Just saw the check engine light come on under maximum acceleration.

It flashes for a few seconds and then goes off.

Vehicle feels like it is running OK, even under heavy load (max acceleration).
Smooth. Nothing noticeable.

It repeats every time under heavy load, and I can trip the light ON anytime I want.

Owners manual suggest this may be a misfire?

if the light goes off, does it store codes that a code reader can see?
Or does it record a code only when light stays on?

Anyone else experience this?

 

Anytime the engine light flashes its misfiring. It will store a p0300 at minimum. It may store a p0301-p0308 as well. Last digit indicating which cylinder is misfiring. If not you need a scanner that would show misfire history counts to see which cylinder is causing problem.

 

Thanks HumTech!

Excellent info. I will pick up a code reader and see if I can pull a code.

What commonly causes a particular cylinder to misfire? Presume it is electrical rather than mechanical problem?
Or computer problem firing at the wrong time?
Could it be as simple as a bad spark pug or wire?

 

If its one cylinder only it could be plug, wire, coil, injector, compression. If its multiple cylinders all on one bank its likely an o2 sensor or restricted catalytic converter. If all cylinders random, could be both cats, o2 sensors on both sides, restricted exhaust. If you have a remote start installed and the installer used one ign coil for the tach trigger and that splice is corroded could be that. Many things can cause the miss. Need to narrow it down to a cylinder or multiple then go from there.

 

Just read your other post on the rattle. Use a rubber mallet and tap on both converters, if one is rattling then its likely the cause of both your problems.

 

Hi Humtech.

For some reason the forum stopped emailing me reply alerts. So replying only now.

I pulled a P0300 code.
= Random cylinder misfire. The code either flashes and goes off, or goes to a solid warning light which sets the code. Does it at about 4500+ RPM.
Nothing to indicate a problem below this RPM. No light.

I do not feel any misfire or vibration, because at that high RPM and speed everything is vibrating anyway.

I can not get the spark plug wires off to check/change the plugs. (vehicle has 65K miles, but thought that is where I would start).
But the stupid heat shields prevent me for getting a good grip on the boot.
I can spin the boots on the plug, but can not pull it off even with my plug boot puller. Nuts.
I do not want to destroy the wires (they are still supple, not cracked, and look like new even though they are 10 years old).

Any trick to getting those wire off without destruction? I prefer not to pay $100 for a set if I break one. Never had a GM vehicle that I could not get the wires off. But I have never had a vehicle with the spinning metal heat/RF shields that prevent getting a good grip on the plug boots either. Frustrating. I asked the GM dealer how they get them off in the shop. They said that they do end up breaking some if they do not slide off the plug easily. They said they have no tricks (but they must have, because this must be a common problem!). Not what I wanted to hear.

There is massive confusion about which spark plug the 2007 takes, platinum or iridium. GM parts lists both. Maybe it depends on whether the coils are Melco or Delphi? Mine are the square Melcos.
The wire type change with the coils so do spark plugs also?
I can not pull a plug to see what is OEM. I am told by GM parts guys it takes a 41-962 (platinum) or the 41-110 (iridium). Also gap seems to be different and range from 0.040 to 0.060 depending on which source I look at.

The owners manual shows a 41-985 plug (12571164) with an 0.060" gap. Some sources on the net say that is a platinum, other say it is an iridium.

Why does this have to be so difficult with GM parts!!


On the other problem - sheet metal tapping noise on startup:
I banged on the cats and exhaust. No rattle. But something is causing it.
It came back even with the ladder-type skid plate pulled off.
Only does it on startup and first few seconds.
Annoying and I have no idea what that issue is or how to even diagnose this one.

 

I have(had) similar tapping at startup. It's the valve train. I was using 10w-30 but the oil cap says 5w-30. I put in 0w-20 and it went away. I think these engines are really sensitive to oil viscosity. Give it a try, it might be your issue too.

 

Hi Mulepic,

The noise is not a tappet noise, I know that noise from other cars.

This is a loud sound like someone tapping with a metal hammer on a sheet of thin metal like a heat shield or something similar.

 

Are any of your exhaust manifold bolts broken? Very common and will tick until warm. Also check passenger manifold. Mine had a 3" crack under the heat shield.

Random misfire codes suck. I chased one for six months. Mine could only be felt at idle and didnt throw a code for months. I had my mechanic troubleshoot it as it was fall and cold out. He changed plugs and wires, cleaned the MAF sensor. He kept going back to the MAF and i felt it was the intake gaskets. By the spring i was at my end with it and told the mechanic to smoke test it. He also water tested it. Truck ran ok for about ten minutes and lost power. Threw a code- knock sensor. I then ordered knock sensors with harness and fel pro STEEL intake gaskets. I would have to change tje intake gaskets to change the knock sensors. So the mechanic kind of did me a favour by soaking my knock sensors. The truck has been rock solid since and no more misfire. I dont think its your knock sensors but intake gaskets are pretty common in the gm motors. Its a crappy plastic gasket.

 

Update: Ended up being a plugged passenger side cat.

The entire assembly (L&R cats) had to be changed as it is all one OEM welded unit.
A couple of thousand $$$ later, and she runs like a champ.