agentruley

364 iron block
799 heads used
lunati dual valve springs
comp harded pushrods
stock valves with a angle job done at by myself
gen 4 rods
flat top pistons
Holley Terminator X efi (no MAF, VE mode)
NNBS 4 bolt intake manifold
92mmtb
t56
custom cam 238/24X at 112 lsa .595 lift

build went together great, put about 2500 miles on it and now there is a misfire while cruising or maintaining speed. if i give it any kind of throttle to speed up even a teeeny tiny bit, the misfire goes away. but if i cruise, and the vacuum shoots up to like 38-35kpa it misfires. I ran pump gas in it for about 2000 miles then mixed 1 gallon of leaded 110 to about 5-7 gallons of 93 for the last 500-750 miles. symptoms just now showed up

things I have done to try and chase it down

replaced intake manifold gaskets
replaced plugs and wires AND coilpacks (over 3 times)
replaced injectors with known good set
checked fuel pressure
changed cruising AFR in EFi, rich or lean doesnt matter
tried to change cruising timing, same symptoms either 17degrees or 40
used temperature gun and found that cylinder #2 is about 100 degrees colder than the rest. at idle, unplugging cylinder #4 and letting it cool down to 300degrees, plug the injector back in and it rises quickly back top 450-470. cylinder 2 never goes past 325-350 degrees. rest of cylinders seem to idle at or above 400.
Did a compression test, all cylinders go to 210psi except cylinder #2, which only goes up to 180-185
did a leakdown test, cylinder 2 seems to not leak at all, and/or behaves like the others
took rocker arms off to inspect push-rods and the arms, seem great, no obvious failures.

Can someone please help me? I am close to having to pull the motor back out. its my first build and I cant help but think i fucked up the valve job on these junkyard 799 heads. could anything in the bottom end like a scoring on cylinder walls cause this? The short block is the most important part to me as I cannot find or afford iron 6.0's because of where I am from. This one came out of a dump truck.
If I did mess up the heads, I will just shoot for something CNC or brandnew. I feel like using junkyard stuff is screwing me over.

jetech

If the leak down check showed all cylinders healthy your heads are apparently good. Why would you run leaded 110 gas?

Che70velle

iTrader: (6)

What does the spark plug on #2 tell you? Try this…next cruise out get it to miss purposely. Get the vehicle off the road and turn it off as quick as you can. Pull number 2 plug after she cools off for a little while and see what it says.
Can you log while your driving and see misfire? Does it throw a code? What are your AFR’s at cruise when this is happening?

agentruley

The ECU is an aftermarket standalone Terminator X, no way to show misfire data. I have done what is mentioned, plugs look fine. everytime.

I ran some leaded gasoline to see if the engine liked the more stable fuel. It is 11:1 cr
Anyone have any ideas? Im guessing a crack in a valve head, or maybe stretched valve or something that would get worse with heat? so a cold leak-down test doesnt show anything.
I can cruise at anything from 12.5-16.X and same behavior. Mentioned before, I got almost 3k miles out of her without any misfires whatso ever. multiple tunes, and exhaust setups, suspension changes etc has happened. was a good running racecar

Che70velle

iTrader: (6)

The fact that the misfire goes away with throttle input SCREAMS that it’s a fueling (tuning issue), but you’ve done your homework there. Sounds to me like you have no choice but to take a look inside. Keep us posted.

Darth_V8r

iTrader: (4)

I would try squiring a shot of oil into #2 and repeat compression test. See if it comes up. That is a classic way to diagnose rings vs valves. If oil improves compression it's the rings.

Have you considered increasing spark dwell?

As Chevelle mentioned above, what does #2 plug look like? Literally every sensor in the car can be wrong. Plugs NEVER lie

agentruley

I can try that test here in the next day or so when the weather is nice, and before I tear it apart.

"spark dwell" Why would running the same values ive been using for the past year just now start to cause a problem? This is me simply asking to understand, nothing more.
plug 2, mentioned in previous post looks great and just like all the rest. every. time. (ive pulled 3 different sets of plugs over 2 dozen times trying to chase this the "old fashion" way.)

dixiebandit69

Do any of y'all think that it might be an issue from the O2 sensors?

The OP admitted to running leaded gas in this thing, and leaded gas and O2 sensors don't play nice with each other...

Darth_V8r

iTrader: (4)

On spark dwell -- Since you can change it in the tune, it is something you can do for free. If increasing dwell kills the misfire, it could be an issue of intermittently weak coil output. That's all I'm thinking. I have had plenty of builds with cranking compression near yours that needed more dwell time to fire reliably. Is it likely to be a magic bullet? no. Is it free to try vs throwing parts at it? yes