View Full Version : Thoughts, opinions, and suggestions on carbon deposits.

01-06-04, 04:08 AM
Disclaimer #1: Some of the procedures in this post require a delicate touch. Some of them also entail using chemicals that, if not used properly, can damage your catalytic converter, your oxygen sensors, or even cause severe and catastrophic damage to the motor. All suggestions are to be followed "at your own risk"... and they are given with the warning that they should only be applied and/or used by professionals. Flamable chemicals can easily be ignited by hot surfaces, and fire can burn you. Be careful, and don't hesitate to seek professional help.

Disclaimer #2: One of the procedures/suggestions in this post requires the disabling of all the oxygen sensors and the removal of the intake air duct. A proper OBD2 scan tool will be necessary to clear any trouble codes caused by this. Removing battery terminals for a certain amount of time may (or may not) clear codes... but will also erase any learned data in the vehicle's computer, causing reduced drivability and/or gas mileage for a short amount of time.

Disclaimer #3: I do not know everything. If anyone who reads this post has additional information or even corrections to offer, then by all means please do so. The advantage of a community of people with shared interests is that knowledge is freely passed from person-to-person. Whether you're a technician or just a do-it-yourselfer, opinions and suggestions are valuable... and I value any and all input to my opinions and suggestions.


I think we can all agree that carbon build-up, no matter where it may be, is somewhat of a problem for an internal combustion engine. When it collects inside of a late-1960's carburated big block, with intake valves the size of a half-dollar coin, the affect of it isn't nearly as bad as it would be on a late model, high-tech, port fuel injected 32 valve engine like the Northstar. (the exception here being crankcase sludge.)

Carbon takes on a few different forms. It can be a slimy, oily film that coats a flat surface. It can be a dry, powdery dust that prevents a spark plug from firing. It can be a tiny particle, almost too small to see, that either holds open an EGR valve, clogs an oil pump pickup tube, or collects on the tip of a port fuel injector, interfering with proper fuel atomization. It can also be a rock-hard collection of tiny pebble-sized chunks that collect in mounds on the backside of an intake valve.

I think it's safe to assume that some of the mechanically inclined here (or at least those gainfully employed in the automotive profession), have at one time or another used various 'home-brewed' techniques to rid a motor of excess carbon. Does anyone remember getting a motor really hot, then pouring a huge amount of automatic transmission fluid into the primary venturis of the carburator? Personally, I used to alternate between trans fluid, reheating the engine (super hot), tap water, reheating the engine, and repeating that for at least a gallon of each type of fluid. I know that sounds drastic... but it works. If the motor has an oxygen sensor, or a catalytic converter... this can easily cause more trouble than it cures.

As some of you (us) already know... getting rid of carbon from _wherever_ it may be is not too dificult. Injector tips clogged with carbon, or intake valves coated with pebble-sized chunks? ... have them properly serviced and cleaned for just over $100. Piston rings saturated with build-up? ... either waste a few cans of Top Engine Cleaner and risk damage to the exhaust system, or use one of the new tool kits available to dissolve the carbon in the ring grooves and oil control rings. Do you think the combustion chamber and tops of your pistons are coated with an 1/8" layer of carbon? ... dump a quart of water down the throttle body after super-heating the motor, and while it's revved to about 3000 RPM. (Top Engine Cleaner would work for this, too)

... See? It's easy.

Here's some bad news: Carbon is a common and unavoidable by-product of many chemical reactions and processes that take place inside of our automobile's engines. You can't magically prevent it's appearance, as even the suggestions I listed above to deal with it won't do that. The best hope is to manage it, and prevent it from accumulating to an extreme.

Here's a little good news: It's really not that difficult to deal with. We can effectively remove carbon deposits, no matter where they are. We can even do that and at the same time not cause damage to our ignition, emission control, or exhaust systems. We can also modify some less-than-desireable driving and maintenance habits which will actually cause our motors to produce less of it.

Let's look at some situations that are common to all port fuel injected engines, and one that is specific to the Northstar... the causes for those situations, and some common-sense (and usually inexpensive) methods to deal with them.

~~ Engine crankcase sludge. This one actually has two parts. The first one is called soft sludge... and it's the thick, gooey substance that gets produced by mixing old, broken-down oil with heat, hydrocarbons (unburnt fuel), and moisture. When motor oil isn't maintained properly, it becomes something that isn't motor oil. Instead of having the ability to lubricate and stick to metal parts, it becomes contaminated with acids and other chemicals that... you guessed it... turn oil into sludge. The second kind of sludge is the result of not taking care of the first kind in a timely manner. When the soft, gooey sludge becomes much harder, and tends to accumulate in clumps... or settles into crevices and recesses... it becomes 'hard' sludge, which is actually mostly carbon. Here's where it becomes dangerous. This hard sludge can easily break away and drop to the bottom of the oil pan, where it gets sucked up by the oil pump and embedded in the mesh screen in the oil pump pickup assembly. I won't go into the results possible when a motor is starved of oil.

A motor contaminated with soft sludge can be cleaned safely by changing the oil every 2000 miles, but it takes time (many thousands of miles). Also deserving a note here is the danger of using any type of 'motor flush' on an engine with a moderate or severe sludge problem. If the flush works well, a huge amount of crud can be dumped into the oil pan... where it may easily clog the oil pump pickup screen. If you decide to flush a crankcase in order to remove soft sludge, make sure to properly evacuate the oil pan and wash clean solvent (or motor oil) through it _before_ starting the motor.

Preventing crankcase sludge is ultimately easy: Change your oil regularly. Make sure your motor reaches operating temperature each and every time it is started. Never run a motor without a properly working thermostat in the cooling system.

Changing your oil regularly will ensure that the motor oil is really motor oil. I know that sounds strange, but remember that old oil breaks-down into something that isn't really oil, and contributes to the formation of sludge.

When a motor is consistently driven for very short distances, and not allowed to reach operating temperature... the moisture that normally condenses on internal surfaces during heating and cooling will never fully evaporate from the crankcase. Motor oil that is constantly exposed to moisture (never evaporating) will break-down prematurely and allow accumulated acids to attack the surfaces in the crankcase. This is especially dangerous in an engine with aluminum components.

A motor running with no thermostat, or the wrong temperature thermostat, or a stuck-open thermostat, will be able to drive extremely long distances without reaching true operating temperature. See the previous paragraph for an explanation of why this isn't good.

FYI... normal operating temperature (coolant) for all GM V-8 engines is between 225 and 235 degrees fahrenheit. An easy way to see what your PCM or VCM thinks is normal operating temperature is to watch the temperature (on a scan tool) at which the electric fan(s) turn on.

Continued in next post due to size limitation...

01-06-04, 04:09 AM
... continuation ...

~~ Carbon deposits in a fuel system. The only place where carbon forms in a fuel system is on the tip of a port fuel injector. The reason for this is due to the close proximity between the tip of the injector and the intake valve. When an intake valve opens, there is a small amount of hot exhaust gases that are 'scavenged' into the area near the injector tip. This is due to something called valve overlap, and it's designed into the camshaft lobe profile. This 'overlap' is for emission control, and performs the same basic function as an EGR valve (which can be the subject of another thread entirely). Throttle body injectors have a much bigger tip, and hence a much bigger hole for fuel to spray out from. This larger hole has the advantage of not clogging due to it's size, and also because it's located very far from the intake valve. That's right folks... if you've ever been sold a fuel injection cleaning on a throttle body injected vehicle, then you were scammed. Any contaminants in a fuel rail or fuel lines can only be grit, sand, or dirt that made it past the fuel filter. I'm not aware of any solvent that will reliably dissolve this... and someone please correct me if I'm wrong.
Preventing carbon build-up on a port fuel injector tip is practically impossible. It can, however, be minimized by using a quality fuel that has high detergent additives. While all the major fuel producers use detergents, the best of them are Mobil and Chevron. Amoco is good, but not as good since being purchased by British Petroleum back in 1998. Also a good idea is the occasional addition of a fuel injector cleaer into a full tank of gas.

If we can picture a port fuel injector spraying fuel... it's very much like a can of spray paint. When the nozzle is clean, the paint (fuel) is atomized into a very fine mist with no oversized droplets of liquid. You don't want liquid paint coming out of the can... and you also wouldn't want liquid fuel coming out of an injector. Liquid fuel will not burn, but instead will be forced out of the cylinder into the exhaust. All this means is that a clogged or dirty fuel injector will cause you to lose power, efficiency, and waste fuel.

While using a good fuel with detergent properties is a wonderful way to slow the build-up of carbon on an injector... it really isn't a great way to clean one that is already dirty (or even clogged). A good idea is to have the injectors professionally cleaned, but be careful as there are many poor quality services out there. Anyone who tells you that a can of cleaner introduced into the intake manifold will clean an injector is lying to you. You should find a service that uses a pressurized solution that is foced into the fuel rail, being substituted for the vehicle's own fuel supply. The three great products that I'm aware of are 'Carbon Clean', 'BG', and the Snap On injector cleaning machine. They each use a proprietary solvent that works wonders for dissolving carbon. They are all O2 sensor safe, and harmless to catalytic converters when used properly. Unfortunately, they should all be used just prior to changing the spark plugs.

~~ Carbon particles and debris in an EGR system. Okay, we all know that an EGR valve is designed to introduce a small amount of hot exhaust gas into the intake manifold. The how and why part is for another thread. We're concerned with carbon getting in there and causing either the passages to clog or the valve to stick. On the Northstar motor, this is especially a problem due to the design of the system. While it works very well, the layout and diameter of the passages gives carbon plenty of chance to accumulate.

Unfortunately, there isn't much in the way of prevention here. Always letting the engine reach operating temperature offers some protection, but the best medicine here is routine maintenance. Having the intake system (throttle body and intake plenum) cleaned every now-and-then is okay... but even better is the specific tool that GM has designed to clean the EGR system. This tool is part of the carbon removal kit that I made mention of before, in another thread.

~~ Combustion chamber and piston surface deposits. The burning of fuel and air causes carbon to accumulate... period. We can't stop that. The design of the Northstar motor, and the fact that it's a free-breather with four valves per cylinder, means that there will be much less carbon in the top of the cylinder (combustion chamber, piston surface) than would be present on another motor without that technology benefit.

Here, again, all that is necessary is some basic routine maintenance. Good fuel and occasional injection cleaning are all that is required to prevent an excess of build-up. To remove build-up that is already there, I have three options that all work wonderfully. One is expensive, one isn't expensive, and one is free.

1. GM Top Engine Cleaner applied in buckets through the intake manifold, and allowed to soak the intake manifold and entire top cylinder for all cylinders. This works great, but can (if not used properly) damage the oxygen sensors and exhaust catalyst, along with causing the spark plugs to become fouled, requiring replacement. This is the 'not expensive' option... and is also the one that carries a danger of fire, engine damage, and/or physical harm to humans. (has anyone ever seen a motor filled with top engine cleaner backfire through the intake manifold? not a pretty sight, I can assure you.)

2. Cadillac's model-specific top-engine cleaning procedure and special tool kit. This service is available at any Cadillac dealer, and has a couple of TSB's associated with it. It utilizes an adapter for EGR system cleaning, and also effectively dissolves all the carbon in each of the ring grooves on all the pistons. There is no need for spark plug replacement. There is no danger to the catalytic converter or oxygen sensors (assuming professional and proper useage is adhered to). When professionally administered, there is no risk of engine damage, fire, or physical harm. Unfortunately, this is the expensive option (around $350, last I checked).

3. Removal of carbon in the top cylinder area by steam cleaning. Yes, I said steam. This also is good for cleaning the back of all the intake valves, if done properly. Run the engine at 3000 RPM until it's hot. Stop the engine. Disable all of the oxygen sensors (simply unplugging the harness connectors is fine). Remove the intake air duct from the throttle body. Restart the motor. Run the motor at 3000 RPM for 2 minutes, it may not run smoothly due to a lack of MAF and O2 sensor signal input... this is normal. Hold the throttle open so that the engine speed is lowered to about 1500 RPM. Slowly pour 16 ounces of regular tap water into the throttle body (both sides and evenly), and at the same time raise the engine speed to keep the motor from stalling. If a stall seems imminent, slow the flow of water to a trickle until it regains speed. After 16 ounces is added, re-heat the engine by running it at 3000 RPM for 2 minutes. Repeat this at least three times. At no time should you let the motor see anything over 3000 RPM. Re-install the air duct on the throttle body. Re-connect all the O2 sensors. Use a scan tool to clear any codes set. If you wish, go borrow a borescope and take a look at your bright, shiny, combustion chambers and clean intake valves.

The reason for disabling the O2 sensors is simple. They are all heated sensors that have heating elements inside of them. These heaters can reach 700 degrees (f), and will break if exposed to massive amounts of water.

If at any time the motor stalls, you'll need to watch-out for an accumulation of water in any of the cylinders. Do not crank the motor, but instead turn it over with a wrench (or socket) on the front crank pulley. This will evacuate any water left in the cylinders and prevent damage due to hydrolock (water doesn't compress).

Well... it's getting late, and I have work tomorrow. I'm sure that I've left some things out of this soapbox, ranting post. I look forward to any input or questions that anyone may wish to offer.


01-06-04, 08:33 PM
Are you serious? There is no reason to do any of this!

01-06-04, 11:09 PM
Are you serious?
Who, me? I suppose so.

There is no reason to do any of this!
Any of what? ... change driving habits to lessen the accumulation of carbon build-up? ... remove carbon from wherever it may be in your engine?

Or are you saying that it's perfectly fine to drive with clogged injectors, saturated piston ring grooves, and stuck EGR valves?

Anyway, I made mention of posting some detailed info on carbon and dealing with it... and I even included some Northstar-specific info.

Take it or leave it, the choice is yours. Personally, I enjoy reading my own posts, so no matter to me. :bouncy:

Thanks for the input?


04-22-04, 01:11 PM
So informative I thought I would revive it from the dead... Anybody have any before/after pics of the 'steam cleaning' technique? I've done this on my 89 camaro once, but never checked the difference.....

04-23-04, 02:39 PM

We use to do some of this back in the 50's (ATF, water injection, etc.).
Ever wonder why they run so good when it's raining? lol!

If nothing else, thanks for the obvious time it took to post this article.

04-25-04, 03:29 PM
wow thanks, very informative.

I just bought a 97 Eldo with 68k on it, it doesn't seem to have any power loss, it was a south fla. car, so a good amount of highway driving i assume one owner, women, plus its hot down there, do you think it would be necessary to do any of this before the 100k tune up? And if I were going to do anything would u recommend the Cadillac sepcific top engine cleaner?

Also the car has aboslutely no leaks whats so ever, what do you think the first part to go will be considering 68k, I've bearings soon, water pump also, what are your 2 cents?

And one more thing I have usually added the product called "engine restore" to my cars when I get them you know the stuff in the silver can, A) is it worth anything and B) would you add it to a northstar?


04-25-04, 10:16 PM
wow thanks, very informative.

I just bought a 97 Eldo with 68k on it, it doesn't seem to have any power loss, it was a south fla. car, so a good amount of highway driving i assume one owner, women, plus its hot down there, do you think it would be necessary to do any of this before the 100k tune up? And if I were going to do anything would u recommend the Cadillac sepcific top engine cleaner?

Also the car has aboslutely no leaks whats so ever, what do you think the first part to go will be considering 68k, I've bearings soon, water pump also, what are your 2 cents?

And one more thing I have usually added the product called "engine restore" to my cars when I get them you know the stuff in the silver can, A) is it worth anything and B) would you add it to a northstar?


I wouldn't expect anything to "go" soon. With the first tuneup interval at 100K just leave it alone and drive it. The Northstar likes a frequent dose of WOT to cleanout the carbon so lean on it hard and frequently. If you do a WOT accel to 60 and see a "cloud" in the rear view mirror then the engine/exhaust is not being kept cleaned out. Keep doing those WOTs until you see no cloud of particulates/carbon/etc... Change the coolant. Drain and refill with fresh 50/50 DexCool/distilled water. The DexCool is good for corrosion protection for 100K/5 years so yours is due for fresh coolant. Don't flush just drain and refilll with some fresh coolant. Change the oil according to the oil life monitor and FORGET THE RESTORE. That stuff is absolute trash and is not even good for lubricating door hinges. You are doing nothing by buying restore except making the persons marketing it rich. FORGET it. You know, somehow the engineers that designed/developed/validated the Northstar engine managed to do it with plain old fresh oil.....no Restore/Slick50/STP/or any other snake oil. They are all trash and are useless. A waste of money and they can lead to severe ring belt deposits. That is how they demonstrate some short term "improvement" in an old engine...they seal it up with a high viscosity additive....that forms ring land deposits and sticks the rings leading to failure. Forget them.