Engine light codes 0302 & 0174
 

I recently purchased a 2004 WJ 4.7L last week from a mechanic who said he was selling it for a customer who no longer needs it...after about the first 60 or 70 miles the engine light came on...after borrowing a code reader these were the 2 codes that came up...when I brought it back to the mechanic...he said it was probably just a bad aftermarket coil ( said he replaced the 1 & 2 coils because both of those codes came on for him )...but after looking into it some more he decided that the coil was probably good and that he thought maybe there is Carbon buildup on the valves so he decided to fog the intake system...I've only driven it about 70 miles since then so hopefully tomorrow the light stays off....any other ideas would be greatly appreciated

Welcome to CF!

There is an FSB (factory service bulletin) about misfire caused by carbon buildup and what to do about it. It seems it's caused by valves that don't rotate from carbon on the stems and so don't clean themselves of carbon on the seats. This seems to appear in engines that don't generally operate above 2500 rpm. That's normal driving to me. I guess you need to wind it up once in a whole to keep it clean.

There is a Mopar cleaner designed for this, but it seems to me SeaFoam would do the same thing. You just get the engine up to operating temp and pour it down the throttle body so that the engine stalls, then wait 3 hours, start it up and blow all the crap out of it. I would assume the Mopar product is cat safe and breaks up the carbon fine enough to not plug the cat.

My GRANDFATHER used to do this with water, or kerosene if it was bad. The theory is the stuff soaks into the carbon, then boils on startup to break up the carbon. That was before we had cats to worry about. I can remember him with the rear end of his 55 Mercury pointed out over the seawall in Boston and fogging out the bay just by charging it before it stalled. He'd rev it up and slowly pour it down the carburetor until it stalled. He wouldn't wait 3 hours, though. Kerosene was the worst! It put out white smoke so thick you couldn't see thru it.

Just a memory from my childhood.

I've driven it for a few days now and the light has stayed off...hopefully that was the problem...my mechanic said i should buy the seaform and do it again inabout a month...i still have a rough idle when its cold...any thoughts on that

Possibly a dirty Idle Air Control (IAC). Take it off the throttle body and clean it with Brak-leen or injector cleaner. The IAC motor controls idle air bleed to stabilize idle mixture. Don't loose the seal where it mounts. I think the mounting screws are Torx. Also, spray the ports on the throttle body.

Thanks...I'll try that tomorrow

Well i cleaned the IAC a few days ago and it seemed to help but when i started it this morning..30 degrees....it had a really rough idle again...when i cleaned the IAC there was a lotof carbon buildup on both the IAC and the intake....icleaned them both very well...i even used Q tips inside the intake...it only seems to happen when its cold...is there something else i should try or should I just spend the money and buy a new one and see if that works?

The only other thing I can think of it the IAT (intake air temperature) sensor. That's a variable resistor that changes value with temp. There is a complete list of ohms values at different temps on page 6-15 of the Haynes repair manual, or a couple of examples are 29,330 to 35,990 at 32*F, or 11,370 to 13,610 at 68*F. It is supplied with 5 volts so you can check that as well. This signal lets the PCM know to richen the mixture at low temps and lean it out when it warms up.

Went to the dealer today to buy the IAT sensor but they have to order it...been having another issue lately...i was driving the other day and the " check gauges " light came on and my oil pressure cage was at zero...pulled over but engine seemed fine so i drove it home...used my other car for a few days and when i went to start my GC today...the gauge worked...bought a new switch today but when i went to put it in...i couldnt get the connector off...then i realized I didn't have a deep 27mm socket anyway...so tomorrow I'll go get a socket and try it all over again...wish me luck..it seems like i always need it

Go to the junkyard and get a couple. Test their resistance value and put one in. See if it helps. If not, you're not out any cash except the couple of dollars you spent at the yard. I sometimes pick up something like that when I'm looking for something else and just pocket it. But that's me. When I remove my part, I generally take the bolts also.