Random misfire at idle 454 vortec

[elevatorman]

Im back with an update have had the rona and plumbing issues so i am back on the truck.
Ive logged all the data i can and ive noticed something weird with number 7 and 4 any time i try too bring the cylinder up tdc the exhaust valves open befor i get there. Ive repeated both of those cylinders numerous times with the same results.
My data below check mark means air heard. X nothing heard.

 

[GoToGuy]

Did you ever replace the O2 sensors, they can wear out, get lazy ,slow to react.
Are you checking these in firing order or just one after the other?
The three digit numbers, are you using a total compression gauge spinning the engine with the starter getting two or three compression cycles on gauge?
Are you using a Cylinder Compression Leakdown test gauge? Input airline no more than 100 psi. ?
This test is where it's easy to understand the concept, but tricky to understand without guidance , experience, what constitutes normal or cause for concern as in worn or defective engine internals.
What do you mean " exhaust valve open before your at TDC " ?
Then your not at TDC. 4 cycle engine. Int. Com. Pwr. Ex.
On aircraft it's easy to see when your approaching TDC, we have the input air closed, as you turn the prop on compression you will see the gauge needle start to move and feel pressure build, stop at TDC, open input air, read differential loss.
On the truck turn the engine by hand, ( long handle ratchet on crank) experienced old dogs can feel by plugging spark hole when approaching TDC, then confirm by piston position visually or by feel, valve movement and or balancer marks.
On 7 and 4 , why no numbers, if your getting 100 plus during dynamic testing , you therefore have compression at TDC.
Your test is telling you your doing it wrong.
Your average is 150 plus, I think your wasting time on a differential Leakdown test. Focus on ignition, fuel related and diagnostic scanned information.
" Blue Streak " is a division of " Standard Auto Parts " a huge international auto parts company. They don't belong to anyone. Many auto parts stores sell their parts.

 

[Schurkey]

ive noticed something weird with number 7 and 4 any time i try too bring the cylinder up tdc the exhaust valves open befor i get there.

How do you know the exhaust valve is "open"? Are you watching the rocker arms? Or are you hearing the escaping air from the tailpipe?

If you're watching the rockers:
7 and 4 are companion cylinders. When one is at TDC-Compression (both valves closed) the other is at TDC-Exhaust (both valves open slightly on overlap, exhaust closing, intake opening.)

If an exhaust valve opens before you get to TDC...you're somewhere on the power stroke going into the exhaust stroke, the exhaust valve opens (power stroke) and closes just after TDC exhaust stroke, between exhaust and intake.

If you're bringing #4 up to TDC and the exhaust valve is closing, you'd be testing #7, because it's coming up on TDC-compression.

If you're listening for air escaping, you've probably got burnt exhaust valves/seats that CANNOT seal properly. You'll need to pop a valve cover to see rocker arm motion to determine TDC-Compression vs. TDC-Exhaust. See when the intake valve closes, towards the beginning of the compression stroke, then bring the piston up to TDC from there.

OR

My standard operating procedure is to set my leakdown tester with a minimum amount of input pressure--10 psi, 15 psi--something like that. Then plug the thing into the spark plug hole. Generally, air can be heard hissing from various orifices, and no pressure showing on the second gauge. As I turn the crank, eventually the hissing stops, and I see pressure on the second gauge. Then I bump the crank the minimum amount possible--the pressure jumps up, and then reduces. Bump again, pressure jumps up, then reduces. Bump, bump, bump--at some point I bump the crank and the pressure DROPS, then recovers. That tells me I've gone too far. Back the crank up several degrees. It's often possible to "feel" TDC through the ratchet handle or breaker bar I'm using to turn the crank. THIS WORKS, but only if the valve leakage is minimal. IF the valves hardly seal, you'd never see pressure on the second gauge, and you're back to looking at rocker arms.

Once I've found TDC via watching the pressure on the gauge and feeling the effort on the ratchet handle, I re-set my input pressure to 100 psi (90 if I were testing aircraft, 'cause that's what the FAA calls for--but the math is more involved and I'm allergic) and read the second gauge for my indicated leakage.

 

[elevatorman]

How do you know the exhaust valve is "open"? Are you watching the rocker arms? Or are you hearing the escaping air from the tailpipe?

If you're watching the rockers:
7 and 4 are companion cylinders. When one is at TDC-Compression (both valves closed) the other is at TDC-Exhaust (both valves open slightly on overlap, exhaust closing, intake opening.)

If an exhaust valve opens before you get to TDC...you're somewhere on the power stroke going into the exhaust stroke, the exhaust valve opens (power stroke) and closes just after TDC exhaust stroke, between exhaust and intake.

If you're bringing #4 up to TDC and the exhaust valve is closing, you'd be testing #7, because it's coming up on TDC-compression.

If you're listening for air escaping, you've probably got burnt exhaust valves/seats that CANNOT seal properly. You'll need to pop a valve cover to see rocker arm motion to determine TDC-Compression vs. TDC-Exhaust. See when the intake valve closes, towards the beginning of the compression stroke, then bring the piston up to TDC from there.

Im watching the rocker arms and the tdc mark on the balancer. But does the timing mark actually matter on any cylinder other than number 1.

 

[elevatorman]

Did you ever replace the O2 sensors, they can wear out, get lazy ,slow to react.
Are you checking these in firing order or just one after the other?
The three digit numbers, are you using a total compression gauge spinning the engine with the starter getting two or three compression cycles on gauge?
Are you using a Cylinder Compression Leakdown test gauge? Input airline no more than 100 psi. ?
This test is where it's easy to understand the concept, but tricky to understand without guidance , experience, what constitutes normal or cause for concern as in worn or defective engine internals.
What do you mean " exhaust valve open before your at TDC " ?
Then your not at TDC. 4 cycle engine. Int. Com. Pwr. Ex.
On aircraft it's easy to see when your approaching TDC, we have the input air closed, as you turn the prop on compression you will see the gauge needle start to move and feel pressure build, stop at TDC, open input air, read differential loss.
On the truck turn the engine by hand, ( long handle ratchet on crank) experienced old dogs can feel by plugging spark hole when approaching TDC, then confirm by piston position visually or by feel, valve movement and or balancer marks.
On 7 and 4 , why no numbers, if your getting 100 plus during dynamic testing , you therefore have compression at TDC.
Your test is telling you your doing it wrong.
Your average is 150 plus, I think your wasting time on a differential Leakdown test. Focus on ignition, fuel related and diagnostic scanned information.
" Blue Streak " is a division of " Standard Auto Parts " a huge international auto parts company. They don't belong to anyone. Many auto parts stores sell their parts.

Im using a leakdown down gauge im spinning over until i hear compression out of my hose inserted into the spark plug hole and then rotating by hand too tdc on the balancer which has been correct it appears for 6 of the cylinders if i go past the mark i wont build pressure.
I am putting 100 psi too each cylinder.
As far as the ecm i have tried too get a different one working but i could not get past the security key too reprogram. So ive stepped back away from that and put the blue streak back in which i didnt put in previous owner had.

 

[Schurkey]

Im watching the rocker arms and the tdc mark on the balancer. But does the timing mark actually matter on any cylinder other than number 1.

For typical Chevy firing order--and other engines using the same firing order and timed off of #1--timing mark on the damper matter for cylinders 1 and 6. When 1 is at TDC-Compression, 6 is at TDC-Exhaust, and vice-versa.

The timing mark on the damper means NOTHING for any of the other cylinders. The rocker arms and/or cylinder pressure tells all. I prefer the cylinder pressure method as described above because the valves are open so little on overlap that they might look closed--or the lifters bleed down and they ARE closed. You really do have to watch them to be sure that they're closed and not on overlap.

 

[Schurkey]

im spinning over until i hear compression out of my hose inserted into the spark plug hole and then rotating by hand too tdc on the balancer

Compression out of the hose, and then only to the top of that stroke, not all the way around to align the damper with the timing indicator.

I am freakin' amazed that when you turn the crank "too far" to align the damper and indicator, that when you finally apply pressure to the cylinder, it doesn't spin the crankshaft.

There's two reasons to have the piston at TDC when doing a cylinder leakdown test:
1. The piston is at the top of the bore, which traditionally has been the most-worn part of the cylinder, leading to highest indicated leakage, and
2. The connecting rod geometry means there's no rotational force on the crank. Get the rod/rod journal offset from TDC more than a few degrees, and pressure on the piston tends to turn the crank so the piston goes down to BDC.


There are some folks who want to do cylinder leakdown testing at ~12 degrees after TDC. They figure that's about where peak cylinder pressure occurs, so that's where testing should occur. But they have to mechanically lock the crankshaft in place or the pressure will spin it, and the piston heads down the bore to BDC. I cannot be bothered to do it this way. Never, ever, not once.

Hazzard Fraught used to sell a "leakdown tester" that according to the instructions, was to be used with the piston at BDC, and 15 psi. Fookin' junk, from a seller famously known for selling Tool-Shaped Objects that aren't actual tools.

 

[GoToGuy]

If you don't think that being a few degree's before or few after TDC matters , hold your hand right here while I bump this propeller a few degrees forward. It WILL hit you hard. If it goes in reverse, the trailing edge is sharp, it WILL send you to the the emergency room.
You got all plugs out, turn the crank by hand, use the hose off the compression tester to feel when it coming up on TDC. Spinning with starter waiting for the whoosh it's already past TDC. There's hardly any crank resistance with all the plugs out.
Using the starter just complicates it. Every time you hit the starter, i.e., fuel pump and injectors ? Coil grounded ? All that excess fuel vapor + wild spark = a very bad day.

 

[Schurkey]

It never occurred to me that he'd be spinning it over with the starter motor.

Yeah, that'd be bad.

 

[Road Trip]

Im back with an update have had the rona and plumbing issues so i am back on the truck.
Ive logged all the data i can and ive noticed something weird with number 7 and 4 any time i try too bring the cylinder up tdc the exhaust valves open befor i get there. Ive repeated both of those cylinders numerous times with the same results.
My data below check mark means air heard. X nothing heard.

Elevatorman,

THANK YOU for following through and capturing the requested data for both
the Compression Test *and* the Leakdown Test. Listen, I'm flirting with running
late for an appt. at the VA hospital, but thanks to the results of your leakdown data
I think I've had a troubleshooting Eureka moment. So to get this out asap for the
GMT400 community to review, here's my thoughts, bullet style:

* The *camshaft* is the mechanical computer that sets the firing order for the engine.
* The PO installed a camshaft featuring the "4-7 swap". (This is popular with the outer orbit motorhead devos for reasons we don't care about right now.)
* The factory firing order that we all know, love, and can recite anytime, anywhere: 1-8-4-3-6-5-7-2.
* Hot, hip 4-7 swap firing order: 1-8-7-3-6-5-4-2. (Note: It does remove the "5-7" adjacent firing & moves it to 4-2? Taming 5-7 hot spot is claimed?)

* A compression test will NOT find a firing order change.
* A leakdown test performed sequentially (every 90° of crank rotation) will uncover a non-standard firing order.
* And if the cam has the 4-7 swap, you will end up with *exactly* the test results that you documented, for you
will end up attempting to test cylinders #4 and #7 in the vicinity of the Exhaust/Intake overlap {360° out} instead
of the both valves closed top of the compression stroke. (!)

While I'm taking care of business, check out something I drew to help you follow the troubleshooting theory:

You must be registered for see images attach

NOTE: I've attached a copy of your test results so that you can use them against the chart above.

Q1: How are your spark plug wires run? I'm assuming the factory firing order to match the cap?
Or did you replace them one at a time, and maybe the PO followed the new cam's firing order?

Q2: Is the wiring harness for the 8 sequential fuel injectors still stock, or did the PO attempt
to modify/hack the wiring harness to follow the new camshaft firing order?

Q3: Could you attempt to prove/disprove the firing order that's currently installed in your truck?

****

More later, gotta show up for my Dr's appt.

Best of luck figuring out exactly where and how much total 'value-added' the PO left behind in the
engine bay besides the aforementioned inline resistor to one of your sensors.

Cheers --