number 5 and 6 cylinder missfire??

[smslyguy]

I have a number 5 and number 6 cylinder missfire on my 2000 gmc 5.3 truck. things i have done so far:
changed intake gasket
cleaned injectors (professional cleaning)
changed spark plug wires
tested injectors
changed coils (on number 5 and 6)
spark plugs were replaced about 15,000 miles ago.
compression test
compression leak down test
cleaned m.a.f sensor

Is it possible that this could be a crankshaft positioning sensor? But if it was, wouldn't it be jumping all over the place?
And aslo if it were a o2 senser wouldn't it also missfire on random cylinders, not just number 5 and number 6?
Any sugestions would be appreciated.

[ChipsByAl]

Just to clarify your diagnostics.

1. Were the injectors balance tested on the engine?
2. Does this misfire occur always? Or does it only do it "hot" "cold" "@idle" "under load"
3. What kind of numbers did you get with the compression check? Static/Dynamic
4. Did you get a fuel sample? Water can wreak havoc, but it is usually only on #2 & #7.
5. Have you disabled another cylinder and checked to make sure that one now shows to be misfiring as well?

Did you perform these steps or was it someone else?
Al

[smslyguy]

Just to clarify your diagnostics.

1. Were the injectors balance tested on the engine?
2. Does this misfire occur always? Or does it only do it "hot" "cold" "@idle" "under load"
3. What kind of numbers did you get with the compression check? Static/Dynamic
4. Did you get a fuel sample? Water can wreak havoc, but it is usually only on #2 & #7.
5. Have you disabled another cylinder and checked to make sure that one now shows to be misfiring as well?

Did you perform these steps or was it someone else?
Al

the injectors were not balance test, just ohms tested.
It always missfires on number 6 and then random on number 5
It do doesn't matter if it is cold. Mainly does it at an idle warm or cold.
i got any wheres from 135-150 psi on the compression test.
I have not disable any other cylinders, just goin off of what efi is saying and i get a po306 code as well.
I preformed all these tests.

[ChipsByAl]

The misfires on cylinder #5 are likely false and just residual from #6 true misfires. Try swapping #6 injector with #4 or #8 injectors and see if the misfire follows the injector or stays with #6. You can do this with the coil and/or spark plug/wires to diagnose with no parts costs. If you perform the tests concurrently, don't move all components to the same test cylinder, mix them up.
AL

[smslyguy]

The misfires on cylinder #5 are likely false and just residual from #6 true misfires. Try swapping #6 injector with #4 or #8 injectors and see if the misfire follows the injector or stays with #6. You can do this with the coil and/or spark plug/wires to diagnose with no parts costs. If you perform the tests concurrently, don't move all components to the same test cylinder, mix them up.
AL

how hard is it to "swap" out the injectors on these vehicles. I would assume it woud be an easy job, but i figured it would be really easy to reck the o rings??

[chevy052500hd]

injectors are pretty easy to swap, the o rings shouldn't get trashed as long as you lube them up good. here is a write up from gm's serivice manual on how to remove them.

[smslyguy]

injectors are pretty easy to swap, the o rings shouldn't get trashed as long as you lube them up good. here is a write up from gm's serivice manual on how to remove them.

switched injectors around and still a missfire on number 6. here is a scan Ib also replaced the fuel pressure regulator, which was fine. and sugestions?

[joecar]

Swap the #6 coil with another cylinder as Al said.

Then swap the #6 plug wire.

Also check spark using this, should all 8 look the same:

[smslyguy]

Swap the #6 coil with another cylinder as Al said.

Then swap the #6 plug wire.

Also check spark using this, should all 8 look the same:

i already done that.

[joecar]

You said your compression test was 135-150 psi spread, which cyl# was the 135 on...?

Also, you can use the DVT controls to shut of any one injector... do this:
- set engine running with some load (e.g. in gear, some throttle, assistant has his/her foot on brake to hold vehicle stopped).
- start logging,
- goto DVT tab, select your PCM,
- on DVT tab, goto Engine Control subtab,
- on Injector Control, click on #1, wait a 3 seconds (count out loud), click on Enable All, ,wait 3 seconds...
- repeat this for all 8 cyls.

When done, look at your log, you should see 8 significant dips in RPM (each 3 seconds in length)...

if a cyl was misfiring then its RPM would not have dropped.

Be Safe when doing this.