number 5 and 6 cylinder missfire??

[joecar]

Did you inspect/test the connector for the #6 coil and #6 injector...?

On each of those connectors, what is the voltage to power, is it exactly the same as the other cyls...? A voltage drop indicates a bad connection somewhere in the circuit...

(hmmm... might be an idea to check all ground connections, regardless of the problem.)

After this, if you did all the tests mentioned in this thread, if the problem is still present, it can be categorized as "hard"...

Do you know anyone who has an automotive lab oscilloscope...?

If you do, get them to look at the injector and coil PCM drive voltage waveform, see if #6 is different...

Also get them to look at the crank sensor waveform to see if it is uniform.

There has to be a reason for #6 misfiring like that, each cause has to be eliminated until you find the one...

[joecar]

You mentioned 125 psi... which cylinder has that...?

[smslyguy]

You mentioned 125 psi... which cylinder has that...?

That was number 6, which was a little odd to me. vrs the rest it is below 15% than the rest. I did test the conection at the coils, at the injector, which was all fine as well. Don't know if you were able to read the scan i had in this forum or not, but number 6 was missfire on the count a lot and also then number 5 and then it would jump to number 8, but number 8 would only missfire like one or two times during the cyle. Don't know if this is residule from number 6 or not. I did not test the crankshaft positioning sensor, though.

I am leaning towards the burnt valve, but wouldn't it be using oil? I will re-do the compression leak down test once again to see if it had changed. I sure has the symptoms of a burnt valve, at least to me.

[chevy052500hd]

Engine Compression Test

Charge the battery if the battery is not fully charged.
Disable the ignition system.
Disable the fuel injection system.
Remove all the spark plugs.
Block the throttle plate wide open.
Start with the compression gauge at zero and crank the engine through four compression strokes (four puffs).
Make the compression check for each cylinder. Record the reading.
If a cylinder has low compression, inject approximately 15 ml (one tablespoon) of engine oil into the combustion chamber through the spark plug hole. Recheck the compression and record the reading.
The minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kPa (100 psi). For example, if the highest pressure in any one cylinder is 1035 kPa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kPa (105 psi). (1035 x 70% = 725) (150 x 70% = 105).
Normal -- Compression builds up quickly and evenly to the specified compression for each cylinder.
Piston Rings Leaking -- Compression is low on the first stroke. Compression then builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.
Valves Leaking -- Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.
If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression, the cause may be a head gasket leaking between the cylinders.


May help you better diagnose it ...

[smslyguy]

Engine Compression Test

Charge the battery if the battery is not fully charged.
Disable the ignition system.
Disable the fuel injection system.
Remove all the spark plugs.
Block the throttle plate wide open.
Start with the compression gauge at zero and crank the engine through four compression strokes (four puffs).
Make the compression check for each cylinder. Record the reading.
If a cylinder has low compression, inject approximately 15 ml (one tablespoon) of engine oil into the combustion chamber through the spark plug hole. Recheck the compression and record the reading.
The minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kPa (100 psi). For example, if the highest pressure in any one cylinder is 1035 kPa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kPa (105 psi). (1035 x 70% = 725) (150 x 70% = 105).
Normal -- Compression builds up quickly and evenly to the specified compression for each cylinder.
Piston Rings Leaking -- Compression is low on the first stroke. Compression then builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.
Valves Leaking -- Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.
If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression, the cause may be a head gasket leaking between the cylinders.


May help you better diagnose it ...

Thanks for the info, I did notice it said: Valves Leaking -- Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.
Which is not the case i have here.

[joecar]

Yes, I was able to read your log file.


Have you changed any components on the rotating assembly (anything betwwen/including pulley to flywheel/flexplate)...?

Maybe try doing a CASE relearn, and then see what you get for misfires.

[joecar]

Do this: remove all the spark plugs, and for each cylinder, bring it to TDC and carefully measure the depth of the piston top from the spark plug hole (i.e. check for bent rod on #6)...

Also, if you can easily remove the valve covers, check #6 for loose rocker or bent rod/valve.

There has to be a reason.

[smslyguy]

Yes, I was able to read your log file.


Have you changed any components on the rotating assembly (anything betwwen/including pulley to flywheel/flexplate)...?

Maybe try doing a CASE relearn, and then see what you get for misfires.

no, nothing of that sort. For the case relearn is it as simple as hitting the "case re-learn" button, or do you have to bring it up to 5,000r.p.m. until the engine stumbles and then shut the ignition off???

[joecar]

CASE relearn steps:
- first warm up engine (ECT must be in range),
- apply brake,
- in Park/Neutral,
- press CASE relearn button,
- bring RPM to above 4000 or 4500 (must do within 10 seconds),
- when it stumbles immediately release throttle,
- let engine spin itself down to idle,
- shut off engine (turn key all the way off) and wait 15+ seconds.

[smslyguy]

CASE relearn steps:
- first warm up engine (ECT must be in range),
- apply brake,
- in Park/Neutral,
- press CASE relearn button,
- bring RPM to above 4000 or 4500 (must do within 10 seconds),
- when it stumbles immediately release throttle,
- let engine spin itself down to idle,
- shut off engine (turn key all the way off) and wait 15+ seconds.

i did the case re-learn with no luck. I also redid the leak down test on number 6, and after increasing the air pressure to 120 psi it held pressure. Although after the test i started the engine and devolped a knock, which to me would indicate a sticky valve. just kinda guessing here?? any other reasons as to why i have the knock now???