is where they start to fail. In one case, the car became a grocery getter so its under an annual mileage limit that my state requires and i live with a P0420. In my 2000 civic, oil burner, it only lasted to about 120,000 mi, I put a magnaflow cat on and it has been fine for awhile now. In all the other cases, accords, it was time to get rid of the car anyway (rust and other problems) so that took care of the problem.

 

my car? I would be looking for vacuum leaks. Look up High Hc and High Nox. the 2 common is unmetered air making the engine run lean or bad cat converter.. do the carb cleaner spray trick. No vacuum leaks? then you may have an internal vacuum leak in the throttle body. Bad TB gasket. Or bad IAC (the pintle getting stuck somewhere in its travel. Put a vacuum gauge on it. at idle anything over 18 is great.. Then move to the cat. Some have had luck cleaning the cat with just soap and water. I have never been that lucky. My old d15b7 with 450k would start acting weird every few months, so I would take it out and run the bejesus out of it, Italian tuneup is what its called, That would blow all the crap out of the cat converter and then it would start running better. Think massive blue/black fart