Are my spark plugs still misfiring?

[72timemachine]

Hello,

I recently blew a head gasket or two a few weeks ago on my 1990, c2500, 5.7 ltr. engine. Upon removing the spark plugs, I found suspicious yellow deposits on cylinders 2 and 4 and while the old head gasket did not look damaged where the two cylinders are closest together, side by side, I was thinking this could be burned coolant which left these yellowish deposits on the ceramic which I have never seen before.

The number 7 cylinder head's gasket in the top right corner had obvious damage and allowed coolant and oil from the nearby oil return hole to enter the cylinder and burn off creating smoke and steam out the tailpipe. I bought new factory spec heads, head gasket set, new bolts, used a bottom tap to clean out the bolt hole threads in the block, resealed the intake, double and triple checked all my head bolts, intake bolts and set the valve lash at 3/4 of a turn once I had all the play out of the push rods. I tightened the rocker arms to the point where you could just roll the push rod with your fingers but no tighter, then I applied 3/4 of a turn to set them. Some say 3/4 of a turn, and some say 1 whole turn. I suppose I can always add the 1/4 turn if needed but I do not have any noises and the truck does run good except for a skip under acceleration which leads to my question.

After doing a lot of reading here and on the internet, I see that there are so many things which can cause a skip...everything from bad grounds to vacuum leaks to everything in between. When I was driving for a very short while with a blown head gasket, I am sure the truck picked up a skip in the number cylinder because when I removed that spark plug, the oil leaking into the cylinder from the oil return hole did leave wet oil on the spark plug.

Since reassembling the entire upper half of the engine, I have been trying to hunt down this skip which you only feel under acceleration. I used a lawn mower to visually prove that each spark plug was giving me spark and I purposely attached the alligator ground clamp to the ground electrode in order to direct the spark to jump across the gap because I thought that if the ceramic was cracked around the center electrode that it may prove there was damage to the plug if it consistently DID NOT fire the spark across the gap to the ground electrode but out of the crack of the ceramic which could possibly be the cause of my skip. I did this with all 8 spark plugs and when I tested them using the magneto of the lawn mower, they were consistent and seemed to fire properly. I understand you can test a spark plug at several different point on the plug, however, an ohm meter will not tell you if the ceramic is cracked, right? I cleaned the plugs as best I could and re-gaped them to .035. the center electrode is still nice and square. I set the timing to 0 after disconnecting the tan and white wire as directed on my tuned up sticker under the hood.

My plug wires are factory GM and only a few years old, however, short of testing them with an ohm meter (10,000 or less ohms per foot no?) I hooked up my timing light to each wire to see if I had consistent pulses of light coming from the gun...they all pulsed consistently. I then used a spray bottle of water to squirt a little water on the hot exhaust port to see if I had a cold or misfiring plug, however, the water burned off immediately on all 8 exhaust ports.

Last night I parked in a very dark location and sprayed my wires, coil and distributor cap with water, I soaked it pretty good and only saw two tiny sparks just one time from two different plug wires which where touching the dipstick and transmission dipstick tube. I plan to insulate these tubes today by zip tying some old heater and vacuum hose to the tubes to insulate them from the wires.

Is it likely I still have a faulty spark plug after running these tests? I have no problem buying new plugs, however, replacing plugs which appear to have no issues is not solving the problem. I do not want to take anything for granted and I am trying to check possible causes off the list. My truck has around 150,000 miles on it and the engine runs very well as I am proactive with taking care of it and do not drive it hard but when you drop the hammer, it really takes off for heavy truck.

Thank you for any help, it would be most appreciated.

[Tom]

Spark plugs causing issues is pretty rare. Did you not replace them when the head gaskets went bad?
I've never heard anyone say a full turn past the pushrod getting resistance. I usually go 1/2 turn past.

[72timemachine]

No Tom, I did not replace the spark plugs. I inspected them, cleaned them, tested them with an ohm meter in several places, visually saw that I had spark jumping the gap using a lawn mower ignition system and a ground wire and had rapidly, sizzling water evaporate off my exhaust ports on the exhaust manifolds to check for a misfire.

I understand what you are saying regarding spark plug failure. If the center electrode is all eaten away (smoothed over and is no longer nice and square) due to age, yeah, it's time to change them. Aside from mine getting dirty, they look brand new, however, it is not going to convince me that everything is fine.
Several years ago I experienced a hair line crack in the ceramic insulator where the spark plug boot is attached...it was leaking spark to the exhaust manifold and causing a skip.


Odd thing is, right after I wrote this thread this morning, I got in my truck to head off to work and the skip is no longer there. The engine is running very smooth and there is no skip on acceleration. It has been skipping since I replaced my head gaskets and rebuilt heads. I am not saying the problem is solved just yet, however, if it returns and I find out it what it was, I will let the forum know.

If memory serves me correctly, on YouTube, Tyrell's Classic Workshop, Ian Tyrell had an older 12 cylinder Lamborghini with several misfires he attributed to fouled out ( I believe too rich) spark plugs. He simply started the car and gently warmed up the engine and one by one the skipping cylinders came back on line and the engine ran fine. He is very hesitant to remove spark plugs from these aluminum heads due to wear it causes the spark plug threads each time you install and remove the spark plugs. Makes sense with soft aluminum and this guy has been working on Lamborghini for a very long time...he does not even use a torque wrench when he rebuilds engines. I'm not saying this is what happened to me, however, it is unusual for things to fix themselves.

Thank you.

[57taskforce]

Based on what I’ve read, I’d bet your valves are too tight, I prefer to adjust them with it running because your can get them fine tuned just right. I only 1/4-1/2 turn past when the pushrod stops tapping the rocker.(too loose) plugs could be an issue and are easy to change but it seems like you’ve ruled them out, I would have replaced them when doing the top end rebuild bit can certainly understand why you didn’t.

[72timemachine]

Update on the misfire. I am using the factory spec spark plugs, AC Delco cr43ts. These are spark plugs with a built in resistor. Turns out that when I started the engine from dead cold in the morning, the truck ran great until it came up to temperature, then it would slowly laps into skipping from then on.

I just happened to spray some water on that, now very hot #7 exhaust manifold and noticed it was not sizzling off nearly as quickly as the others. I then pulled the plug wire off the spark plug and did not notice any change in the engine's idle.

Once the engine cooled off, I removed the #7 spark plug and inspected with a very bright light which illuminated a very tiny micro crack which appeared to be highlighted by some black discoloration within this micro crack (approximately 1/16" in length) because I could not wipe it off, it seemed embedded in the crack. I ran my fingernail over it and could feel something so with that I used the sharp corner of a utility blade to try and get a better sense of what I feel is a very tiny crack. This may have been causing the misfire or perhaps the plug was somehow internally damaged, I'm still a bit unsure. I replaced this spark plug with a new one and the misfire is now gone and the engine is running smoothly.

Has anyone experienced a resistor with the spark plug causing a misfire if I did not in fact have a cracked spark plug?

[Palf70Step]

i HAVE HAD PLUGS GO BAD, RESISTOR AND NON RESISTOR TYPE FOR REASON UNKNOWN TO ME, BUT WHEN i FINALLY FIGURED IT OUT AND CHANGED THEM PROBLEM SOLVE.

PS I am not shouting, just to lazy to catch the shift lock and retype everything.

[72timemachine]

Nice to hear I'm not alone on that, thanks, nice c/10 you have there.