Okay my 95 Neon has been throwing a O2 sensor and multiple misfire sensor code. I replaced the O2 sensor but the multiple misfire code remains. I did some reading and basically it could be ALOT of things. Wires, fuel pressure, bad injectors, plugs, cam position sensor, yadda and yadda and yadda. So I pulled my plugs and they are black with carbon. I pulled my injector rail and the tips of the injectors are black with carbon. It's been smelling like it's running really rich so I'm not to surprised by the carbon build-up. I'm going to start checking all the possible causes on Sunday but I'd like to clean up the carbon build-up also. I've read about people misting water into the intake, so I guess that's a possibility. I've also seen seafoam mentioned a lot. So how would you go about cleaning up the carbon deposits? If I used seafoam, which product and how do you use it? Is it just a add to gas and drive type thing? I would really like something that isn't a "over the next several tanks of fuel" type of thing. I picked up a newer fuel rail for the Neon with a fuel pressure damper on it, so I'm going to swap that in also.

Thanks!

'95 = OBD2?
If so, and if you have a scanner that shows live data, you can look at all the values of your various sensors. One that would cause it to run rich would be a bad temp sensor. Not only will it run rich, but the ECU will never go into closed loop mode if it thinks the engine is still cold. A scanner can tell ya if it's gone into closed loop mode or not as well.

If everything seems ok there, I'd be looking at the EGR system. Assuming it has one. Its gonna screw with your air/fuel ratio, which can cause your misfires, if it's not working right.

Either way, I'd worry about finding the source of the problem before getting rid of the carbon.

When it comes time to do that, I'd suggest pouring a rablespoon or two of seafoam down each spark plug hole, crank it over briefly with the spark plugs out (and a towel or something over the holes to catch the mess) to push it into every nook and cranny in there, let it sit over night then take it for a good hard drive on the highway for half an hour. That should be enough to get rid of the worst of it. You can put some more seafoam in the gas tank to continue cleaning things out more slowly.

I have a scanner, although I'm not sure if it does live data. It came with a CD and a usb cable, but I'll grab the instruction booklet and check into those features. Yes, OBDII. I picked up a working EGR valve when I picked up the newer fuel rail with the damper on it. I figured I was there at the pick n pull for someone else so I'd grab a few "possible" bad parts for me. The guy at the front desk of the one I go to owned a Fiero and just recently had the engine blow so it's out in the yard being picked over. Anyways we talked about Fieros several times when I went there and bought parts for mine. Now he gives me really good prices on stuff, usually misc items priced at about $5.00 a piece. For instance I picked up 2 unbroken front turn signal/driving lights for the Neon, a aluminum AC hose for a Accord, the fuel rail and injectors for the Neon, EGR valve for the Neon and a very good condition door panel for a 02 Mazda Protege5 all for $30.00.

I'll check the temp sensor. Would that effect the temp gauge? Because that appears to be working fine. And yes I plan to figure out the reason for the multiple misfire first before cleaning up the carbon but I want to know what I should do to clean it up as I'm sure it's pretty bad judging by the plugs and injectors.

Do I buy the seafoam tank additive and use that in the cylinders? I'd like to get this problem cleared up so I can get it inspected and registered again. School is about to start and a second car is really helpful while my kids are in school. Still, I don't want to sink to much into it as I'm planning to put it up for sale around Febuary and find myself a 2nd gen Eclipse or Talon (non turbo, so oddly enough it will have basically the same engine as the Neon just with the intake and exhaust reversed on the head) or a VW Corrado.

[This message has been edited by Khw (edited 08-11-2013).]