The problem is #4 and #7 cylinders. They both have good compression and pass a leak down test.
I assume since it is showing - fuel trims that means it is getting fuel but not firing.
- fuel trim means the computer is removing fuel that would be supplied by the base programming, based on O2 sensor readings indicating richness. The computer is in "Lean Command".
I can check with a timing light and it has spark to the plugs.
Spark TO the plugs is not necessarily spark across the plug gap--which is what counts.
PULL THE PLUGS, especially #7 and #4, see what they look like. Fouled? Rounded (worn) electrodes? Excessive gap? Cracked porcelain? Clean and pristine?
Checking spark with a timing light is a half-assed way to assess spark quality. Use a spark tester calibrated for HEI.
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it
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There are other styles, and there are non-HEI versions of the style I prefer, and linked-to. The one I linked to is an HEI-calibrated unit.
Normally runs around -18% LTFT and near 0% STFT. If im accelerating at a moderate throttle maybe 3/4 the fuel trims runs about -2% LTFT and +2% STFT. Means vacuum leak i think.
-18 on the fuel trims? The LAST thing I'd expect is a vacuum leak.
Actually, it's really surprising to me that the computer can be in lean-command when the engine is misfiring--which generally leads to the O2 sensors reporting a false-lean condition. That should put the computer into
rich-command.
How ancient are these O2 sensors? What does the scan tool show for voltage/crosscounts?
I did however get this today. New GM injector harness. Mine that is installed is brittle and has broken injector plugs where they lock on to the injector.
Since im already 1/2 way there changing the harness it's not that much harder to change the lower intake. Just a couple more hours really.
Instead of changing the intake, research your O2 sensors, and see if you can find a fuel leak into the intake manifold--leaking injector, leaking fuel pressure regulator if the big-block has the regulator in the manifold (not sure it does.)
Maybe the injectors for #4 and #7 aren't firing because of poor electrical connections; the misfire may go away when you install the new harness and get the injectors securely "plugged-in".
Also, something else i notice and i don't know if it means a thing but there is a alternate firing order 18736542.
My engine is missing on #4 and #7, Hmm seems like a odd coincidence. I don't know what injector harness i used either off the core engine or the original
It's a long shot for sure but maybe a new stock GM wire harness is the fix.
It is possible the cylinders are fine and just getting a injector pulse at the wrong time. Does seem like a extreme long shot though as that would mean someone repined the core engine injector harness for some reason and i ended up with it installed on my engine.
The OEM 7.4L did not use that firing order. Some aftermarket camshafts have that firing order, and someone could have re-cammed the engine, or re-pinned the harness.
Pull a valve cover. Either one, or both. Verify the camshaft firing order based on valve positions. Either #8 is followed in 90 degrees of crank rotation by #4, or by #7. Either #5 is followed in 90 degrees of crank rotation by #7, or by #4.
Then verify the firing order of your (old) harness. Does it match the cam?
Or just swap #4 and #7 plug wires at the distributor cap. Does the engine run better? Seems unlikely since you say the idle is good as-is.