I am working on my parents truck. They had the CEL come on, the code is p0300 random misfire. The cylinders that are misfiring are #1, 4, and 6. I just finished doing a complete tuneup minus new o2 sensors.. All new Ngk plugs, Bosch wires, Gold tip distributor cap and rotor, changed the oil and filter, air filter, swapped out the brake and power steering fluid, cleaned the throttle body and put seafoam through the brake booster line, changed both PCV's. The thing is just used seldomly so I thought I would give a good once over. The new plugs and wires did not help??

The truck has about 125000 miles and if there is any other info needed please let me know. I got code erased once I did the tune up but you can feel the misfire so I know it will be back..

 

Redline fuel injection cleaner or new injectors. Check for manifold leaks with a can of starter fluid.

 

+1 to the above. Random misses can be a bit of a PITA to track down. My money is on a crack in a vacuum line. They can be aggravating to find, and tough to fix without doing more damage to heat weakened plastic barbs
Go slow and careful.

 

Great, thanks for the ideas. Now how do I check for a leak with starter fluid??

 

Smoke is a good way to find a vac leak...

 

Spray it around the intake or where ever you think there may be a leak and if the engine RPM pick up, your drawing in air. I'd bet you have some dirty injectors though.

 

Spraying a flammable liquid around an engine bay is never a good idea.

BTDT....lost some facial hair.

 

Use common sense and be easy on the trigger.

 

Here's something to check.

Pull the intake ductwork & air cleaner stuff off so you have access to the throttle body.

Get your head over the top of the TB, open the butterfly and with a flashlight, look into the intake so you can see the bottom.

If this is the manifold I think it might be, you will be looking for oil inside the manifold on the bottom. There is a flat plate bolted to the bottom of the intake that closes it off from the lifter galley. The gaskets suck and fail causing a lean condition/vacuum leak. If you see oil down there, you need to pull the intake manifold off, remove the cover plate, clean it and seal it with silicone. Then reinstall it and that should help your misfire.

If the intake is a solid casting with no lower cover plate, this does not apply.

(It worked brilliantly on my old '01 5.9L)



See the square gasket in the picture? That is for the lower cover I'm referencing. Silicone works better without the gasket.

 

Thanks for the guidance I will keep you posted with what I find. I cleaned that up with the Seafoam pretty good so I dont know if that will throw me off with a visual..

Did you have a misfire or code before you did this?

After doing more research I feel it may be faulty injectors or egr, or intake leak..

 

No codes, just an unexplainable stumble and oil usage without any visible leaks or smoke. I figured it out and fixed it before the motor had 30k miles on it, though. Longer run time could lead to worse leaks.