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P0300 Random Misfire Code

19K views 9 replies 8 participants last post by  carbonburner 
#1 ·
Just wanted to add my experience for others searching this P0300 random misfire code issue. 1998 Astro Cargo van with 155k.

My van ran pretty good most of the time it would really only give me the code when going uphill at freeway cruising speeds. The SES light would come on and when the van really started acting bad SES would blink. Van would start spuddering and lose power. At first it was only when carrying a load but after awhile it would do it everytime I went up this hill on the freeway even without a load. One time it was so bad I had to pull over and shut down the van and give it a minute before firing it back up and it ran fine.

My mechanic couldnt narrow it down so I finally brought it to the dealer. They diagnosed it for about 5 hours and came to the conclusion that it needed a valve job. Quoted me something like $2800 for that or $6500 to swap out the motor for a new. Its a work van and I didnt have the liberty of not having a van for even a week so I went out and bought a low mileage 03 instead.

A few things I had done prior to this was swap out the plugs wires cap and rotor, all AC Delco. Cleaned up the Map sensor, repaired a faulty gasket, cant remember if it was head gasket or intake. It was basically patched up somehow with a technique of running some chemical thru the cooling system and flushing it a dozen times until it hardened up where the repair was needed. It failed a leakdown test prior and passed afterwords so I guess it held up. Still gave me a random misfire code after that. Also put in a new distributor gear assembly even though on the old one the teeth looked fine. None of that was a fix.

Thats my story I appreciate all the info on this site and so wanted to share my experience with others trying to track down this issue. Hopefully your story has a happier ending.
 
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#2 ·
Some food for thought. I've had 7 Astros since 96 and they have all been 1996 or newer and I did all the work on them myself, so I pretty well know what can go wrong with ignition. I would have approached your problem like this: I always start with a cap and rotor. Usually when they are shot, you can see the coil wire on the coil arcing to the coil body. While I have the cowling and glovebox off, I always take a needle bottle and put motor oil on the plug tines for the cam position sensor plug on the distrubutor, the plug to the ignition coil, and the plug to the ignition control module next to the ignition coil to disolve surface oxidation and rustproof. I always make sure that the coil wire to the distributor cap is routed not near anything that is grounded. That huge ignition voltage will punch through a coil wire to a crossmember if leaning against one. I would have replaced the ignition coil next. Sounds like yours was breaking down from heat. Denso makes one that is better insulated from wire post to coil frame. Next I would have replaced the ignition control module next to the ignition coil. I did once get a cap and rotor that only lasted 30 days. Cheap Chinese junk. I get the 5 year warranty cap with brass contacts at Autozone and replace it every year (4 times for free).

Another note: Behind the passenger front wheel is a shroud over the wheel A frame. If that shroud is torn or out of place from being tucked straight up over the A frame, your tire will spray the ignition coil with water when it is wet out, because the coil is right above that shroud.

A flashing SES light with loss of power is usually a serious ignition electrical breakdown. Hope this was helpful. Ken
 
#3 ·
Had much the same problem with my '02 going up good hills with a load, then without a load. Turned out to be an almost plugged fuel filter. Plenty of fuel for cruising or power on flats, not enough for a load like a hill.

Replaced the fuel filter and everything's been perfect since. And my fuel pump instantly quieted down, too. Which reminds me, it's time to replace the fuel filter again.

If your problem was a specific cylinder or two, I'd say fuel injector, plugs or wires. But because it's a P0300, I will say filter, coil or ICM. It's a situation that's affecting all cylinders, not just one or two.
 
#6 ·
The problem is when I start pulling hills the van sounds like it is straining, it has a loud like rev noise, sorta like I am towing more than I should with the van, but I am not towing anything behind it. After about a minute, more or less, the van starts to sound normal, and starts picking up power. The OP at some time replaced the cat with just a normal cat, like the universal kind on ebay, not like the factory that came off it. The van idles, and runs good until you start pulling a hill. Also getting p0420 code at times, along with the brake and ABS light comming on at times. It is a 2004 GMC safari AWD. Thanks.
Ok, Let first start by telling you what I have done to the van already. I just had new AC Delco plugs, wires, cap, and rotor put in. Replaced the fuel filter, and a o2 sensor also.
 
#9 ·
I know this is old but everyone needs to start doing better diagnostics rather thatn just replacing stuff. Cut the system in half helps, if there is a random mis fire it is pretty easy to check to see if your coil and everything upstream of that is working by disconecting the wire going to the middle of the cap and crank the motor over, its well over 1000 volts and will arc to ground, it will be obvious if the sprak is there, next check the cap etc.
 
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