1996 Bonneville SE 3.8 VIN K

The issue turned out to be a broken valve spring on #6 intake. A $5 fix. It'* the first time I've ever experienced an intermittent fault for this problem.



I get a consistent P0306 cylinder 6 misfire until the engine warms up. At warmup the misfire goes away and the MIL stops blinking. I've checked or done the following:

- switched around plugs
- switched around coils
- switched around ignition wires (no contact with O2 sensor in the rear)
- UIM is new, LIM gaskets new (updated aluminum type)
- switched around fuel injectors
- checked for vacuum leaks and replaced worn hoses

I use Autotap and the issue always involves #6, along with a generic P0300 code. I'd suspect a head gasket but a forum search suggests that this is really rare. The O2 sensor shouldn't be involved because the engine is in closed loop when the problem occurs. Any suggestions/advice appreciated.

can you change number 4s wire with 6s and see if the misfire moves to 4 rather then 6?

Any coolant loss issues? How did the plugs look when you swapped them? When you searched for the head gasket issue, did you come across this techinfo article?
http://www.bonnevilleclub.com/forum/...e=article&k=38

I switched #4 and #6 wires. The misfire was still limited to #6. One thing I forgot to add was that 3 years and 20,000 miles ago this car had a severe internal Dex-Cool bath when the UIM failed. I blew everthing out and the car has run fine to this point. I'm wondering if the Dex-Cool in the cylinders may have weakend the head gasket to the point that it'* just now failing when the engine is cold. The car has 122,000 miles.

The car was recently losing coolant internally and that'* why I changed out the LIM gaskets this past weekend. The old LIM gasket was clearly failing and I could see evidence of coolant loss. The UIM was still in good shape. It'* too early to tell if replacing the LIM gaskets has arrested the internal coolant loss but the cold misfire persists as before.

The plugs were replaced at the same time as the UIM. They still look good and switching them around doesn't make a difference. The misfire is always at #6.

Might be time to do a compression test. Any chance of a valve sticking while it'* cold?

Quote: "Might be time to do a compression test. Any chance of a valve sticking while it'* cold?"

A good thought on the sticking valve. I could remove the valve cover and crank the engine with the fuel pump and PCM fuses removed to check.

Has anyone ever started a 3.8 with the valve cover off? I used to do this on solid tappet Chevys for the valve adjustments.