Error code P0306

[xlh1200hd]

I have a 1996 Pontiac gran am gt with 130k miles. I have replace the plug wires and plugs. Replace the coil pack for cycl. 3 and 6. I keep having to reset the ecm. With the light on or off the car runs great. The only other thing is you start to smell fuel when in traffic but on the open road it is fine. looking for some ideas of what it could be. Also looking to find out if there is a ecm upgrade that is out there that will fix the problem.

[rixGAphx]

Originally posted by xlh1200hd
I have a 1996 Pontiac gran am gt with 130k miles. I have replace the plug wires and plugs. Replace the coil pack for cycl. 3 and 6. I keep having to reset the ecm. With the light on or off the car runs great. The only other thing is you start to smell fuel when in traffic but on the open road it is fine. looking for some ideas of what it could be. Also looking to find out if there is a ecm upgrade that is out there that will fix the problem.
P0306 = Cylinder 6 misfire detected.

As far as this PCM is concerned, 'misfire' can be caused by:
* Ignition Failure (plug, wire, coil, ignition module, or PCM); OR
* Fuel Injection Failure - Too little fuel (lean mixture) for combustion, or too much fuel (rich mixture) for combustion.

I'm 99.99% certain your problem is the latter - a rich mixture; as evidenced by 1) your ocassional P0306 code and 2) your 'smell of gas'.
YOUR #6 INJECTOR IS STUCK OPEN!!!
Right now, the injector is ocassionally spraying so much fuel into the intake runner that the mixture is too rich for combustion.
This does 2 things: 1) causes misfire, and 2) sends enormous amounts of unburned gas, CO and HC into the exhaust.

Some of this unburned stuff is consumed in the exhaust stream just as the header meets the #4 exhaust runner, thereby lowering the amount of oxygen in the exhaust.
The O2 sensor tells this to the PCM, which LEANS the mixture to the other cylinders in an attempt to achieve 'ideal' O2 exhaust levels.
The result is the other 5 cylinders are running too lean, and that results in HOT burning (sounds weird, but it's true) which can burn the exhaust valves and cause overheating of the coolant.

I've been dealing with this condition in my '96 3.1 for over a year now, and have ended up replacing EVERYTHING above the block as a result. $$$$ :banghead:

FIX IT NOW!!
Attempt 1: Run a couple bottles of name-brand off-the-shelf Fuel Injector cleaner thru the tank.
Attempt 2: If (1) doesn't work, consider a professional FI cleaning, the $50 +/- kind.
Attempt 3: If (1) and (2) don't work, the only solution is to replace the injector. It can't be reasonably cleaned by removing it, and it can't be feasibly rebuilt outside of a very specialized shop.
'Rebuilt' injectors are available for about 2/3 the cost of quality new injectors. Quality new injectors are about $110 EACH at NAPA, $150 to you as a customer if a shop does the work.
Thing is, as I found, that after replacing the faulty #2, two others failed and I had to go in again. I threw in the towel and replaced all 6, since it was likely at 9 years/95k miles the other 3 would also soon fail.

It's a PITA 'cuz the UIM and Fuel Rail must be removed to access the injectors, but it's the only solution.

And while you're down that low thru the crap, definitely replace:
* The two little 1/2" diameter x 2" coolant hoses that heat the UIM/Throttle Bore; and,
* The t-stat;
Finally, consider doing the LIM gasket at the same time, since you'll have done 1/2 the prep work for the LIM gasket.
* * * *

Spend the bucks now, before everything goes to hell in a handbasket very quickly.
Or sell the car and pass the problem onto somebody else. Give him our web address. :D :D

Good luck.
-Rick