Dropped Cylinder Help

[Eastonfrench]

One LAST thing I need to fix on my project (for now).

So, at an idle, i feel a stumble in my truck. It felt like a dropped cylinder, so I did the following to diagnose it:

Pop the hood with the truck running.
Begin unplugging spark plug wires from the cap and rotor (manually killing cylinders one by one).
Each one i unplug, the engine would almost die. I would plug it back in, it would shake less, and I move onto the next plug.
When I pull the number 8 cylinder, there is NO change in idle.
THis confirms, i believe, that I am running on seven cylinders.

Now, Why?
I switched spark plugs with one from another cylinder: no change.
I tried switching plug wires: no change.
I used an in-line spark plug tester to ensure i had electricity getting to the plug: check.
To confirm the last point, I can also hold the plug near metal and crank the engine and get spark to arc to metal, so we definitely have spark in the cylinder.
I did a compression test on the cylinder: compression was great, about 135 psi.
To attempt to check for fuel, I took the plug out (and unlpugged the distributor to prevent ignition) and cranked the engine. There was visible spurts of fuel shooting out the plug hole.

SO, I think, and correct me if I am wrong, that I have compression, spark, and fuel. If this is the case, why is it that I am not getting any action out of cylinder number 8?

[Irish_Alley]

could it be something with the cap/rotor? you may be getting spark but it may not be one thats "hot" enough

[Eastonfrench]

Thanks for the response.

I popped the top off of the cap and rotor (which was replaced about 5 months ago) and checked to make sure the points werent all messed up. It looked fine to me (all points looked identical).

I suppose it could be a weak spark, but everything is new past the distributor.

[bd]

Recheck the firing order: 1-8-4-3-6-5-7-2.  Cylinders 8 and 5 are running mates.  Did killing #5 make any less or more change than the other cylinders?  Did you check the compression in all cylinders or just #8?  Does it pop back through the carburetor if you throttle the engine rapidly?

Check for vacuum leaks by spraying some Berrymans B12 or similar carb cleaner around the carb base gasket and along the intake manifold margins where it joins the cylinder heads, etc.  Temporarily remove the vacuum lines from the manifold fitting located behind the carburetor and cap the fitting to see if there is any improvement.  Do the same thing with the vacuum line that connects to the brake vacuum booster.  Remove the EGR valve and flip it over.  Without depressing the pintle valve off its seat, spray some B12 around the pintle to see if any solvent bypasses the valve into the adjoining chamber.

[crx88ia]

SO, I think, and correct me if I am wrong, that I have compression, spark, and fuel. If this is the case, why is it that I am not getting any action out of cylinder number 8?

Not trying to sound like a smart ass but if you have all air fuel and spark its working, if its not working then it's missing something. Are you sure its fuel coming out and not oil or coolant? Spark is probably the problem like you said.

[blazer74]

All else fails pull the valve cover and see if you have equal valve rocker movement from #8 to the others on that side with the other cylinders.

Broken rocker, pushrod, possible.
At worst wiped cam lobe but you should have other symptoms with that.

[Eastonfrench]

I am going to my old man's tool shed this weekend and my brother and I will dig into it. I am going to pop the valve covers off and see if anythings sticks out

[Eastonfrench]

Okay. Did some more compression testing. My bad cylinder pushes 115 psi. The others are all 150-150 psi.

Did a wet impression test by squirting some oil in the bad cylinder and compression testing, and the bad cylinder that previously showed 115 went up to 150 psi. Thoughts?

[crx88ia]

Bad Ring; maybe try some MMO or Seafoam in the bad cylinder through the spark plug hole and see if the ring releases with solvant. Not to much, you don't want to fluid lock the cylinder. Let it drip down then change your oil prolly. Crank it a bit with the distro unplugged to clear the stuff out.

[jg1977c20]

sounds like rings in that cylinder - look up leak down test - just use the compression test hose with the shrader valve removed put #8 at top dead center and put about 80 psi into the cylinder , listen for air escaping - ehaust pipe . carb . valve cover , dip stick tube also pull plug from #6 and make sure air isnt escaping into that cylinder - this will confirm the problem more than a compression test

[VileZambonie]

You've been messing with that cylinder, the compression will go down since you've been feeding it fuel and not firing it. Is the choke wide open and a battery charger hooked up during your compression test?

Have you placed a vacuum gauge on it? Any loss of coolant? With the spark plug out have you pressurized the cooling system?