good ole P0303 misfire code [Archive] - Forum


View Full Version : good ole P0303 misfire code

12-01-2012, 10:00 PM
sigh. :)

So, not even 2 months ago i did the LIM gaskets (Felpro) on my '99 GT, found the bolts in the manifold weren't even finger tight and water was just pouring out. Trace amounts made it into the oil. Decided to do it myself thanks to the AWESOME guide found here at

Not even 2 weeks later the water pump went, spewing out all the new coolant.

A week after that the bleeder tower decided to start leaking profusely.

A few weeks later driver side brake caliper seized.

Earlier this week driver side window guide system broke, not sure what exactly but there was a crunch and the window fell down inside the door --- didn't shatter or anything, was able to pull it back up and keep it there with tape --- looks very stylish......... ;)

Now, today on my way home the SES light starts FLASHING...... hence the sigh.

Was driving fine on the highway until i stopped at the exit. The car started shaking, almost violently, and fighting not to stall. Stepping on the gas made the shaking a bit worse and it had almost no power and was just stuttering and almost choking. Felt like it was misfiring pretty bad. Weird thing is, it seems almost fine and normal once i get over 50 km.

Finished the drive home, very carefully, and nursed it up the driveway. I changed the plugs (NGK) when i did the LIM. It hasn't had any oomph since the tranny was done a few years ago. Would bog down when i try to accelerate hard for the past couple years before eventually kicking in.

Plugged in the odb II reader and gave me the code in the subject.

Am i hoping too much to think it could simply be plug wires or coil pack?

I've read a few other very similar posts, didn't really see anything exactly the same. I'm hoping something i've added could trigger an idea specific to this.

12-02-2012, 08:32 AM
Flashing SES light is evidence of a severe misfire, and the shaking and loss of power would coincide with that. There are several possible causes, including bad plugs, wires, or coils... but a plugged catalytic converter is also a common cause of this. Might want to get it checked out.

12-02-2012, 10:21 AM
thx for the reply.

Ya, with my luck it'll be something $$$$ that i can't simply do myself to save the $$$$.

New cables and changed plug in cyl 3 with known good one.

Still p0303.

Now to find out if i can swap the 3/6 coil pack with one of the others and get the misfire to move.

12-02-2012, 12:02 PM
New coil pack.

Still P0303.

Weird that it seems to run just fine while revving higher and SES stops flashing on the highway.

12-02-2012, 07:37 PM
Are the plugs iridium ? Is there any coolant loss ? Also the window guide clips are about 8 to 15 bucks. And are pretty easy to change. A guy on eBay well them and they epoxy together for 17 or so A bad injector will cause that code too

12-03-2012, 09:07 PM
There isn't any mixing/loss of coolant or oil AFAIK.

Before i did the LIM gasket i'd lose all the coolant withing 30min of driving, had a few jugs in the trunk so i could get to and from work. Turned out to be a failed gasket and all 4 outside bolts weren't even finger tight --- also no detectable mixing of fluids.

Haven't even looked at the window yet.

Broke down and took it to the shop to get it checked out.

If it's more than $300 after diagnostics i'm done with it. Really sucks seeing as I was so proud of doing the LIM and everything actually working after :).

Feels like the car is telling me to let it die. lol

Ya, i'm leaning towards Cat or injector too.

thx for the reply.

12-04-2012, 07:59 PM
Very low cylinder 3 compression test.

Shop thinks it's probably piston rings.

oh well, it wasn't a completely horrible car for 8 years.