Engine is missing on cylinder #4???

[1Byounkin]

I have a 1991 explorer. The truck will not pass emissions because the HC is too high. The engine has a miss, if you remove the #4 plug wire, with the engine running, than the motor will smooth out. Replace the #4 injector. Engine still has a miss. Check compression in all cylinders, all read 150psi and never drop pressure sitting. I have NEVER HAD A CHECK ENGINE LIGHT ON. The computer and all components have been checked. All good, no errors. Still have a miss. Tried a different wire for #4. Same thing. Used a smoke machine to check all the vacuum lines. All good no leaks. Even changing the plug with no effect. Everything looks and checks great, but still have a miss in #4. Unplug the plug wire it smooths out. The plug is wet but will not burn if you light it. Put pressure on the coolant system, not much drop in pressure, like 1psi drop. When you check the emissions it will range for 1300 to 500 hc. The limit in TN is 220 hc. fuel pressure is ok. The engine runs great has power but just has this miss that is causing unbrunt gas go though the exhaust. Which make the HC read high.

Any ideas---

 

[KobeJ05]

you test the coil pack?

 

[1Byounkin]

all have great spark.

 

[KobeJ05]

how does the plug look? i have a miss on cyl 4 too but i have oil on the plug.

 

[1Byounkin]

the plug on #4 was black with a little carbon. After a whlie the plug became wet, but can't be lit. If it had gas on it then it should catch on fire. The rest of the plug looked like the engine is running lean. On #4 the plug looks rich.

 

[nexis]

you said that you remove #4 plug wire and it smooths out, replace #4 injector and it acts up again. are you pulling the plug wire,or the injector wire? If your pulling the injector wire, i would almost suspect that something is wrong with that injector thats causing it to stay open too long, or not fully close. start off with a seafoam treatment, or what ever brand of fuel injector cleaner you like, see if that helps the situation any. If that helps the problem, then you have 2 options at that point, replace the injector, or try to soak it in cleaner for a hour or so. However soaking it in fuel injector cleaner can damage it. so you may end up having to replace it anyway.

The problem really sounds like a issue with the injector on that cyl. Its wet and carboned, the rest look lean. Sounds like the O2 sensor is detecting a rich run due to the gas being dumped into #4 and leaning out the rest

And dont think that if it has gas on it it should catch on fire. it takes the right fuel to air mixure to produce ignition, too much gas will not burn, and will foul out a cylinder very quickly.

 

[1Byounkin]

there is a new injector in #4 and still have the same problem. unplugging the wire from the #4 plug on the spark plug. Basically I'm just going to have the top of the engine rebuilt. Reseat the valves. Reface the valves. Replace the valve seals. On all 6. Each cylinder is being check for any leaks. This would eliminate any problems. Everything checks out and is working great. I believe it is something that is mechinal that the computer does not control. All injectors are good.

 

[J-RodG]

Thats what happened to me! I had a miss on cyl 4 and it turned out to be a cracked head! I had all the same symptoms and did all the same tests with no resolve so i just replaced the heads! Runs great now!

 

[1Byounkin]

Well found my problem. A cracked valve. Heads look great. Refaced all the valves and did the seats.