My 454 Rebuild

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.

I wouldn't think it's that its the wrong firing order, because the injectors are pinned to the harness so they match the computer.

What you could do to verify is continuity check from the ECU to the injector plugs and make sure they're correct that way. This would ensure at least the wiring is correct.

I'm not sure how they are set up in that connector. Maybe #7 and #4 are next to each other and someone oopsied and you're stuck with someone's mistake?

Not understanding what you're saying fully.
My harness is correct to the injector plug at the manifold from the ECU. Checked more than once.
I do not know about the harness from the plug to the injectors. I will be rechecking it all again when i change the injector harness.
I have been over and over and around this problem so much i think i need a new set of eyes on it as im just out of ideas. I have paid 2 shops already with no real results. I need someone good like Ivan from Pine Hallow auto diagnostics.

Spareparts said:

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.

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- 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".

Spareparts said:

I can check with a timing light and it has spark to the plugs.

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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

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.

Spareparts said:

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.

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-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?

Spareparts said:

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.

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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".

Spareparts said:

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.

Click to expand...

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.

@Schurkey
- 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".
Yes i would agree from what i understand.

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.
Spark plugs look like they are firing, Not fouled for sure and maybe look slightly lean.
I agree with the spark tester but it is a quick down and dirty way to be reasonably sure they are getting spark when a proper checker is not available. Also, the spark will jump a decent distance and is a nice hot blue spark with a SNAP to it.

-18 on the fuel trims? The LAST thing I'd expect is a vacuum leak.
I don't know why that is stuck in my mind and maybe im not understanding fuel trims.
If it's getting to much fuel the computer goes - on fuel trims
If it is lean the computer is + fuel trims.
Thinking this through if unmetered air was entering the cylinder it would be lean and the computer would add + fuel?


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 assume that if it's misfiring then the O2 sensors would think it's running rich since the exhaust appears to have unburnt fuel in it leading to the O2 sensors to report to the ECU to lean out - the fuel trims?
The 4, O2 sensors are near new. Not GM but they were not the cheap ones either.
I can not tell you what the voltages are but when i watch it the voltage does move on all 4 sensors. Crosscounts?

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".
Could or could not be misfiring because of the injector harness, Unknown at this point.
Injectors and regulator have been changed a few times. Fuel pressure is 58psi with a new ACDelco FPR.
I have less than a year old set of 5o injectors and presently an installed set of reman stock injectors. Neither the FPR nor either set of injectors changed a thing.
Everything tried seems to make the situation a little better at first but in reality nothing really changes.

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.
I have no idea what was done to the core engine. It could have had a different firing order cam and harness re pinned for all i know, but that is an extreme long shot and i highly doubt that is the problem. More of a strange coincidence. I did not learn about the alternate firing order till last week.
The idle is rough and has misfires.
I theorize that if the harness was repined then the injectors are firing at the wrong time and a small gas charge is sitting on top of the intake valve's and sucked into the cylinder as a pool of gas and not a mist causing the misfires' in turn causing the O2 sensors to read as rich telling the ECU to go - fuel trims. Possible?
I admit it's a long shot though.

Here is a question i thought about on the way home from work today.
Both STFT and LTFT react immediately to throttle inputs. They will both swing from -18% to a +% depending on throttle position.
I understand STFT will act that way normally but shouldn't the LTFT remain steady and be very slow to react with small changes over time?

Spareparts said:

-18 on the fuel trims? The LAST thing I'd expect is a vacuum leak.
I don't know why that is stuck in my mind and maybe im not understanding fuel trims.
If it's getting to much fuel the computer goes - on fuel trims
If it is lean the computer is + fuel trims.
Thinking this through if unmetered air was entering the cylinder it would be lean and the computer would add + fuel?

Click to expand...

Yes. Low O2 voltage (excess O2 in the exhaust system) causes the computer to go into rich-command, trying to correct the lean condition.

Spareparts said:

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 assume that if it's misfiring then the O2 sensors would think it's running rich since the exhaust appears to have unburnt fuel in it leading to the O2 sensors to report to the ECU to lean out - the fuel trims?

Click to expand...

They're OXYGEN sensors, not "Fuel" sensors. When the cylinder misfires, the oxygen that SHOULD combine with the hydrocarbon fuel exits into the exhaust "unburned". That O2 is sensed by the O2 sensor, generating a false "Lean" signal to the computer--which enters rich-command trying to correct the false "Lean" indication.

Spareparts said:

The 4, O2 sensors are near new. Not GM but they were not the cheap ones either.
I can not tell you what the voltages are but when i watch it the voltage does move on all 4 sensors.

Click to expand...

Time to connect a scan tool and find out what the sensors--including the O2 sensors--are doing.

Spareparts said:

Crosscounts?

Click to expand...

Some engine computers/scan tools report the number of times the O2 sensor crosses from "Lean" to "Rich", and from "Rich" back to "Lean". Higher numbers are better. A TBI may have single-digit crosscounts at idle, and perk up to the 20s or perhaps higher with more throttle/rpm/load. O2 sensors get "lazy" with age/mileage; they don't react as quickly as a new sensor should.

Newer vehicles/scan tools just show a graph of the sensor voltage, you can see whether the sensor is active or lazy based on how fast the voltage changes.

Spareparts said:

The idle is rough and has misfires.

Click to expand...

Spareparts said:

rock solid at idle 600rpm.

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So the VACUUM is "rock solid" but the engine misfires?

Spareparts said:

Here is a question i thought about on the way home from work today.
Both STFT and LTFT react immediately to throttle inputs. They will both swing from -18% to a +% depending on throttle position.
I understand STFT will act that way normally but shouldn't the LTFT remain steady and be very slow to react with small changes over time?

Click to expand...

1. "Long-term" fuel trim is still a matter of a few seconds.
2. As the throttle position and vacuum as shown by the MAP sensor changes, the computer is switching from one fuelling cell of it's programming to another fuelling cell--which may have different fuel trims in memory.

Spareparts said:

They said there are steady misfires on 4 and 7...

...3 push rods were stuck in the lifters somehow and i have no idea how. Had to use pliers to pull them out. Both on #7 and 1 on #4. They are not bent and appear to be fine.
I did not think to first take the intake off to see why they were stuck unfortunately.

The lifters once removed all appeared fine but i noticed about half were pumped up and the other half i could easily push the plunger down almost bottoming out.
So i don't know what to think about all that.
None of the pushrods had oil in them. Like when you first pull them out usually there is a little running out. These had none.

Click to expand...

Interesting coincidence? 4 and 7 had stuck pushrods, and also have the misfires.

I have NO idea how pushrods can get stuck in a lifter.

Spareparts said:

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 assume that if it's misfiring then the O2 sensors would think it's running rich since the exhaust appears to have unburnt fuel in it leading to the O2 sensors to report to the ECU to lean out - the fuel trims?

Click to expand...

They're OXYGEN sensors, not "Fuel" sensors. When the cylinder misfires, the oxygen that SHOULD combine with the hydrocarbon fuel exits into the exhaust "unburned". That O2 is sensed by the O2 sensor, generating a false "Lean" signal to the computer--which enters rich-command trying to correct the false "Lean" indication.
Trying to wrap my head around this new way of thinking.
A misfire will introduce unused oxygen into the exhaust. The O2 sensor will detect to much oxygen and command the ECU to add more + fuel?

Spareparts said:

The 4, O2 sensors are near new. Not GM but they were not the cheap ones either.
I can not tell you what the voltages are but when i watch it the voltage does move on all 4 sensors.

Click to expand...

Time to connect a scan tool and find out what the sensors--including the O2 sensors--are doing.
Tomorrow i will try to post a video of fuel trim voltages.

Spareparts said:

rock solid at idle 600rpm.

Click to expand...

So the VACUUM is "rock solid" but the engine misfires?
Yes it will vary a little up/down with engine rpm. If i crack the throttle open it will drop a little for a second then back to normal. When i snap the throttle open then closed it will drop under 19 then climb a little over 19 till it returns to idle.
By rock steady i mean the needle does not flutter at all and is steady at idle.

Spareparts said:

Here is a question i thought about on the way home from work today.
Both STFT and LTFT react immediately to throttle inputs. They will both swing from -18% to a +% depending on throttle position.
I understand STFT will act that way normally but shouldn't the LTFT remain steady and be very slow to react with small changes over time?

Click to expand...

1. "Long-term" fuel trim is still a matter of a few seconds.
2. As the throttle position and vacuum as shown by the MAP sensor changes, the computer is switching from one fuelling cell of it's programming to another fuelling cell--which may have different fuel trims in memory.
Understand.

Interesting coincidence? 4 and 7 had stuck pushrods, and also have the misfires.
I have NO idea how pushrods can get stuck in a lifter.
Over 50 years working on engines and i have no idea either. Wish i had though to take the intake off before i removed the pushrods.

Spareparts said:

Trying to wrap my head around this new way of thinking.
A misfire will introduce unused oxygen into the exhaust. The O2 sensor will detect to much oxygen and command the ECU to add more + fuel?

Click to expand...

Exactly right.

Spareparts said:

Tomorrow i will try to post a video of fuel trim voltages.

Click to expand...

Fuel trims are not displayed as "voltage".

My scan tool shows them as numbers, where 128 is "no correction", numbers less than 128 are lean-command, and numbers greater than 128 are rich-command. The scale goes from 0--256, but I don't think I've ever seen numbers less than ~90 or higher than ~160.

Other scan tools show a percentage of correction. That's where you get "-" and "+" numbers.

Spareparts said:

By rock steady i mean the needle does not flutter at all and is steady at idle.

Click to expand...

That's not what I'd expect from an engine that misfires at idle. But that's affected by how "dampened" the vacuum gauge is. A severely-dampened gauge would give fairly steady readings even if the instantaneous vacuum was varying.

Easiest way to get around that is to look at MAP sensor voltage.

Schurkey said:

My scan tool shows them as numbers, where 128 is "no correction", numbers less than 128 are lean-command, and numbers greater than 128 are rich-command. The scale goes from 0--256, but I don't think I've ever seen numbers less than ~90 or higher than ~160.

Click to expand...

Pretty sure ALL OBD2 are in percent, either 0% = no correction, or 100% = no correction.
128, as far as I know is OBD1 only.