What would cause heavy carbon buildup?

1990 RENIX with 111k miles, 4.0

I seem to have a problem with heavy carbon buildup on my valves or piston tops. I have had a ticking that drove me crazy for the last two years, finally cured it with two cans of Mopar Combustion Chamber Cleaner down the intake for the last 3k, now it is slowly creeping back.
I'm assuming theres heavy carbon buildup on the piston tops or the valves causing some sticking. Every time I run a can of combustion chamber cleaner through it, everything is quiet for a good while then slowly comes back.
What would cause this recurring problem of carbon buildup?
I have checked all the plugs and they all show even wear and greyish tan coloration.
New T-stat around a year ago, it runs at 210 all the time.
The mileage is still good, plenty of power. I recently did a long highway trip and flogged it hard for 13 hours straight, it didn't burn a drop of oil and returned good mileage even though I had a 450lb load in the back.

What is causing this?!

Do you drive with a very light foot, and never step on the gas?
There is a saying "blowing the carbon out"

If use of the combustion chamber cleaner reduced the noise, then yes the problem is probably carbon related.

You've done what you can. Chemicals may not be enough for a complete remedy. Barring tearing down the engine to physically remove the carbon, this may become more of a "characteristic" of your engine rather than a problem.

Is the ticking related to the amount of gas you are giving it, or is it a constant problem?

I had a ticking, and it turned out to be a cracked exhaust header.

I drive with a heavy foot now, but babied it for the last few years. Also, I bought the jeep from the dealer, previously owned by an older guy who lived on an island, hence the low mileage, and not really any freeway driving.

Assuming everything else is in decent working order, the EGR valve is known for causing trouble. It shuttles exhaust gasses back into the intake stream (the theory borders on sound, but there are better ways to do it. And, it's not really necessary on our low-compression engines...) in an effort to "dampen the fire" and reduce NOx (which doesn't work for us - it merely increases HC and CO.)

Easiest way to clear carbon? Load a spray bottle with clean water.
Disconnect the exhaust in front of the catalytic converter. Tie the cat out of the way with some wire.
Remove the rubber tube from the throttle body.
Have a helper hold a middlin-high idle (~1100rpm or so,) and shoot a spray of water - on the finest mist you can manage - about once every 15-20 seconds until you stop seeing black clouds come out of your exhaust. These black clouds are the carbon that used to be deposited in your engine, and they're why you tied the cat out of the way.

Shut down engine.
Let cool.
Reattach cat, use new gasket.
Reattach intake hose.

Return to service.

This is probably the best way I've found to handle carbon deposits in your engine - you're literally steam-cleaning them off of the internal surfaces. Doesn't do much about the EGR crud crapping up the intake, I'm afraid - but it does clean out the chambers rather well

Just some questions to throw out there. With the RENIX, you have 2 temp sensors. Your gage could show good (hot), but your computer sensor could still think it's cold and make the mixture too rich. If the exhaust is leaking (making a tick) does the 02 sensor make it run richer? Too rich a mixture, worn rings, or low compression should be the only things that would cause carbon build up.

What grade of gas do you run?

scottmcneal said:

What grade of gas do you run?

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good call throw some premium in it every so often and that will clean alot of the crap out

Sherman_thetank said:

good call throw some premium in it every so often and that will clean alot of the crap out

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I don't run premium unless there is no mid grade available,... it do not help the 4.0L. However, I find that most stations low grade causes engine ping and reduce performanance. Try to get the carbon out as per 5-90 then shop around your neighbourhood for gas that make the 4.0 happier.

clunk said:

I drive with a heavy foot now, but babied it for the last few years. Also, I bought the jeep from the dealer, previously owned by an older guy who lived on an island, hence the low mileage, and not really any freeway driving.

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My guess is that your carbon problem is a result of buying a vehicle that was not broke in properly. To fix it, you will likely need to rebuild it, with focus on having the cylinders honed, not buffed.

The fact that your plugs look normal and it's not burning oil tells me that there is nothing catastophic wrong with the engine.
Unless what it's doing right now is causing you sleepless nights, I'd just continue to drive it and perform regular maintenance per the owners manual.

After decarbonizing your engine, run this stuff on a regular basis. You only have to run about an ounce for every fill up, and it maintains a clean engine and fuel system. I use it in my jeeps, and it works. http://cgi.ebay.com/ebaymotors/Yama...mQQcategoryZ124102QQihZ021QQitemZ310120073467

PS, you can get it at any Yamaha outboard dealership.

Cheap gas like ARCO will cause this..Sorry but i have seen it..I have found that running fuel like chevron And texaco has helped mine alot..My mpg went up a little, but it does run better...Ping and knock is gone...
What i found to be funny is that my diesel motorhome does better on ARCO diesel fuel..Went from 10 MPG to 12 MPG...Also burns cleaner...