99 vortec 5.7 missfire

[darrellward1]

i own a 99 suburban k-1500 with 124000 mi, the vehicle is driven daily without many problems. but recently i developed a missfire condition that only shows up after med to hard accelleration.i brought this to my local dealer for diagnosis and was told i had a bad dist cap or wires. i changed them and still have the problem.the pcm shows a p0300 and a p0305 (though not at the same time) i purchased an obd2 scanner to retrieve the codes myself and have been able to clear them. i can drive the truck normally and it will run fine,but if i accellerate hard i can get it to throw the codes every time. i wonder if i have a bad valve spring or bad advance springs in the distributor? any help would be greatly appreciated.

[1999 Tahoe 4x4]

P0305 means you have a misfire in cylinder Nr.5

 

This is quite a well discussed chevy topic on the net. I have sometimes misfires on clinder Nr.5 as well, but it occures rarely so I don't worry about it. First I would make sure to use good quality wires and check for proper (proper = according to GM) wire routing. Wire Nr.5 shall not cross or touch wire Nr.7. This is very important. Then I would check for proper distributor adjustment for proper base timing. You will need some professional scan tool like Snap-On MT2500 for that. To my best knwoledge you won't be able to use a timing light to set base reference timing. The distributor can be mechanically adjusted with a regular 13mm wrench the same decades old way. According to the 1999 Factory manual, to set your reference timing properly you HAVE TO REV THE ENGINE at around 1500rpm. Your professional scanner will display the degrees. Adjust the distributor to 0 degree and torque the 13mm bolt at the leg of the distributor.

[95Sierra2500]

Another thing to check for is carbon-tracking. I had this issue on my Grand Prix where it would only misfire under load, and it turned out that my #3 plug was carbon-tracking. You'll know if you are or not by pulling the boot and checking for thin black lines on the outside of the ceramic. Doubt it's your problem, but it's simple to check for, and alot cheaper than bying a prof. scanner.