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.

Were the injectors balance tested on the engine?
Does this misfire occur always? Or does it only do it "hot" "cold" "@idle" "under load"
What kind of numbers did you get with the compression check? Static/Dynamic
Did you get a fuel sample? Water can wreak havoc, but it is usually only on #2 & #7.
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.

Were the injectors balance tested on the engine?
Does this misfire occur always? Or does it only do it "hot" "cold" "@idle" "under load"
What kind of numbers did you get with the compression check? Static/Dynamic
Did you get a fuel sample? Water can wreak havoc, but it is usually only on #2 & #7.
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.

[chevy052500hd]

check the spark plug for cracks, hell even replace it 15,000 miles is a short period but I have seen ones bad, I have had to replace them at 10,000 on my truck when I had the supercharger on. If not, it must be mechanical, sticking valve, or the start of a head gasket failure.

[smslyguy]

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.

I would assume the the number 6 is the correct missfire. I also have a po306 code along with the number 6missfire on the scan. Not really sure what this is supposed to accomplish? Are you saying that maybe this is a false missfire?

[smslyguy]

check the spark plug for cracks, hell even replace it 15,000 miles is a short period but I have seen ones bad, I have had to replace them at 10,000 on my truck when I had the supercharger on. If not, it must be mechanical, sticking valve, or the start of a head gasket failure.

swaped the number 6 plug for the number 4 plug still number 6 missfire. If it were a start of a head gasket why would it still have compression and also hold compression??

[smslyguy]

check the spark plug for cracks, hell even replace it 15,000 miles is a short period but I have seen ones bad, I have had to replace them at 10,000 on my truck when I had the supercharger on. If not, it must be mechanical, sticking valve, or the start of a head gasket failure.

actually now that you mention it number 6 was a little low on the compression test. it was 125 and number 2 was also lower than the rest at 130

[chevy052500hd]

It may be possible that the head gasket is breaking down slowly, they did this on my duramax, I started noticing the coolant system was still pressurized when it was stone cold, then slowly started feeling a miss, and now it is eating coolant. It's not a complete failure yet just slowly starting .... been seeing this more on cars with aluminum heads and iron blocks in high mileage situations. Any condensation under the oil cap?

[smslyguy]

It may be possible that the head gasket is breaking down slowly, they did this on my duramax, I started noticing the coolant system was still pressurized when it was stone cold, then slowly started feeling a miss, and now it is eating coolant. It's not a complete failure yet just slowly starting .... been seeing this more on cars with aluminum heads and iron blocks in high mileage situations. Any condensation under the oil cap?

none and it doesn't use any coolent either.

[joecar]

Get a block test kit from part store, $50, it tests for presence of combustion gases in coolant.

[smslyguy]

Get a block test kit from part store, $50, it tests for presence of combustion gases in coolant.

If it were a "burnt" valve it wouldn't hold compression in that cylinder. In this case it does hold. And if it were leaking combustion gases in the coolent it would be evaporating the coolant or boiling it. and it doesn't do that either.

[joecar]

Did you also do a leakdown test which is a bit different than a compression test...?

[joecar]

If it were a "burnt" valve it wouldn't hold compression in that cylinder. In this case it does hold. And if it were leaking combustion gases in the coolent it would be evaporating the coolant or boiling it. and it doesn't do that either.Yes, the later part of the thread steered toward ignition components...

it might even be your PCM... but first you have to eliminate all possibilites... you need 2nd and 3rd "opinions" from different test tools... i.e. compression test, leakdown test, block test kit should all agree with each other.

[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???

[joecar]

If a valve was sticking during the leakdown test, you would have detected air escaping into the intake or headers...

Sounds to me like that piston has a piece broken off the top of it (above the ringland)...

Have you checked for bent con-rods and bent pusahrods...?

[smslyguy]

If a valve was sticking during the leakdown test, you would have detected air escaping into the intake or headers...

Sounds to me like that piston has a piece broken off the top of it (above the ringland)...

Have you checked for bent con-rods and bent pusahrods...?

and yes, in this case i did have a little bit of air goin into the intake. none in the exhaust. I didn't pull the valve covers off yet, i was goin to try some seafoam in the oil to see if this helps with the sticky valve, if i do have a stuck valve that is. also removed the valve covers and inspected the valvetrain, which all looked good.

[Maverick88]

My dad's ranger had a constant number 4 misfire and had many of your described symptoms. It turned out to be a receding valve on that cylinder. We changed the heads out with some from a scrap yard and it runs like a champ now. The valve was actually receding back up into the head a little bit and this caused the misfire.