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Old 04-04-2005, 05:05 AM   #1
mmoore181
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Default Misfiring; P0300


Hey all, new to the board. Just bought a used 2000 Saturn SC2, manual transmission. It ran great for the first few weeks, but now I am having problems with misfiring.

I ran an OBD II scan and came back with P0300, which is a general/random misfire. The code was pending (whatever that means?).

The problem is this: at highway speeds (usually in 4th or 5th gear), the car will studder and misfire under a light load. If I take my foot completely off the gas, the problem goes away. If I floor it, the problem goes away. Only when I apply a little pressure to the gas pedal does the misfiring occur. Some days are better than others.

Since I bought this thing used, I don't really know when it had its last tune-up, etc. I replaced the air filter, which was really dirty, so it might've been a while since the previous owner tuned it up.

Before I take it into the shop or do some home repairs, I was wondering if anyone had any tips or advice. Anything is greatly appreciated.

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Old 04-04-2005, 05:49 AM   #2
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first off I would check the sparkplugs and plug wires. Make sure all connections are tight, no corosion, plugs are not fouled or worn and are still at the proper gap.
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Old 04-04-2005, 10:02 AM   #3
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I'd also reccomend something else. Pour a jug of injector cleaner in your gas tank, it may help your car a bit as well.

Make sure that your Saturn uses the specified NGK spark plug. The 1.9 Saturn engines would run poorly when people used the wrong type of plug.

Anyway, if your plugs and wires are still good, look upstream, to the ignition coils. Sometimes if the car has gone too long without good plugs and wires, the coils will get too hot and be damaged.

Good luck, and tell us what you find when you work on your car.
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Old 04-04-2005, 07:37 PM   #4
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Default Took the car in..

I took the car in to the dealership and they said my pre-cat was cracked and that I could possibly need a new catalytic converter. Of course they want a substantial amount of $$$ for the job.

My question (for any of you who might know) is this: could a cracked pre-cat be causing my misfiring problems described above, and if so, is it something that I need to replace immediately to avoid engine damage, etc.?

Thanks for the replies I've already gotten.
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Old 04-05-2005, 12:37 AM   #5
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Default Re: Took the car in..

Quote:
Originally Posted by mmoore181
I took the car in to the dealership and they said my pre-cat was cracked and that I could possibly need a new catalytic converter. Of course they want a substantial amount of $$$ for the job.

My question (for any of you who might know) is this: could a cracked pre-cat be causing my misfiring problems described above, and if so, is it something that I need to replace immediately to avoid engine damage, etc.?

Thanks for the replies I've already gotten.
If your pre-cat is damaged, your engine will run hot, and use far too much fuel. With the poor running at WOT, I'd believe that the cat is damaged.

Don't pay a dealer's price. Muffler shops replace catalytic converters and precats for less.
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Old 04-05-2005, 12:51 AM   #6
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Thanks for the help, Artificialist. I also pulled the spark plug wires and noticed that one of the plugs has oil around it. Seems like a valve gasket cover -- is that a fairly easy job to do at home? I'm new to all this, so I apologize for the rookie questions
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Old 04-05-2005, 02:36 AM   #7
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Quote:
Originally Posted by mmoore181
Thanks for the help, Artificialist. I also pulled the spark plug wires and noticed that one of the plugs has oil around it. Seems like a valve gasket cover -- is that a fairly easy job to do at home? I'm new to all this, so I apologize for the rookie questions
I would do the VC gasket not to mention make sure that there is no carbon tracking on the plugs and the wires from firing down the oil soaked plug (s) before the even thinking about the pre cat. This is your best bet for the misfire. Easy DIY job.
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Old 04-06-2005, 06:42 AM   #8
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Thanks for all the replies. I am going to go ahead with the VC gasket and get new plugs and wires, and I'll post if it solves my problems.
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Old 04-09-2005, 09:25 PM   #9
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Default UPDATE

Ok, so I got the car running in tip-top shape again.

First, I changed the wires, plugs, DIS and valve cover gasket. This was apparently causing the misfire.

But then a new problem developed -- the car stumbled quite badly on acceleration, and jerked violently. After doing a back pressure test, I found out that it wasn't the pre-cat (like the dealer said), but the main cat itself.

So I had that replaced at a muffler shop, and viola, the car is running better than ever w/ more power. It's like a new engine.

Anyway, thanks for all those who replied. Maybe this info will help somebody diagnose their problems someday.
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Old 04-10-2005, 04:49 AM   #10
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Was the main cat bad because it was plugged? If it was then the precat was likely not cracked but the converter material was likely broken up and gone down stream to block the main one. I very much suspect that all you have left of the pre cat is an empty shell--if you are lucky. It will be very worth while removing the pipe from the exhaust manifold/precat and checking to see if there is any left in there. If there is then get it replaced before what is left goes down stream. Not to mention if you are in an emmission testing area you will likely not pass your next test.
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