Loaned my daughter truck and it was running pretty good for 161K miles. Last week, check engine light came on and stayed on. Engine started to run rough and when pressing gas, would bog down even further. She took it to place and had diagnostics down. They said error code P0308 showed up, with potential causes of: spark plug, plug wire, coil, vacuum leak, injector fault or high or low fuel pressure. Said Cyclinder 8 was misfiring. I can check plug and maybe vacuum hose loose, but I don't know how to check coil, injector fault or high/low fuel pressure. Anyone able to provide some guidance would be appreciated. Not planning on driving truck, as I don't want to ruin it. Check engine light did blink a few times but basically stayed lit.

 

Sounds like a clogged cat to me.

That can take out the numbers 7 and 8 spark plugs pretty quick.

Unscrew the pre-cat O2 sensor and see what happens.

 

Some new plugs will probably give some immediate help, but like dude said, you got a clogged up cat and probably a blown plenum gasket. Is it using a lot of oil? Read the plenum sticky at the top of forum.

 

welcome to df.
replace the spark plug.
inspect the plug wire, replace if bad.
consider replacing cap and rotor if they're old.

you can test for clogged cat by temporarily removing the front O2 to relieve pressure. you'll get a new CEL for no O2, but if it runs any better at all, it indicates a cat problem. but - if your misfire is #8 only, i doubt that it's your cat, as that would affect all cylinders.

you can swap injectors between #8 and another cylinder to see if the misfire moves. this would confirm the injector.

if a new spark plug and cap/rotor didn't fix it, i'd run a compression test to see if something serious has developed, like a cracked head or head gasket. check for loss of coolant, check for coolant in oil.

 

what is clogged CAT and where is it located? What is blown plenum gasket and where is it located? I am not technically sound so need some simple guidance please. How do you check front o2 sensor and where is it located? not losing radiator fluid, but is using a little oil, not yet excessive (1 qt in a week). Do smell gas fumes. Not smoking. Test showed cyl 8 misfiring. tks

 

Certainly sounds like blown plenum gasket to me: https://dodgeforum.com/forum/2nd-gen...um-thread.html

The gasket being shot can also lead to the cat being clogged (catalytic converter). It's probably been bad for a while now. Using a quart of oil every week is pretty excessive...

 

CAT- catalytic converter. Blown plunum- the bottom of th intake manifold and gasket goes bad, allowing oil into the intake. Results in excessive oil use (1qt a week is definitely excessive in my book), excess oil in the intake results in the cat becoming clogged with all the excess oil being burned in the combustion chamber. The O2 sensor is the oxygen sensor installed in the exhaust pipe just in front of the cat. The cat looks like a muffler with a big heat shield on it and is mounted at the front of the exhaust close to the engine. The O2 sensor screws into the exhaust pipe and can be unscrewed to allow freer breathing of the engine. Unscrew the O2 sensor, disconnect it or hang it out of the way. Start the truck And see if it runs better. If so, your cat is clogged. Read the plenum sticky mentioned above.

 

tks to all that posted for me. I appreciate it. One last question....can anyone tell me what it might cost me to get the plenum gasket replaced? Ballpark is fine. I know CAT is between $150 to $250 or even more for part only. The plenum gasket is around $15 at NAPA. Not sure about 02 sensor.

 

I bought my cat on Amazon for like 70 bucks and had it installed locally. When I had my plenum replaced it was $562 at a local shop.