In our previous article, we discussed carbon build-up in gasoline direct injection (GDI) engines and why it happens. In this article we will discuss best practices for identifying and diagnosing carbon build-up in GDI engines. Carbon builds up in engines over time as part of the combustion process and this starts to cause problems such as misfiring, clogging, incomplete combustion, engine warning lights, reduced / sluggish performance, higher fuel consumption, emission test failures and drivability faults. Carbon build up occurs in port-injected engines and is especially prominent in gasoline direct-injection (GDI) engines.

Carbon build up along with engine drivability concerns are long term effects and compound problems of PCV failure, low tolerance piston ring blow by, & leaking valve guide seals. This combination is a recipe for disaster.

The valve shown right is from a Mercedes-Benz with just about 65,000 miles. This carbon build-up will most likely go unnoticed until drivability symptoms surface.

Mercedes-Benz carbon build-up

Early signs of carbon build up are sometimes difficult to spot, but there are signs. Most vehicles will begin to exhibit a rough idle when cold, slight shake at idle and sometimes an extended crank. Usually clearing up after 5 – 10 seconds. All of these symptoms usually remedy once the engine reaches operating temperature. In other words, once warm, you will not notice the issue. Keep in mind, it doesn’t take a lot of carbon build-up for these symptoms to begin to appear.

As the build-up worsens or increases, you will begin to notice the earlier minor cold idle issues are now more prominent. They will evolve into a consistent hard start when cold. You will notice a rough idle and possibly have a malfunction indicator lamp (MIL) illuminate with a P0300 random misfire stored, or multiple cylinder misfire due to incomplete combustion.

The engine will continue to develop carbon build-up and with that build-up will bring more symptoms. A hesitation when accelerating, some describe this as a sag when the throttle is depressed. Not like the familiar tip-in hesitation, this sag is different in feel. On turbocharged vehicles, the engine may not reach an appropriate RPM with enough volumetric efficiency to spool the turbocharger. For example, you’d expect to see the turbocharger spool up around 1400 – 2800 RPM. And instead it will not provide the expected boost or spool in this range. In extreme cases, you can see mass air flow faults, however with these fault codes come all other symptoms, so we are usually checking for build-up before chasing the mass air flow fault codes.

We’ve laid down the symptoms, but now what, how do we begin our testing? This is where old fashion seat of your pants comes into play, does it act like or feel like carbon build-up? Then it probably is.

The trouble is, mass air flow readings will be mostly normal, which in turn says (fuel) trim numbers will mostly be normal. You can use advanced techniques, like an oscilloscope, monitor MAP or boost sensor at idle and compare the cyclic variations (air pulses moving around). Be sure to compare the same cylinder to itself during multiple combustion events.


The waveform shown below is a tailpipe pressure reading, showing combustion efficiency issues caused by carbon deposits.
This is before a carbon cleaning.

Waveform showing carbon build-up

The waveform shown below is a tailpipe pressure reading, showing combustion efficiency restored.
This is after a carbon cleaning.

Waveform showing confirmation of repair after carbon cleaning.

Still though, the easiest way to confirm it is carbon, is to LOOK. Use a borescope, remove the spark plug, and with the intake valve at the lowest point of its travel, look for carbon. If you don’t have a borescope, you can grab yourself the ATS borescope, but if you don’t have the budget, there are phone app borescopes available with lesser quality cameras and options, but they work just well enough.

The ATS iC – INSPECTION CAMERA is a medical grade video camera that delivers superior images on your Windows pc, tablet, or Android phone display.

Simple USB-C borescope. Resolution is not the best. But can record video and photos. Does a decent job of showing carbon build-up.

Once diagnosed, you can remove the carbon build-up from GDI engines without disassembling the engine, using the ATS 3C Intelligent Induction Cleaner. As well as prevent future build-up with preventive cleanings every 30,000 miles. Questions, comments? Ask away, we would love to chat carbon with you.

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