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Ford/2002 Ranger 4.0 SOHC misfire



  Just bought a 2002 Ranger with 4.0 SOHC engine.  Has a misfire at idle and low rpms when engine is cold.  After it runs for 5 minutes and warms up, the misfire is gone but still idles kind of rough.  I hear that rough idle is common on these.  I am a mechanic but not too familiar with 4.0 SOHC engines. Check engine light comes on, scanner says code P0304 #4 cylinder misfire.  Has new motorcraft plugs, plug wires, and have verified that spark will jump an inch and a half at the end of the plug wire even when misfiring, so I don't think coil pack is bad. Checked all vacuum hoses and fittings, and EGR valve.  I heard that intake gaskets are a problem on 4.0, but possibly only on OHV and not on SOHC models.  Can you verify this?  Am going to do a compression test next.  Have heard it is common to have broken valve springs on these engines, can you verify this also?  Only has 113,000 miles and has no blow-by.  Runs very good while driving, and only misfires and triggers the code when engine is cold and not under load.  Under load the misfire is either gone or not noticeable.  But when idling cold you can hear a definite misfire.  No other codes display, and it is always #4 cylinder.  No lean codes or anything else. Is it possible it is a different cylinder than #4 even though the code is for #4? If there was a vacuum leak, wouldn't there be lean codes, and why always the same cylinder?  In my experience, a bad injector causes misfire whether hot or cold, do you agree? Does not have timing chain rattle or any unusual noises.  Is there a way that I can find out if the timing chain update has been done on this truck?  I heard about an update to fix timing chain tensioners and install an oil plug, but have no idea if it has been done or if it even pertains to 2002 models.  Hope you can give me more information on that also.

Thanks for your time.  I used to volunteer on this site also, so I appreciate what you do.

ANSWER: Wow I wish I got these kinds of descriptions on all cars LOL  if it's only happening at idle you are probably leaning towards a broken valve spring as I have ran into this before . You can try spraying brake clean around intake the upper when cold as these intake gaskets are known tongi bad as well.  As far as the update. If you look at the throttle body on the left cylinder head, right if you are looking at it, you will see what looks like. Plug screwed in to cylinder 4 . The plug is about 1-1/2 inch in diameter round and sticks out of the head about 2" pointed at the throttle body . So if you can do a leak down test, you are probably gonna find a broken valve Spring but at least it's an easy on to get to .  But lastly don't rule out a bad coil . Easiest way to check  those. Usually they will crack on the bottom when they go bad . Please keep me posted

---------- FOLLOW-UP ----------

QUESTION: Thanks for the reply.  I see on the top front of the left cylinder head what looks like a hex plug with a round thing that sticks out about an inch and a half. It points upward toward what looks like a coolant temp sensor on the back side of the thermostat housing under the throttle body.  This must be what you are referring to.  So if it has this plug, what does that actually mean?  Is this the plug that I heard is related to the timing chain update?  What does this plug actually do and what is it there for?

  I won't be able to work on it now until the weekend to try and find the cause of the misfire. The left valve cover has a lot of stuff over it, so it won't be as easy to remove as I thought.  The fuel line to the injector fuel rail runs over the top of it.  What is the best way to get that out of the way?  All the other stuff looks pretty obvious such as coil pack, EGR vacuum valve, wiring, etc.  If I do finally get this figured out, I will let you know what I found .    Thanks a lot.

So that plug actually has a tensioner on the end of it to add tension to the tomong chain guide as far as the fuel line pretty sure it has flexible connections on each side so if you rice the two screws holding it in place you should be able to tie that side of the rail up with the injectors still attached  


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brian howard


I can answer most question from drive ability to electrical, electronics and almost all check engine light concerns from 2000-2013 vehicles


I am a Ford Certified Master technician. I specialize in drive ability concerns, but can help with most concerns from steering to suspension and climate control relating to Ford And Lincoln/Mercury products from model year 2000 to current

Ford Certified Master Technician

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