GXP running bad, but no codes. Not looking good.

[skregal]

My GXP is running pretty bad. It is a misfire where the engine takes
a quick hard stumble every ten seconds or so.

The SES light is not on, but I grabbed my code reader just to be sure.
So I confirmed no trouble codes are set.

I then ran the car with the reader telling me the operational data and it
says I have lots of misfires on Cyl 1, Cyl 2, Cyl 3, and Cyl 8,
Other operational data seems normal as far as O2 sensor info and
other sensors. Fuel pressure is at 40lbs. and holds good when running.

Cylinder 1 has about 1000 or more misfires listed. I'm not sure why
the reader can tell there are misfires but the car is not throwing any codes.
You can definitely tell the misfires are happening.

Edit: forgot to mention that throttle body + intake gaskets and spark plugs were replaced
30K miles ago. Some coil packs were replaced under warranty a long time
ago, at least 75K ago or more and I'm not sure which ones.

[CMNTMXR57]

There are only 2 things it can be;

1) Fuel
2) Ignition

Sort those out and you have your answer. While you are getting a misfire count, what I am also seeing is two odd cylinders and two even cylinders, which keep it running semi-smooth and "balancing" it out.

My guess is that it's ignition. These coils use a spring loaded contact in the part that goes over the plug. So what I might suggest doing, move the coils from cylinder 5 & 7 to 1 & 3... DO the same for 4 & 6 on the other bank. If the misfires move, you know it's the coils. However, that two cylinder on each bank thing has me wondering something like PCM issues.

Fuel pressure is good, but the real only way to tell if it's a bad injector is to run an injector balance test with a scan tool with a fuel pressure gauge attached to the fuel rail and allowing the tool to fire each injector sequentially. A working injector should drop the PSI a few and rebound, then the next cylinder, the same. When you get to one that doesn't drop the PSI, there ya go.

[skregal]

I think you hit on a good approach. I don't know the age of all the coils and I am
pretty comfortable that the fuel (and air) problems are too recently fixed to
start giving me issues again. Time to swap coils and see what happens.

[96 SSEi]

the PCM didnt set a dtc because it didnt find the misses outside of its 'normal range' yet, I assume.if you check the pending codes, it may be there.

[skregal]

OK so started swapping coils.

1st. The rear bank has the black epoxy and the front bank has the white.
I assume the dealer changed the back bank. Because the harness plugs
are different I could only swap on the same bank.

2nd. I swapped cylinder 6 with cylinder 2 on the front bank. Decided to
start the car and see what the scanner said. The car ran worse and actually
set a code (P300 random misfire).

3rd. Shut off car and swapped cylinder 1 with cylinder 7 on the back bank.
Cleared all trouble codes before I started the car. When I then started the
car it ran pretty good. No SES light. Looked at operational data and it was
saying no misfires on any cylinders. Let it idle long enough to close loop and
then started giving it revs on the gas pedal. During good revs it will still
misfire on cylinder 1 and now cylinder 7, but only for a few seconds then the
misfires will go back to 0 and stay there until I rev it hard again.

So the engine is running much better but I am still a little baffled as what to do.
The rear bank of coils are the newer ones, probably has 70K on them. The front
bank of coils look original with 175K on them.

I don't want to throw parts at it. Does this engine use an ICM?

[skregal]

Actually took the vehicle on a test drive and it is not running good
after the coil swap. Still misfiring on mostly cylinder 1 and some
on cylinder 2. Both have a different coil and still the misfires are
on the same cylinders. DTC code is being set for P301 (misfire 1st cylinder).

So if not coil then next (I guess) I have to look at the injector?
Since my cheap scanner will not fire injectors, I guess I could swap those
to different cylinders. Major pain.

[96 SSEi]

I would first inspect/replace plug, and then move on if needed

[skregal]

Pulled cylinder #1 spark plug and it looked fine. Put it all back together
and it runs perfectly like a champ. Drove it to work 60 miles and it
runs like a scalded cat. I don't know what the problem was, how
it started, or how it went away. ????????

I'm guessing a poor ignition connection related to plug, coil, or injector on
cylinder #1.

[skregal]

Update: So the vehicle is still intermittently misfiring and running poorly.
Sometimes it runs just fine for a day, which made me feel like it was
an ignition connection that wasn't tight and randomly acting up.

But because I switched coils and plugs I am slowly changing my mind
about the most likely cause. Now I am leaning toward head gasket,
which of course means the death of the car.

The vehicle doesn't blow white smoke when you start and the coolant
level has not gone down to a point where I need to add any.
However, it is always cylinder #1 that is misfiring, and the plug looked
fine, but was slightly lighter (light tan) than the other plug I pulled.

Maybe the vehicle is leaking enough coolant into cylinder #1 that it will
stumble and misfire for a short time, then clear up if the vehicle gets
driven for a while. Somedays it runs good if it hasn't sat too long to
soak the cylinder. This is my current theory. I need to do some
more testing, I guess, compression test and coolant pressure test.

It sucks because I have just put a bunch of new parts on this car, and
I am not going to fix a head gasket. I have six cars (2014 Malibu not listed)
and this one takes more of my time and energy than all the others combined.

[harofreak00]

I would still try a different coil.

[96 SSEi]

If the plug from #1 miss is tan, it is highly unlikely that there is a coolant leak : it would be contaminated and rusty.
I would replace the plug: it may have internal damage: you can test it with a multimeter

so you have no codes other than 301?

[skregal]

Yes, no codes other than 301. I can easily put a new plug in there.
I have already swapped the coil with another cylinder, and the misfire
stayed on cylinder #1. I could buy a new coil for about $50 or an
entire bank for $150. I hate to throw parts at it.

[96 SSEi]

Yes, no codes other than 301. I can easily put a new plug in there.
I have already swapped the coil with another cylinder, and the misfire
stayed on cylinder #1. I could buy a new coil for about $50 or an
entire bank for $150. I hate to throw parts at it.

I SEE, I would swap again with a working no miss cylinder and swap the plug also.

after no joy on the swap, I would do a compression test -if compression is good,I would move to injector diagnosis.