1995 Mustang V6 - Cyl #3 Misfire - Help!

thank you for the help general..
Dcm I will go back and check all vacuum connections and lines.. as well as fuel injector #3 connector.
So the amount of cylinder leak down that I have shouldn't cause this skip?
and being that cyl 3 is leaking down through the intake when all others were leaking down through the oil filter cap area?
Just digging...
And learning as I go.
Thanks again y'all for the help!
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I am not familiar with how a leak down test is run. Based upon your compression I would not expect a miss.
Try the test General Karthief just mentioned.

Have you tried to clean the MAF sensor wires? Any air leaks between the MAF and the throttle body?
Disconnect then plug back in the various sensors on the engine and air intake (dirty contacts?). Do the same with the engine computer.

Bent or damaged push rod? If you pull the valve cover, check the cam lift for #3 and compare it to the others.

Spray WD40 on the shaft for the EGR in case it is sticking.

Turn on your lights and heater fan on full.. Make sure you have at least 13.5V at idle. Had low voltage problems in the past causing a rough idle.

Running out of ideas.


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I wasnt able to do as much this afternoon, but..
I redid the leak down test on cyl 3. It does without doubt leak back through the intake.
I did find a small vacuum leak and repaired it, and it made little difference. The only difference I can tell is that the skip isn't quite as bad but still clearly there. It's like it magnified the skip.
I sprayed every vacuum line and associated parts with starting fluid, and I am now fairly confident I have no further vacuum leaks.
This is as far as I got though.
I have cleaned the maf sensor not too long ago, but I will surely do it again.

something keeps telling me to focus hard on cyl 3, possibly the intake valve damaged or not seating properly. With the amount of backfires I am getting through the intake, and knowing that cyl is losing compression through the intake.

I just hate to tear that side down... again... head gaskets aren't cheap.. don't have 50 miles since I changed them.

but it is what it is I guess..
what do you guys think on my intake valve theory? (or worse, cam lobe, but I don't want to even think about that right now)
The backfiring puzzles me. I would not think 90PSI compression would do this.

I would triple check the entire firing order and separate #3 plug wire from all other plug wires.
Start at the plug and check the position next on the coil. Start at the coil and check the position next at the plug. Make sure your diagram for the coil pack is correct and you are looking at the pack from the correct direction with relation to the input plug. Seen this mixed up before on the 3.8.

Backfiring is often caused by running to lean.

Another thought is put an old fashion (needle type) vacuum/pressure gauge on the intake as close as possible to the intake. Supposedly if you have a bad valve it will oscillate by a couple of inches of vacuum every time the valve opens and closes.

View: https://www.youtube.com/watch?v=SdlNwm8OHco

There are vacuum lines under both front fenders which you can not access without removing the inner fenders. The passenger side has lines going to the fuel charcoal canister which are known to have issues. The drivers side is mostly the speed control which you can pinch off under the hood.

Do you notice a skip while idling? If so, try to pinch off lines leading off the engine and see if it changes.
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New intake valve and push rod on cyl 3.

Cyl 3 misfire still present.

I have not tested fuel pressure yet.
But correct me if I am wrong- if my fuel pressure is a bit lower than expected, wouldn't that cause other cylinders to misfire at some point? instead of specifically #3 all the time?
I have checked the injector, using a stethoscope, I listed to each injector for a time, and cyl 3 injector sounds exactly the same, firing at the same rate as any other injector. I did replace the injector o rings when I rebuilt the top end the first time.
Yes the cyl 3 misfire is the only code it's showing.
What is the compression after you replaced the valve?
Did you check the cam lobe when you had it a part?
Did you apply Dykem and lap the valve to verify you have full contact between the valve and the seat all the way around?
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I think I have found the problem.
I was focusing way too much in the wrong areas.
I took cylinder 3 plug wire off the spark plug, and tested for spark, as I have done in the past. it does have spark. but, I remove cylinder 2 spark plug wire, testing for spark. plug wire #2 had a much much stronger spark and ark than did wire #3. #3 is very weak.
so I have ordered a new ignition coil, again, as this one was new also. but this time, I ordered a motorcraft brand.
it should be here tomorrow, and I will post the results.
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