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Removing carbon build up on pistons?

My 87 Acura Integra with 170k gave SMOG reading in CA as below:

Speed Max Meas
15 760 875
25 542 535

A m/c tells me it is the result of carbon build up on pistons.

What is the fix?

Incidentally it gave the following in 2010 - upon retesting without any repair it passed:

Speed Max Meas
15 760 656
25 542 678

Note: Recent oil change (Mobil 1 High Mileage), tune up, Thermostat, new PCV valve, NGK Plat spark


Updated: It is for “NO” - Nitrous Oxide

I am not sure what the numbers mean, not sure about the carbon buildup theory, but seafoam has a number of uses.

I just changed engine oil - if I use seafoam in the gas tank, am I required to change the engine oil again? I put expensive oil and matching oil filter.

No. You would only have to change the oil if you put SeaFoam or any other solvent in your oil. Normally you would only add solvent to your oil about 5 minutes (engine time) before you change the oil, but that is old school, most people do not add any solvents to their oil on modern vehicles.

I doubt that you have carbon build up on your pistons in a modern fuel injected engine. I would suggest that you might need a new PCV valve if you still have the original in there.

Most likely your oxygen sensor is getting old and is not as responsive as it used to be. That will cause HC emissions to rise above normal. In your vehicle, this is probably not monitored by the PCM but they usually have a port that you plug a voltmeter into to manually check its operation.

I assume those numbers mean that you’re failing because of a NOX problem; a.k.a. oxides of nitrogen.

You might consider having the EGR system checked.

“I doubt that you have carbon build up on your pistons in a modern fuel injected engine.”


The measurements above are for “NO”

PCV valve is also new
HC is normal range - just the NO is out of range for low speed

It looks like you are just slightly above the limit at 15 mph, 875 to 760. It must not be necessary to pass the 25 mpg limit for NO b/c you didn’t pass it in 2010 but you still passed the smog test then. So focus on the 15 mph NO.

I think both the N and O are coming from the intake air. Air is 70% nitrogen, 20% oxygen. Under high heat the two must can combine and form NO. NO is the chemical nitric oxide, and is an air pollutant. Seems like you have from among three problems

  • Carbon build up – this stuff forms hot spots which are considerably hotter than the rest of the cylinder and piston surfaces, so it could form NO in those spots.

  • EGR malfunction. The EGR is supposed to open enough to cool the cylinders for the main purpose of preventing nitrogen oxides from forming, like NO. If the EGR isn’t working properly, this could result.

  • Catalytic converter is on the fritz. The cat is sort of the back-up system to remove any remaining nitrogen oxides that come out the exhaust manifold and turn them into (I guess) N2 and O2 where they go out the tailpipe to live life anew. No harm, no foul.

So what would I do in this situation? hmmm … well, first off I’d want to make sure the ignition system is working 100%, so I’d install a new distributor cap, ignition rotor and spark plug wires, assuming that is how yours is configured, an electronic ignition using a mechanical distributor to move the high voltage around. I’d make sure the battery and alternator are in good condition, and I’d charge up the battery with a battery charger just before the test. A 10% decrease in voltage to the ignition system could result in a 20% less energetic jolt at the spark plug. I doubt that will solve this problem, but then you’ll be starting with a known good ignition system.

Next I’d test the EGR to make sure it is working at least minimally. On my Corolla, what I do is hook up a vacuum pump to it, apply some vacuum, and it should stall the engine at idle. Even if it does, I’d still remove it and take a closer look. Any signs of carbon deposits there, I’d replace it. And I’d clean out all the EGR ports – the holes through which the exhaust gas recirculates into the intake manifold.

Next I’d make sure the vacuum system that controls the EGR is working. That varies a lot from car to car so you’d have to look at your service manual. My Corolla has three tiny vacuum ports at various points in the throttle body which feed the EGR system, and the computer gets involved too. There’s also an EGR temperature sensor on the Corolla, so if you have one too, make sure that is working.

Finally I’d do what I could to insure the mixture isn’t too much on the lean side. Lean mixtures result in higher combustion temperatures. So check for small vacuum leaks, maybe remove & do a clean-up of the inside of the throttle body.

To remove unnecessary complications that might have unintended consequences, I’d revert to plain dino 10-30 oil, and a copper ngk plug, maybe the v-groove version for a little better igniting spark.

If all that doesn’t do the job, next up would be a new cat.

Those carbon deposits – if you actually have them enough to be causing this problem, which I’m dubious – are super hard and practically impossible to remove without disassembling the engine. I wouldn’t worry much about those. Best of luck.

Edit: I’m presuming the engine air filter is in good shape.

The first thing to do is check if there are any codes stored in the OBDI engine management system

When you have useful information, then you can decide in which direction to go.


@Tester Do we have a difference of opinion on what constitutes carbon build up? Before I retired from the Navy in 1990, I could not afford a new car, or even anything remotely newish, so thanks to the hobby shops, I rebuilt the engines in many a 60’s and early 70’s vintage cars and my definition of carbon build up was not what I sw in that video. Those pistons were clean.

@sciconf If you follow the EGR tubing, there is a tube going ot the exhaust manifold. These can clog up with carbon, reducing the exhaust gases that are being recirculated. Remove the EGR valve and spray down both tubes, one to the intake and one to the exhaust with a carburetor cleaner. This could solve your problem.

The pistons that you think are clean reflect the condition of the intake valves.


Yeah, had to clean a lot of junk off the valve stems too.


In regards to those reading from 2010 . . .

Those were the FIRST readings, and it failed

Upon retesting it, the numbers were within specs, and it passed. OP didn’t post the SECOND readings

That is how I understand things to be, based on what I read

But let’s forget about my interpretations for a minute

I perform smog tests at work, and I’m in California

If a car fails ANY part of the test, it fails

Tester has provided proper data for you to use here. Back when I was a kid…I used to play with “Water Injection” never saw such shiny pistons in my life…before or since. But there are many other ways of decarbonizing a piston…but why on earth would you be suffering a 1950’s engine malady on this fuel injected engine.

Like Tester said prior…you shouldn’t have this condition with modern fuel injection…which your car HAS and if the Fi system was to blame…you would never have passed a single smog test EVER.

God I LOVE “Mechanics” that spew things like this…the primary reason they say things like this is because they are clueless…the other reason is to confuse and obfuscate and give you something to chew on.


Do the pistons in that video have enough carbon to trigger higher NOx? It doesn’t look like it to me. As @Keith noted, they look clean to me.

Carbon buildup on the backside of the valves (non-combustion chamber side) should have no effect on NOx.

I would look to the EGR circuit and make sure that’s functioning correctly.

On a related note, has anyone used water in their past to remove carbon from pistons? E.G., pouring it down a running engine?

Water injection definitely will clean your pistons… Now you need to properly meter this water…and in the proper amount it works wonders. But dont let the thing inhale a ton of the stuff…small amounts over time… Say a half gal a day or less…slowly slowly over run time. The pistons will shine like a new nickle.

I dont think this OP has any issues with carbon on the pistons…He was just told something to keep him busy.

I doubt very, very seriously that any carbon on the tops of pistons has anything to do with failing an emissions test.
Maybe the EGR passages are clogged up for the most part.

Pistons can carbon up if the engine is running on the rich side of things and/or due to oil consumption. Water can certainly clean them but that doesn’t mean dump a quart down the intake and hope for the best.

@Tester - I got the part of recovering the codes. But your video link and the comments - unsure what you are saying.

@db4690 - so what are you saying should be the issue?

@GeorgeSanJose - yes @db4690 is correct - I did not post the values when it passed - it passed without any work done to the car.

So adding 2 cans of Seaform the full tank is not going to solve the issue?

If there’s carbon on the pistons, there’s carbon on the intake valves.

High NOx emissions is a result elevated combustion temperatures. This can be caused for a few reasons.

The EGR circuit isn’t functioning.

The engine is running hot.

The engine is running lean.

The problem isn’t with the catalytic converter because your vehicle has a two way catalytic converter. And two way catalytic converters don’t reduce NOx emissions The vehicle has to have a three way catalyst in order to reduce NOx emissions.

If there’s a lot of carbon on the intake valves, this carbon can act like a sponge and absorb some of gasoline that’s supposed enter the cylinders. So at low RPM’s when the injectors are providing less gasoline, and the carbon is absorbing some of the gasoline, it leans out the mixture to the cylinders. This then causes the combustion temperatures to increase, which then causes increased NOx emissions.

So if the high NOx emissions is caused by carbon deposits, the Seafoam will remove the carbon deposits.

And if you’re not already doing so, I would recommend that you use Top Tier gasoline from now on to prevent future carbon deposits.


@Tester - There has never been a check engine light ever - so the m/c said that OBD1 has nothing to give.

This car has FI but does not have an EGR

M/c wants to check the Oxygen sensor on their Oscilloscope and go further with diagnosis. He says when engine gets hotter it will give more NO!

I could try seafoam too - tank is full now - so I was told to add two cans.

I am already using ONLY Costco gas for years - so perhaps there is no carbon then. M/c thinks on this car the throttle body does not get dirty.