Cylinder 4 misfire/ P0304 Code - Ranger-Forums - The Ultimate Ford Ranger Resource

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Old 09-17-2016
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Join Date: Feb 2016
Location: Hopedale
Posts: 4
Cylinder 4 misfire/ P0304 Code

2005 Ranger 3.0 Litre V6: Have been battling a check engine light for a few months.
Codes keep coming up as ; P0304: misfire first 1000 revolutions & P0317: cylinder 4 misfire. Engine has about 90K.
Changed plugs and wires first. no difference
Changed Coil pack. no diff
Changed injector on #4. no diff
Changed cam synchro, and sensor. no dif
Pulled the #4 plug after about 10K miles and it "looks" beautiful. Nice ash greyish. Not white, not black.
Misfires at idle, when sitting at a stop light, and unless I keep the RPM's up at the light, check engine light will start to flash, and engine will go into a "low power mode." Must shutoff, and restart. Once moving down the road, runs fine, no misfire, but tends to ping a bit under heavy load, like timing is advanced.
I have not done compression test yet. Have to get tester.
Been reading alot here from others that are experiencing same symptoms. Most have done compression tests, and found no abnormalities.
Is comp test best way to go from here? Or?
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Old 09-18-2016
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Join Date: Jan 2014
Location: Vancouver, BC
Posts: 7,279
Welcome to the forum

Reads like you have eliminated most other things as the problem.

Your 2005 falls in the 2004-2006 range for bad exhaust valve seats on the 3.0l Ranger engines.

A TSB was issued for that:

First step would be to remove all 6 spark plugs and then do a compression test for all 6 cylinders, engine should be cold.
See where #4 is at when you have the average from the other 5

You can add a teaspoon of oil to #4, #5, #6 and then redo compression test on all 3
Compression numbers will go up on all 3 but not as much on #4, because the oil seals the rings, but not the valves, so if #4 is still low then it is a valve issue not a ring issue, kinda like a poorman's leakdown test
A leakdown test on #4 would confirm bad exhaust valve seat because you could hear air escaping from #4 by listening at the tail pipe, you could do this first if you have the means

What happens with this is that the exhaust valve seat would start to recess/sink into the head so there was no longer a good seal between valve and seat, so compression in that cylinder would go down enough to cause a misfire at lower RPMs, as RPMs increase there is less time for the air in the cylinder to escape thru the exhaust valve seat gap, so compression increases enough for cylinder to fire.

If you do have this problem then BOTH heads need to have new seats installed or swap for new heads.
This didn't happen to ALL 2004-2006 3.0l Rangers, so there was no recall, Ford, of course, doesn't say why it happens on some, but my guess would be some seats were out of spec and were installed, this is why BOTH heads should be repaired/exchanged, because other exhaust valve seats in your engine will be from same batch as the one that is failing at this time.

And in the compression test you may find 1 or 2 other cylinders that are lower but not as low as #4, those seats are also failing but haven't recessed enough to cause a problem yet.

Last edited by RonD; 09-18-2016 at 11:30 AM.
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Old 09-19-2016
Thread Starter
Join Date: Feb 2016
Location: Hopedale
Posts: 4
I was afraid you might lead me to this. I will do the compression test/leak down test and advise. Thanks for your reply. D
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