Questions and Answers

Eclipse Carbon Buildup? Q. Hi. I hope you can help me with my problem. I have a 1996 Eclipse RS with 60,900 miles on it. It is a 2.0 liter. I had my head gasket replaced about a month ago, and since the day I brought it back from the mechanic, a bizarre problem has occurred.

Carbon keeps getting caught in my system, so the mechanic says, causing the car to buck wildly at idle (stopping at lights, etc.) - the idle jumps like crazy and the car smells of exhaust and fuel - really struggles to run. Often once shut off, the car is hard to start when this problem creeps up. Sometimes I can drive the car really hard and the problem goes away. The check engine light ended up coming on, and I replaced the EGR valve.

Well, here I am - the problem occurs about once every month (was happening once every couple of days.) I have taken it to the dealership and they say they cannot find anything wrong with it. Could it be my idle speed sensor? Or is it really carbon getting lodged in my system from the car running so rich before the gasket fix? Any advice would be appreciated.

Suzie

A. This sounds like kind of an odd problem. If the problem is indeed carbon, as the mechanic says, and after driving it hard it runs better, that would tend to support that conclusion.

One problem with fuel injection is that because the injectors spray the fuel at the intake port, carbon builds up on the intake valve. The poorer the quality gas, the more carbon tends to build up. Now what happens when a sufficient amount of carbon builds up, it acts like a sponge, absorbing the fuel until it is holding all it can. Since some of the fuel that's supposed to be going into the engine is being absorbed, the computer tries to compensate by richening the mixture.

Now what happens is that this carbon has all the fuel it can hold, all of a sudden it reaches a point where it just dumps it all back into the engine. Now the engine has this big blob of fuel it can't deal with, the computer sees all this extra fuel and leans the mixture to compensate and now you have all kinds of drivability problems such as you describe.

There are additives you can put into the gas to help break up the carbon and clean the valves, but if the problem is as bad as yours seems to be, the best solution would be cleaning the valves by hand. This can be done by removing the head and valves and sand blasting them clean or by using a walnut shell blaster.

The walnut shell blaster can be used with the head still on the car and removing the intake manifold to gain access to the valves. As the name implies, the walnut shell blaster uses crushed walnut shells as the abrasive to clean the carbon off. The crushed walnut shells will not damage the engine if it is not completely cleaned out like a glass bead or sand abrasive.

Of course I'm assuming the cylinder head was resurfaced and the proper head gasket was installed. Also the timing belt was installed properly. Personally, when I do a head gasket I always look at the valves and if they have a heavy load of carbon, that's when I clean them.

Additional Information provided courtesy of ALLDATA