Engine Low compression on back 2 cylinders...


im not that much of a idiot
Oct 25, 2019
Houston, TX
The cylinder by firewall, driver side and passenger side, had lower compression than the rest. Ie: 30&90 Vs. 140-150 for rest. I read somewhere if adjacent cylinders had low comp. It was a head gasket. I did notice some blue smoke under hard acceleration, nothing crazy, but visible for sure. None today tho. ?. Its still running quite strong, so i really doubt im near a whole cylinder down (the 30 psi cyl.) Head gasket, not a biggie, but im not tearing whole thing out and rebuilding yet if its running strong enough for me. I think i wasnt sealed on the comp. Checker somewhere. And the plug looked fine. I put in new MSD iridium plugs anyway.

Just lookn for ideas and making a 1st post. What yall think?
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Mustang Master
Feb 19, 2017
At 30 psi you would have a definite misfire. I'd retest and see where your at.


Mod Dude
Apr 6, 2011
Hillsborough county
And replace those iridium plugs with hotter spark copper ones.. seriously huge waste of money. The newer materials are to extend plug life not create better spark. There is a reason that performance engines (read racecar) use copper plugs. They are cheap and just plain work. The advent of platinum plugs was so the wasted spark systems of old did not eat the copper plugs up and now manufacturers want 100k before a tune up is needed, hence the iridium plugs.
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im not that much of a idiot
Oct 25, 2019
Houston, TX
glasgow- cool, good to know. It's not misfiring. It runs strong for stockish 302 so im not worried about it yet. Im lazy and for some reason that was the hardest plug hole to get my hand right to get threaded in. Once i get a lot of smoke or power loss i'll know i have a real problem.

a91- Only $25 extra. Good info tho. It says more spark, better cold start. The electrode is like maybe .03-.04. The autolites i pulled out was like ..080 gap. lol. (rough eyeball measurments). It actually felt stronger and more peppy. Could be placebo like.

2 things i didnt like on the MSD plug tho. 1. the end of the electrode has a way smaller diameter that is .05 long say, and i can see that getting burned out and opening the gap up. 2. the plug is like near 1/2'' shorter, up top, and using the plug wires i got (msd, came in back seat of car) puts your wires a :taco: hair off the header plate that bolts to the heads.


i'm familiar with penetration
Mod Dude
Feb 18, 2001
+1 on the plugs. Copper plugs are the way to go. Iridium is hard and resistive. Great for lasting 100k miles on a modern ignition, but copper works best on these ole fox cars and their 80s ignitions

And 0.080” gap on your old plugs was way to big. Probably a very weak spot and down on power. Set your gap to 0.052”-0.056”


im not that much of a idiot
Oct 25, 2019
Houston, TX
Right on. I was being careless and did'nt even check the gap on new plugs. I was like it's close enough. Checked the old plugs, they were at .055-.06.


im not that much of a idiot
Oct 25, 2019
Houston, TX
Done burnt a damn spark plug cable up, going to cylinder 4. Slacking on paying attention, and apparently was sitting on header. Not thru the insulation, but damn close, so still in use for now. Gave it less slack via spark plug wire holder adjustment. Did a oil change, and added 1 quart of the lucal oil heavy duty stabilizer. Seemed to work fairly good surprinsgly. Had it billowing smoke when i came to a stop couple days ago, did the oil change, and it mostly went away under low rpm. Makes me think its a valve seal more than a piston ring. Iono, that cylinder 8 could be shot. Was also missing out a smidge into 3rd gear shift low rpm. and pretty much went away also. +1 for lucas oil products. lol. But still throwing some smoke up top 4-6k rpm. Gonna run it till it :poo:s on me.

Im fckn pissed right now. Tried to break loose my lower control arm bolt to install these lower torque box reinforcements and could'nt get it loose.. Used a impact with swivel socket and all, broke my swivel, which i was intending on breaking something, cheap harbor :poo:. Mf'er not coming loose with my rooky ass mechanic skills and patience. Did'nt wanna break my dad's nice matco wrench or strip the bolt so i stopped. lolol.
Last edited:

Wayne Waldrep

Before I post a pic, do you have one of yours?
15 Year Member
Apr 14, 2003
Cuba, AL
Once I got older and decided I didn't like to always have to manhandle everything, I made me some new rules for myself.
1. Always clear out enough room around your work area so that you can get full leverage on whatever you are working on. This cuts down on poor angles and repeated attempts which only wear me out and piss me off.
2. Use PB Blaster on everything....preferably way ahead of time.
3. Never get in a hurry. It only frustrates you and doesn't get the job done faster.
4. Don't try to "make do" with something that isn't correct for the job. Tool or part. You'll regret it later....or maybe even right now!
5. If all else fails, put it down and try again tomorrow.....or get a bigger hammer, either way. :nice:
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im not that much of a idiot
Oct 25, 2019
Houston, TX
Ya totally on all the above.

Will switch to PB. I might take it to a shop and get them to break it loose. Then snug it up for drive home, so i know i can break it loose easily.

Actually, I'll try heating it up first and see if that helps. Ill pb it day or 2 ahead also.
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