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Several tests done- whats causing my misfire? 2.3L

cavity.

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1997 Ford ranger 2.3L w/ 100k miles
Slight misfire at idle (warm or cold), once every 300 miles or so I will get a P0302 code (misfire cylinder 2)

The car has:
New Plugs/Wires
New injector on cyl 2
Fresh oil, oil filter, air filter
New Catalytic converter

I've tried:
different spark plugs on cyl 2
Pulling either of the plug wires from cyl 2 makes idle rougher
both plugs at cyl 2 have spark
Vacuum test is unsteady -- points to ignition issue
Compression is a little low on cyl 2 (155 dry, 170 wet) (other cylinders have 168-175 dry), but im not sure this is enough to cause the steady misfire

So my two questions are:
Could this low compression be enough to give me a consistent misfire?
Could a coil pack be faulty if it is still giving visible spark (tested at idle)?

Thanks for any help, i really appreciate all the posts here.
 


RonD

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While engine is running both spark plugs fire on compression stroke and exhaust stroke, so even with one coil failing engine wouldn't miss.

make sure spark plug wiring on coil packs is correct, easy to reverse these or the wires at the spark plugs.

There are 2 coils in each coil pack
1 and 4 spark at the same time, as do 2 and 3, they are "matched pairs" to balance the engine, so 1 and 4 share a coil, as do 2 and 3

So a coil issue on 2 should also effect 3

You could have a sticky valve on 2, won't show up on compression test well, jumpy vacuum also indicates that.

You can test for it at tailpipe using paper, video here: https://www.youtube.com/watch?v=PI5bBV0ZFek
 

tomw

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I had thought that the spark plugs on the intake side of a dual plug head did not fire all the time. Meh.
If the plugs on the passenger side fail to fire, you will have misfire, as they are used 100% of the time the engine is running.
You have enough compression on #2 that the engine should fire. You may have valve(s) that are receding into the seats. That will cause intermittent misfire(I do not know why).
You can use a timing light on the offending cylinder to insure the plug is firing. Given that 1-4, and 2-3 fire together, a problem on one plug of the pair, might cause the other plug to either not fire, or fire poorly. The 'waste' fire(exhaust stroke) of one cylinder is an 'easy' fire for the coil, so the spark goes on through to the other plug(compression) which takes a bit more voltage and then does fire.
The plugs have their easiest time when an engine is running at idle, compared to being under load climbing a hill, for example.
If you have a steady miss, in general it is mechanical, where an intermittent will likely be fuel or ignition. The valve recession (receding) is the one exception to the rule that I have heard of, and it occurs in both the 3.0 and the 2.3.
I do not know which years were afflicted with the valve problem...
I would try swapping both plugs and wires between 2 and 3. Or even to a complete swap from the center two to the outer two to see if the problem follows.
You can also try swapping the coil packs, side to side. Be sure to get the wires installed properly, or you can create more problems. Check the wire position against a known correct installation, or a manual.
tom
 

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