Question for the expert mechanics here
 

My Dodge Dakota has about 20000miles on a total rebuild 4.7 w/automatic. It’s been running fine but recently I started getting a check engine light. Code reads PO305 (misfire detected cylinder 5 ). I’ve replaced the plug and the coil, and the injector. Compression is good. Still getting the light. At idle it feels fine but under light acceration you can feel a slight miss.
I’m at a loss as to what to do next. Any suggestions ?

Precise compression testing is only as good as the accuracy of the test. This may vary with the test.
Oil can cover ring leaks up.
I guessing those might not appear under idle/WOT conditions.

Could it be:
Intake manifold/gasket leak?
Head gasket?
Valve sealing?
Sticky lifter?
Particles in fuel line to cylinder 5?

[disclaimer: not expert mechanic]

Only 20K miles? I’d check for rodent chewing through wiring also. Check that power is getting to coil..

What's the fuel trim doing?

Fuel pressure is fine at all rpm points.

Start at the plug and work backwards. Have spark? Reliable spark? If not, go to the wire. Then to the coil. Then the wiring to the coil. If you've fired the parts cannon at it and it's still not working then I'm betting the wiring. The tiniest little nick can throw them off.

Pulled the trigger on the parts cannon for all the obvious culprits. Then I proceeded to play part roulette to see if the problem would travel. But it remains on #5. I can hear the injector activate and I get a consistent signal on the o’scope for it. Coil on plug and that fires consistently.
The only thing to do next ( that I can think of ) is to lift the valve cover and see if it’s a bad valve or lifter

So you’ve switched injectors with another cylinder and still #5?

Bad valve or lifter should show up on comp test. Did you leak down test engine?

No the lambda control..... any cheap scanner should tell you if its adding or subtracting fuel. "Fuel trim", it'll be a percentage reading. If it's adding fuel at idle range there could be a vacuum leak somewhere..... do you have a scanner?

No. Haven’t done a leak down test. I don’t have the gauges and have never done one before.

See if problem follows after switching injectors.

I would think a leakdown of that cylinder would be my next step. Aren't those early 4.7 motors famous for dropping the valve seats out of the head ? It is also possible that you have some abnormal cam wear. Easy enough to pull the valve cover, and make sure you are getting full lift .

I’m not aware of any valve seat problems with the 4.7 but certainly couldn’t rule that out. I had the heads done when it was rebuilt so I guess it would be prudent to look closely at that. It would be hard to imagine cam wear occurring this quickly but I suppose a failed lifter is a high possibility.
I guess I’ll be removing a valve cover next.

No. No scanner

Poor or dirty contacts with the coil wire can cause problems under load. Check to make sure the connectors are tight and there is no corrosion.

Flux capacitor.

Single cat exhaust? Might be a plugged cat. Has it been replaced over those 200,000 miles? Also, check oxygen sensor...

He only has 20 K miles

Actually he has 20K on the rebuild and an unknown # of miles on the rest of the truck likely including the cats. But as the problem seems to indicate a single cylinder, then the cats are not a likely issue as that would impact the entire bank (or the entire engine if single cat).

I'm not a mechanic, nor did I sleep in a motel 6 last night. But I do seem to recall these engines would occasionally develop cracks in the cylinder heads. That could manifest as a miss that is not related to fuel or ignition issues. Really need to do a good compression/leakdown on this one.

angela

I agree with Fred - start with cranking compression, then leak down test