'98 Toyota almost didn't pass smog test


I have a '98 Toyota 4Runner with a 3.4L V6 and 155,000 miles. I took it in for a smog test and it almost didn’t pass. The HC at 15 mph was 50 ppm and the maximum allowed is 51. The average for passing vehicles is 8 ppm! At 25 mph it was 19 ppm with the average being 6 and the maximum 35 ppm. The CO and NO were above averages also but well below the max. It’s got a lot of miles but I have done regular maintenance and always use name brand gas since it was new. It gets about 19 mpg, about the same as it always has. What could be causing the elevated emissions?



The following, supposedly from Quantas Airline, reminds me of your dilemma. You car ALMOST didn’t pass.

After every flight, Qantas pilots fill out a form, called a “Gripe Sheet,” which tells mechanics about problems with the aircraft. The mechanics correct the problems, document their repairs on the form, and then pilots review the gripe sheets before the next flight. Never let it be said that ground crews lack a sense of humor. Here are some actual maintenance complaints submitted by Qantas’ pilots (marked with a P) and the solutions recorded (marked with an S) by maintenance engineers.

P: Left inside main tyre almost needs replacement.
S: Almost replaced left inside main tyre.

P: Test flight OK, except auto-land very rough.
S: Auto-land not installed on this aircraft.

P: Something loose in cockpit.
S: Something tightened in cockpit.

P: Dead bugs on windshield.
S: Live bugs on back-order.

P: Autopilot in altitude-hold mode produces a 200 feet per minute descent.
S: Cannot reproduce problem on ground.

P: Evidence of leak on right main landing gear.
S: Evidence removed.

P: DME volume unbelievably loud.
S: DME volume set to more believable level.

P: Friction locks cause throttle levers to stick.
S: That’s what they’re for.

P: IFF inoperative.
S: IFF always inoperative in OFF mode.

P: Suspected crack in windshield.
S: Suspect you’re right.

P: Number 3 engine missing.
S: Engine found on right wing after brief search.

P: Aircraft handles funny.
S: Aircraft warned to straighten up, fly right, and be serious.

P: Target radar hums.
S: Reprogrammed target radar with lyrics.

P: Mouse in cockpit.
S: Cat installed.

P: Noise coming from under instrument panel. Sounds like a midget pounding on something with a hammer.
S: Took hammer away from midget.

On to your problem:
How long has it been since you’ve done a good tune up. Plugs, plug wires, or anything else that can cause a miss, will elevate your unburned hydrocarbon count, as will a plugged air filter.


The catalytic converter is getting old. Changing it will reduce the emissions even if the fuel injectors are putting out too much fuel.


How long have the spark plugs been in there?


About a month before the test I replaced the spark plugs and air filter and added some injector cleaner to the gas tank.


How many miles does it go between needing a quart of oil? HC goes up the more the engine uses oil.


Since it’s a high mileage engine, it’s probably burning some oil. Not enough for visible smoke; but, enough to increase emissions. You can get an idea of how well the engine is sealing by performing compression and vacuum tests. A mechanic could interpret those values for you. There are tests of sensors and actuators, but, don’t worry about them until it gets worse.


I change the oil every 5k miles and never have to add oil in between changes. Anyone else heard of catalytic converters wearing out?


150k is easily time for a new catalytic converter. O2 sensor, catalytic converter. Pair them up and you’re going to see a large improvement. Does your warm up quickly and maintain a steady temperature? Have you ever replaced the thermostat?


I have to respectfully disagree with the cat converter as the primary suspect for a number of reasons.

First, reductions in unburned hydrocarbons (HC) come from complete combustion of the gasoline. That comes from good, strong spark properly timed and nice fine injector spray properly metered. Reduction of unburned HC is not the function of the cat converter.

The cat converted primary stage, the ceramic substrate plated with platinum palladium/rhodium, when heated weakens the bond between the nitrogen and oxygen atoms reducing NOx. The second stage does allow passing carbon monoxide molecules to bond with additional freed-upp oxygen atoms and become CO2 (carbon atoms bond with other atoms extrememly easily). But it does not seperate the hydrocarbon molecules…its function isn’t really combustion (if it does that’s called a “backfire”). And, any 1996 or newer vehcle has a downstream oxygen sensor that monitors the function of the cat converter and will trip a check engine light if the converter is “spent”.

In short, while a new cat converter may reduce CO and NOx a bit, I think the real solution is to look at those things that affect metering, injection quality, and ignition. Those are the real culprits in high unburned hydrocarbons. Since there are no CEL light indications, I’d start with a thorough tuneup including new plugs and go from there.


A new “Cat” will reduce ALL the measured emissions dramatically. They don’t last forever.

In older cars without catalytic converters, a “normal” reading would be HC-250ppm and CO-2.5%.

HC is unburned fuel, not a “rich” mixture (which causes high CO).

Not all the fuel (100%) ever burns in an internal combustion engine. Tiny amounts cling to metal surfaces inside the engine and escape combustion and are expelled in the exhaust. Part of the Cats job is to burn these HC molecules…

An injector with a slightly degraded spray pattern can also cause this because the droplets of fuel are not properly atomized and can not burn completely…


OK, it has new spark plugs; but, probably old spark plug wires. If the engine rocks, just a little, during idle, it could be from weak, or missing (intermittently) spark from old, leaky, spark plug wires. The engine computer will NOT turn on the check engine light for a little misfire (under 2%). The engine computer turns on the check engine light when emissions reach 150%; but (hopefully), not before.


Did you take it on the interstate for a good twenty mile run to clear out the cats before the test? Also did you sit in line for a extended amount of time idling? sometimes that can cause the car to go back into a closed loop mode making the car run less clean. I think some Saturns had that issue.


Also, ever replace the oxygen sensors? Even if the CEL is not on, there is a good chance 10-yr old sensors may be out of whack, and need to be replaced.


High HCs is going to be unburned fuel or possibly oil consumption given that the engine has 155k on it. If the engine has ever gone through overheating this can ruin valve seals and/or piston rings. A lot depends on how much oil you’re burning.

Just curious, but does this vehicle have an automatic transmission, and if so does it idle very smooth while in DRIVE with the brake applied?

Low compression, weak valve spring, bad injector spray pattern, etc. could cause this problem also, but this usually shows up as a rough idle to some extent. Sometimes it may be subtle and hard to notice.

With a 155k miles, step one in the shop would be a compression and/or leakdown test just to rule out a mechanical fault.


While I agree that some unburned hydocarbons escape the cylinders, I respectfully generally disagree that the cat converter should be changed here.

A rich mixture causes unburned fuel, higher HC. As well as higher carbon monoxide.

While some unburned hydrocarbons may be bits that clung to the cylinder walls, they’re primarily the nuclei of fuel droplets. Only the surface HC molecules in contact with the oxygen burn, so the droplet burns down in layers, like peeling an onion. If there’s insufficient oxygen or teh droplets are too large, they simply don’t burn all the way through the droplets in the limited amount of time available.

Older pre-cat engines did not have the sophisticated multiport injection systems of modern engines…they burned rich, thus the high HC readings.

I agree about the injector.

But I still maintain that, looking at the readings, the place to start is with a good tuneup. The engine is running fine, passing emissions (though the HC level is high), and no codes are being stored. Cat converters fail by either contamination of the catalyst or deterioration of the substrate causing areas to break away and reducing the surface area that the exhaust stream contacts. Perhaps erosion of the catalyst is a failure mechanism as well, but I can’t honestly say I have any evidence to support that. When they do fail on an OBDII engine, the downstream oxygen sensor output is recognized by eth ECU as not sufficiently changed from the upstream reading and a code is stored.

While we ultimately might end in disagreement, I stand by my post. I still think that the place to start is a good tuneup. Fresh plugs, wires and filters should reduce the HC levels.


since you’ve heard from the resident mechanics, this simple diyer asks, have you had the timing belt (chain) changed recently?

something has changed. what has been done to the vehicle before that? has the vehicle been in a major accident?


Thanks for all the comments. I checked my records and found that I replaced the O2 sensor 60k miles ago. I replaced the timing belt 30k ago. The wires, cap and rotor have never been replaced. Spark plugs come out clean (ash colored) when I replace them. My last smog test 2 years ago had similar readings (high HC). The engine idles smoothly in drive - no miss, just a little tic from the valves. Besides replacing the spark plugs and air filter, what else can I do to tune it? Should I take it in to have it scoped to see if I have a week spark somewhere? Or should I just replace the wires, cap and rotor?


I’m still curious about oil consumption. How much per 1000k miles?

Define “ash colored”. The plug tips should be light tan in color and ash-colored, to me anyway, denotes dark gray or black.

If the vehicle had similar high HCs 2 years ago and this occurred after a timing belt change, I wonder if it’s possible that someone could be off a tooth on the cam timing. That could cause it.

A scope is a great resource to have and use and it can tell quite a bit about what’s going on inside the engine while it’s running. If those are the original plug wires then I would definitely replace them. If the cap and rotor appear to be fine visually then those 2 items might get a pass. Other than carbon tracking or burning not much can go wrong with a cap and rotor.

You might also consider a possible vacuum leak, which could be verified with a vacuum gauge. A slight vac. leak means the ECM is going to try to compensate for it by going richer; at least to a certain point.

Hope some of that helps.


www.centuryperformance.com/spark2.asp has a good photo comparison section for “reading” spark plugs.