P0300 And P0102 Codes On A 99 5.7

99yukon350 · July 17, 2009

I have a 99 Yukon 350 with 166,000 on it. I have dual exhaust on it no cross pipe. It does have 2 cats. I started it up one morning and check engine light came on and ran real bad. It showed no signs of issues before that start up. It is not just doing this on idle, it is at any rpm. I checked the code and it was P0300. The cap, rotor are less than 1 year old, the wires are less than 2 years old, the O2 sensors are less than 40,000 miles. I just replaced the plugs today, #8 was pretty black compared to the others. I tried a different coil and that didn't make a difference. The vacuum is good at idle and at 3000 rpm. The oil pressure reads good, it constantly runs at 165 degrees but I did notice recently that the volt gauge is not right in the middle at 14. The alternator is 3 months old. It runs just a little over half way between 9 and 14 unless there is the load of the A/C, lights, or putting the windows down then the gauge goes down. The battery is 3 years old and a 770 cca.

The 300 code was the only code it would put out until now. It now will spit out 102. For a minute it was also putting out 182, 66. I have not had a problem getting it to start and it has not stalled out. It will back fire a bit while driving it. It seems like one side exhaust blows harder but the other side blows hotter.

Where should I go next? I'm trying not to put too much money in it as I have been looking for a newer truck.

bill190 · July 17, 2009

The battery should measure 12 something volts with just the ignition on (not started), then jump up to 13/14 volts after starting. This would be the alternator kicking in and charging the battery. Measure at battery with voltmeter (multimeter).

Several of those codes could have been caused by low voltage. Electronic gizmos do not like low voltage like below 9 volts. And the further you get from the battery, the lower the voltage will be!

So I would check your alternator, the wires going to the alternator, and the connectors at both ends of the battery, ground to body and ground to engine block.

Be aware there are fusible link wires and the wire itself is a fuse, so a wire can be bad and would need to be replaced with the correct part number replacement for proper protection.

How to do a voltage drop test on wires and wire connectors...

http://www.engine-light-help.com/voltage-drop.html

I would fix this problem first, then see if the other problems go away?

99yukon350 · July 21, 2009

I figured out what was causing the volt gauge not to read 14 when running. (corrosion between the two postive cables). This did not fix the misfire.

I tested the spark plug wires. With the tester set to 20k ohms, the longer ones maybe 18 inches read 2.5-2.8. The shorter ones read around 1.5. I have read several things from anything under 6 is bad and should be replaced to the lower the better. Which is true?

bill190 · July 21, 2009

Getting it straight from the "horse" (GM Factory Service Manual that is)...

All the service manual says is to check for an open (Infinite resistance), cracked insulation, and then check to be sure it is sparking.

Then common sense to me would be to use stock (factory specification) spark plug wires. And that if one wire was radically different from the others, then that would indicate a problem with that wire.

With that said, if you had high resistance spark plug wires installed and one wire was low resistance, then I wouldn't think that would be good?

But if they are all low resistance wires, then I wouldn't think a few ohms would matter?

I have read that ACDelco plugs (stock) are the best. And that there can be problems with other types.

But if my engine is missing, I replace the whole works anyway (preventative maintenance). New spark plugs, new spark plug wires, new distributor cap, new rotor. Then everything seems to hum along for quite awhile after that.

Anyway for the misfire problem now that the voltage is OK, here are additional checks...

http://www.random-misfire.com

Here is a page on spark plug wire testing with an emphasis on the insulation...

(The idea being that the spark "leaks out" before getting to the plug when it has cracked insulation.)

http://www.trustmymechanic.com/auto-repair...plug-wires.html