Random misfire at idle 454 vortec

[elevatorman]

Also i did briefly manage too get the truck too idle perfectly 10 seconds. While the truck was running i turned on injector balance test too cylinder 1. It was the smooth as glass.
Im really starting too wonder if the driver for injector 1 is lagging. All the injector wiring ohms 0.2 ground side also. Balance test shows all 8 bleeding down the same.

 

[Road Trip]

Ok so i was initially not performing the leakdown correctly i was under the impression that you apply x amount of air and watch it hold without a constant air source. So that being said i repeated the process correctly and at 100 psi inlet side cylinder pressures were all at 90 psi or 88. Air was observed slightly at crank exhaust and throttle body not sure what too make of it.

Thanks for coming back with that status report on your test procedure.

Note: Just about everyone reading this has hit similar snags while familiarizing
ourselves with getting accurate results from compression and leakdown tests. Good
on you for figuring out how the sideways data was generated -- that's how we
all gain the experience we need to proceed down the path towards the goal of
truly successful DIY. :0)

****

Alright, I decided to stop and reread the entire thread. (In a subsequent reply
I will summarize all that you've tried so far in order for everyone to have a
consistent troubleshooting perspective.)

In order to make troubleshooting progress, we first need to figure out which
of the 2 possible scenarios that your engine bay falls into:

1) Engine is mechanically sound, and it's ability to control air flow at all speeds & loads is a steady constant.

(Intake manifold not leaking to crankcase or atmosphere, Intake & Exhaust valve
seating is 100%, piston rings 90+%, not pressurizing the cooling system, etc.)

In Scenario #1, the engine is the victim of engine management getting it wrong, whether
it's ignition or fueling anomaly.

2) Engine has marginal/intermittent ability to control air flow, due to worn valves, seats, guides, etc.

O2 sensors are Oxygen sensors, not Complete Combustion sensors. As Schurkey is quick to
point out, good combustion uses up the oxygen that cylinder was fed, so little/no oxygen is pushed
past the exhaust valve. On the other hand, a misfire = the oxygen that went past the intake valve
into the cylinder now leaves the cylinder via the exhaust valve & right by the O2 sensor.

This is what he means by a 'False Lean'. Which the computer (mis)interprets as a need to
add fueling to the Long Term trims. If you think about it, a misfiring cylinder on a bank ends up
changing the fueling strategy for the other 3 cylinders on that same bank. And given the
fact that we are operating in a feedback (closed) loop, the troubleshooting view gets pretty
foggy/counterintuitive in a hurry.

In Scenario #2, the engine is the perpetrator of the engine not running smoothly, and
the engine management is the victim in this scenario. And no matter how much we
attempt to fix a correctly-working engine mgmt setup, it still won't operate properly.
(For example, the VCM engineers will tell you that it's a prerequisite/assumption that the
engine is leak-free at all possible interfaces *before* you can expect their system to
act/react (interact) properly in normal use.

****

A lot has been done so far to make sure the Ignition and Fueling are as new. Renewing
the intake manifold gaskets to try and resolve the fuel trims was a good step. Also
the replacement of the timing chain was something that I would have also done in your
shoes. CMP <> CKP correlation is verified at 0° is a solid.

And I am assuming that the MAF, MAP, and front O2 sensors are at least good enough
to date that they don't cause the SES (Check Engine Light) to come on. (Or is the
light always on because of the deleted cats/rear O2 sensors, so other codes could be
hiding behind this?)

And when relying upon the Check Engine Light, we have to keep in mind that (per the
EPA rulebook) ...the VCM coders don't have to turn the light on until the problem is
severe enough that is causes the tailpipe emissions to be +50% higher than what the
vehicle calibration would normally output. Guess what -- in the real world this means
that your big block can be a little rough/stumble a bit at idle with no light showing. (!)

****

Given all of the above, in order to not chase my troubleshooting tail, if this truck was mine
I would want to do all I could to prove to myself that the engine is mechanically right.
Because if I don't then I run the risk of trying to fix an engine mgmt system that's
twisted itself up into knots because the root cause of the intermittent misfiring is the
engine itself. (!)

So I'd recommend that we gather enough data that we can follow it to wherever it takes us.
I scribbled up a quick Compression + Leak-down Score Card that I'm hoping that you
will fill out and post the results. (See attachment #1.)

And in Attachment #2 is how I accuratize my observations while performing a careful
leakdown test. You see, the sound of high frequency air hissing can bounce around
off of all the different hard surfaces in an engine bay, leading to my inability to discern
if a low-level leak is from the 454's oil filler tube, throttle body, or even the radiator
opening. (The exhaust pipe is a lot easier...unless of course it's rusted out.)

But when you add the paper towel tube to the testing, and hold it to your ear
while placing the other end in the aforementioned locations, it becomes a no-brainer.

You know, when I ask a shop how did the compression test go, and they say 'good',
all within 15% of each other, with no further details, my head drops a little in
disappointment.

On the other hand, when a different shop has it all written down, and they say
that 6 of the cylinders were reading 175psi, but the remaining 2 were only 155.
And when their observations are written down on a piece of paper, and you see
that the 2 weaker cylinders are #3 & #5, and that they are physically *adjacent*,
well now we have some actionable info. No doubt that there's a good chance that
the head gasket between the 2 weakest cyliinders is on the way out.

The point I'm trying to make is that if you were to take the time to map out the
Compression Test PSI as well as the Leakdown Test % loss on a cylinder by cylinder
basis, and further add precisely where you pinpointed the source of any leaks during
the leakdown test...well then we will really know IF your engine can actually run smoothly
while being fed a emissions-compliant diet.

IF so, then we can go ahead start trying to sort out if you have aged/lazy O2 sensors, do
you have a marginal MAF, what's the true health of the wiring harness, does your
VCM have leaky electrolytic capacitors affecting your A/D converters *or* your
injector drivers, etc., etc.)

Because if you have one or more intermittent/marginal kinda burned exhaust valves
in place, then there's no need for worry about anything in the paragraph above
before we first fix the underlying mechanical issues.

I don't mind a problem to fix, but I hate a mystery. And you've already worked
hard enough on your big block GMT400 that I'd like to help sort this out if at
all possible. Fill out that score card and let us know what you find.

Tag, you're it. :0)

Fingers crossed. Cheers --

 

[Schurkey]

i repeated the process correctly and at 100 psi inlet side cylinder pressures were all at 90 psi or 88. Air was observed slightly at crank exhaust

"Crank exhaust"? You mean crankcase ventilation--the oil fill cap, or the PCV grommet, or somesuch?

and throttle body not sure what too make of it.

Leakage at the throttle body indicates the intake valve is not sealing.

The first thing to do is to lightly "pop" the intake valve (or the valve-tip end of the intake rocker arm) in question with a rubber mallet/plastic-tipped hammer. The hammer knocks the valve off it's seat an eighth of an inch, and the valve spring slams it back. This crushes any carbon deposits that "might" be holding the valve slightly open.

If this doesn't seal the valve, the next step is to loosen the rocker arm adjuster nut a half-turn and retest. MAYBE the valvetrain wear is more than the hydraulic lifter's plunger travel. Or maybe that rocker wasn't adjusted properly to begin with.

If that doesn't fix it...you'll be wanting to plan for cylinder head removal to resurface the valve and seat, and to assure that the guide is in good shape.

You didn't mention noise at the tailpipe, or bubbles in the radiator. I'm guessing the exhaust valve is sealing OK, and there's no cylinder leakage into the cooling system.

 

[Carlaisle]

Also i did briefly manage too get the truck too idle perfectly 10 seconds. While the truck was running i turned on injector balance test too cylinder 1. It was the smooth as glass.
Im really starting too wonder if the driver for injector 1 is lagging. All the injector wiring ohms 0.2 ground side also. Balance test shows all 8 bleeding down the same.

Are you running the black box ECM? If memory serves, it only has two injector drivers - one for each bank of cylinders.

 

[Road Trip]

Are you running the black box ECM? If memory serves, it only has two injector drivers - one for each bank of cylinders.

Maybe you are remembering the stock '95-earlier 2-injector TBI 7.4 setup?

Here's the Fuel Injector wiring harness I found in Vol. 3 of the '99 FSM for the '96+ Vortec 7.4 motor:

You must be registered for see images attach

Note: By default the VCM drives the individual injectors by following the firing order. (Sequential multi-port mode)

My understanding is that the computer has the capability to revert to batch-fire mode if
certain inputs go bad. (ie: Good or good enough is better than all or nothing.)

For folks still learning about reading electrical circuits, note that power is *always* applied to one
side of all 8 injectors (with key in RUN or START positions) ...so each injector is opened by the
VCM *grounding* the other connection. It takes current flow for the injector to be opened.
(This is why that elsewhere in this forum folks have found marginal/damaged fuel injection
harnesses were causing a problem. If you are down to a single strand still working you may or may
not have enough current to turn the coil inside the injector into a strong-enough electromagnet
to cycle the moving bits open against the closing spring.) EDIT: See attached.

One last thing -- in the last few GMT400 years, GM adopted the convention of putting the +12v
source(s) at the top of each page, and the circuit ground at the bottom. I personally find this
much easier to digest than the multi-page spaghetti of the older style schematics. :0)

So, the OP was on the right track when he was verifying the continuity of the wiring harness
to each of the 8 injectors.

Hope this clarifies what's in place on the OP's '97.

 

[Carlaisle]

Maybe you are remembering the stock '95-earlier 2-injector TBI 7.4 setup?

Not sure what I'm remembering now, but it's clearly not the stock L29 ECM.

 

[elevatorman]

Thanks for coming back with that status report on your test procedure.

Note: Just about everyone reading this has hit similar snags while familiarizing
ourselves with getting accurate results from compression and leakdown tests. Good
on you for figuring out how the sideways data was generated -- that's how we
all gain the experience we need to proceed down the path towards the goal of
truly successful DIY. :0)

****

Alright, I decided to stop and reread the entire thread. (In a subsequent reply
I will summarize all that you've tried so far in order for everyone to have a
consistent troubleshooting perspective.)

In order to make troubleshooting progress, we first need to figure out which
of the 2 possible scenarios that your engine bay falls into:

1) Engine is mechanically sound, and it's ability to control air flow at all speeds & loads is a steady constant.

(Intake manifold not leaking to crankcase or atmosphere, Intake & Exhaust valve
seating is 100%, piston rings 90+%, not pressurizing the cooling system, etc.)

In Scenario #1, the engine is the victim of engine management getting it wrong, whether
it's ignition or fueling anomaly.

2) Engine has marginal/intermittent ability to control air flow, due to worn valves, seats, guides, etc.

O2 sensors are Oxygen sensors, not Complete Combustion sensors. As Schurkey is quick to
point out, good combustion uses up the oxygen that cylinder was fed, so little/no oxygen is pushed
past the exhaust valve. On the other hand, a misfire = the oxygen that went past the intake valve
into the cylinder now leaves the cylinder via the exhaust valve & right by the O2 sensor.

This is what he means by a 'False Lean'. Which the computer (mis)interprets as a need to
add fueling to the Long Term trims. If you think about it, a misfiring cylinder on a bank ends up
changing the fueling strategy for the other 3 cylinders on that same bank. And given the
fact that we are operating in a feedback (closed) loop, the troubleshooting view gets pretty
foggy/counterintuitive in a hurry.

In Scenario #2, the engine is the perpetrator of the engine not running smoothly, and
the engine management is the victim in this scenario. And no matter how much we
attempt to fix a correctly-working engine mgmt setup, it still won't operate properly.
(For example, the VCM engineers will tell you that it's a prerequisite/assumption that the
engine is leak-free at all possible interfaces *before* you can expect their system to
act/react (interact) properly in normal use.

****

A lot has been done so far to make sure the Ignition and Fueling are as new. Renewing
the intake manifold gaskets to try and resolve the fuel trims was a good step. Also
the replacement of the timing chain was something that I would have also done in your
shoes. CMP <> CKP correlation is verified at 0° is a solid.

And I am assuming that the MAF, MAP, and front O2 sensors are at least good enough
to date that they don't cause the SES (Check Engine Light) to come on. (Or is the
light always on because of the deleted cats/rear O2 sensors, so other codes could be
hiding behind this?)

And when relying upon the Check Engine Light, we have to keep in mind that (per the
EPA rulebook) ...the VCM coders don't have to turn the light on until the problem is
severe enough that is causes the tailpipe emissions to be +50% higher than what the
vehicle calibration would normally output. Guess what -- in the real world this means
that your big block can be a little rough/stumble a bit at idle with no light showing. (!)

****

Given all of the above, in order to not chase my troubleshooting tail, if this truck was mine
I would want to do all I could to prove to myself that the engine is mechanically right.
Because if I don't then I run the risk of trying to fix an engine mgmt system that's
twisted itself up into knots because the root cause of the intermittent misfiring is the
engine itself. (!)

So I'd recommend that we gather enough data that we can follow it to wherever it takes us.
I scribbled up a quick Compression + Leak-down Score Card that I'm hoping that you
will fill out and post the results. (See attachment #1.)

And in Attachment #2 is how I accuratize my observations while performing a careful
leakdown test. You see, the sound of high frequency air hissing can bounce around
off of all the different hard surfaces in an engine bay, leading to my inability to discern
if a low-level leak is from the 454's oil filler tube, throttle body, or even the radiator
opening. (The exhaust pipe is a lot easier...unless of course it's rusted out.)

But when you add the paper towel tube to the testing, and hold it to your ear
while placing the other end in the aforementioned locations, it becomes a no-brainer.

You know, when I ask a shop how did the compression test go, and they say 'good',
all within 15% of each other, with no further details, my head drops a little in
disappointment.

On the other hand, when a different shop has it all written down, and they say
that 6 of the cylinders were reading 175psi, but the remaining 2 were only 155.
And when their observations are written down on a piece of paper, and you see
that the 2 lower cylinders are #3 & #5, and that they are *adjacent*, well now we
have some actionable info. No doubt that there's a good chance that the head gasket
between the 2 weakest cyliinders is on the way out.

The point I'm trying to make is that if you were to take the time to map out the
Compression Test PSI as well as the Leakdown Test % loss on a cylinder by cylinder
basis, and further add precisely where you pinpointed the source of any leaks during
the leakdown test...well then we will really know IF your engine can actually run smoothly
while being fed a emissions-compliant diet.

IF so, then we can go ahead start trying to sort out if you have aged/lazy O2 sensors, do
you have a marginal MAF, what's the true health of the wiring harness, does your
VCM have leaky electrolytic capacitors affecting your A/D converters *or* your
injector drivers, etc., etc.)

Because if you have one or more intermittent/marginal kinda burned exhaust valves
in place, then there's no need for worry about anything in the paragraph above
before we first fix the underlying mechanical issues.

I don't mind a problem to fix, but I hate a mystery. And you've already worked
hard enough on your big block GMT400 that I'd like to help sort this out if at
all possible. Fill out that score card and let us know what you find.

Tag, you're it. :0)

Fingers crossed. Cheers --

Thank you very much for this breakdown. I will be doing the test over again over the next few days and i will report the results back.

 

[elevatorman]

"Crank exhaust"? You mean crankcase ventilation--the oil fill cap, or the PCV grommet, or somesuch?


Leakage at the throttle body indicates the intake valve is not sealing.

The first thing to do is to lightly "pop" the intake valve (or the valve-tip end of the intake rocker arm) in question with a rubber mallet/plastic-tipped hammer. The hammer knocks the valve off it's seat an eighth of an inch, and the valve spring slams it back. This crushes any carbon deposits that "might" be holding the valve slightly open.

If this doesn't seal the valve, the next step is to loosen the rocker arm adjuster nut a half-turn and retest. MAYBE the valvetrain wear is more than the hydraulic lifter's plunger travel. Or maybe that rocker wasn't adjusted properly to begin with.

If that doesn't fix it...you'll be wanting to plan for cylinder head removal to resurface the valve and seat, and to assure that the guide is in good shape.

You didn't mention noise at the tailpipe, or bubbles in the radiator. I'm guessing the exhaust valve is sealing OK, and there's no cylinder leakage into the cooling system.

Im a little confused on what is acceptable for the leakdown. Any air coming from the intake and exhaust is bad?
Or if your readings still fall within a 10% drop your good too go.
Also the air observed from crankcase was at the oil fill tube.

 

[Road Trip]

Im a little confused on what is acceptable for the leakdown. Any air coming from the intake and exhaust is bad?
Or if your readings still fall within a 10% drop your good too go.
Also the air observed from crankcase was at the oil fill tube.

A small amount of air leakage past the piston rings is inevitable, normal & acceptable.
(Note: The lower the better, especially for racers chasing after the last possible erg.)

On the other hand, no leaking from the intake or exhaust valves is acceptable
during a Leakdown Test if your goal is to drive a truly smooth running engine.

If all 8 cylinders pass with this static testing, then we can then discuss a technique
to verify proper dynamic valvetrain operation while idling. (Think measured
response in troubleshooting land.)

Saying it another way, a Compression & Leakdown test can identify a physical
issue that's so bad that it shows up with immobile parts...but it doesn't mean that this
completely verifies valvetrain operation while in motion.

Even so, the results of these tests provide a valuable foundation for further troubleshooting.

Good questions. Glad you are paying really close attention to all this.

Cheers --

 

[Schurkey]

As far as I'm concerned--and others may disagree:

I accept ZERO leakage past an intake valve.
I accept ZERO leakage past an exhaust valve.
I accept ZERO leakage into the cooling system.
ANY leakage there is cause for concern if not outright teardown 'n' repair.

There is always leakage past the rings into the crankcase. The amount varies with about a dozen factors. Since there is NO law/regulation or "industry standard" for the construction of AUTOMOTIVE leakdown testers, the indicated leakage on the tester gauge can vary from one brand or model of tester to another, when testing the same cylinder.

There are three ways to make sense of leakdown tester indicated leakage results:
1. You've tested engines of similar bore size to the one in question, you know how "your" tester reacts to good engines and to bad/leaking/worn-out/damaged engines.
2. You take a page from the AIRCRAFT leakdown testers which are regulated by the FAA so as to produce consistent results among all brands/models of AIRCRAFT leakdown testers. Buy a leakdown tester sold for AIRCRAFT engine usage, for cylinder bores having a diameter of 5 inches or smaller (which suits virtually all automotive engines.) Aircraft leakdown testers for cylinders having a larger-than-5" bore use a bigger internal orifice, (.060 vs. .040) they'll show incorrect results on smaller-bore engines including automotive engines.
3. Use a "Master Orifice" similar or identical to what Teledyne/Continental specifies for their aircraft engines. It's nothing more than a calibrated orifice that "leaks" the maximum amount they allow their engine's cylinders to have. You plug the Master Orifice into your leakdown tester, and let's say it registers 79% leakage. You remove the master orifice, and plug the tester into your cylinder like normal--any reading better than 79% (or whatever the Master Orifice produced) is acceptable.

The second leakdown tester I bought came with a Master Orifice built-in, and made to Teledyne/Continental specs. The only issue I have with it is that I think it allows more leakage than I'm comfortable with--their specs are too generous/lenient. I use the Master Orifice, but then I only allow about 2/3 the leakage that the MO produces as my maximum allowable leakage.

I have heard that some folks make their own Master Orifices from some brass fittings and a Holley main jet. I don't know which size main jet is appropriate.

This is the leakdown tester I bought, although it was a decade ago. I didn't get the plastic case, or the tubular extension with mine. And it normally ships with an 18mm spark-plug adapter when our engines have 14mm spark plugs. They'll ship with the 14mm if you tell them...or you can buy a variety of spark plug adapters to fit cars, trucks, aircraft, motorcycles. Just let them know.
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