I hope somebody outhere has a little help here.

I have 99 EXPY 4WD. I am experienceing a P0300 code (multilpe cyclinder missfire). I have changed all sparkplugs, all coilpacks. I still get the code after driving when it warms up. It does miss, like a coil pack. It will miss more when it is cold, after it has warmed, I get the code and it will miss a little less. Obviously, it should after it gets hotter. I have taken it to 3 different dealers and 2 independents. I get the "deer in the headlight look" after a couple of days.

Any suggestion would be greatly appreciated.

 

I assume you changed your plug wires too?

 

No plug wires on a 5.4. The coil packs are direct to the plug.

 

Hmmmmmmmmm, An old trick I tried that seems to work on occasion. It has to be really dark, start you engine and look for arching in and around the spark plug areas. This was my reference to the wires. I also found a bad distributor this way too. Might be worth a try. Also a friend was having a similar problem on a Chevy and we found that one of the boots was not fully pushed down onto the spark plug. Hope this may help.

 

I definitely will try that. You just may have hit on something. That is what we used to do with mags, back when I raced super modifieds. Thanks for the reminder. I will try!!!

 

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Although this is a good technique to find a spark "leak", it probably won't work on the COP (coil on plug) system. There is no high voltage current running through the coil pack wires, all the high voltage is generated right at each individual coil. There is no place for spark to "leak" to ground, and in the remote possibility it did, you would get an individual coil pack misfire, not the "multiple" codes you are getting. Since you have replaced all the COP units, I would look at yor ECM connections. Even though the ECM is located inside the vehicle, there is a large connector through the firewall. On the passenger side, behind the battery, look for a elec. connector with a bolt securing it. It has about 30- 40 wires coming out of it. Check that it is fully seated, the bolt is tight, and the wires all look good. Then look in the passenger footwell, at check the ECM itself. Does it look secure, or has it gotten wet ? Check the connections on the unit itself. All the coils lead directly back to the ECM, so there is not much else that could cause a misfire, especially on mult. coils. Finally, try resetting the ECM, by disconnecting the battery ( + and - ) for 15 - 20 minutes. After reconnecting, your gauges should peg out at startup, letting you know the reset was a success. Good Luck !!

 

I tried the battery disconnect. I got the reset but to no help. I will look and reseat the plugs on the ECM. I have had horrible visions that is the ECM.

Hey, Thanks