P0300 Random/Multiple Misfire

[marcatq]

I'm posting this because I had this problem and didn't ever find a solution online, although it seems to be a regular problem.

If you have a problem with an intermittent misfire, check your distributor gear. If it has too much lash it can cause the spark to jump to one of the rotor screws, burning out the rotor.

It seems that GM from time to time has a problem with their case hardening process. Once the hardened surface is worn through the softer metal wears rapidly. The teeth on my distributor gear were almost half gone.

Installing a re-manufactured distributor solved the problem.

I recommend you also install a magnetic drain plug to catch the material shed by your cam gear and distributor gear wearing into each other.

This is also a good time to evaluate just how much your vehicle is worth. If the new distributor gear wears off, you camshaft is bad too. Then comes the question of where do you stop repairing.

Mine has held up for about 3000 miles so far, but I don't drive this vehicle much.

[old_master]

If the distributor is not centered in the bore of the intake manifold, it will cause the distributor gear to wear. The driven gear, (on the distributor shaft) is slightly softer than the camshaft, making it sacrificial. The distributor bore on the intake manifold is larger than the distributor housing to allow for a slight adjustment of camshaft retard. Years ago, ignition timing was adjusted by rotating the distributor. On 4.3L OBDII engines, rotating the distributor does not affect ignition timing. Rotating the distributor on these engines is how camshaft retard is adjusted. If camshaft retard is off more than 2 degrees, it will cause the spark in the cap to jump a larger gap, (and can also set a P0300). The hotter spark is what causes the terminals of the cap and rotor to burn. This is quite common with aftermarket caps as the placement of the terminals can be incorrect. If camshaft retard is correct, and the cap and rotor terminals burn, you have a faulty distributor cap. If you have an AC Delco cap and the terminals burn, camshaft retard is off.

[marcatq]

Thanks for the info on the sacrificial distributor gear. Prior to replacing the distributer, the rotors were lasting less than 800 miles. (The last one only made it 150 miles.) I've now got at least 4000 miles on the rotor so I think its cured. The first distributor gear lasted 135,000 miles, so I don't think I have an alignment problem. The position of the distributor is set by the hold down and doesn't appear to be adjustable at all.

My purpose in posting this was because I found a solution that seemed to be missing from the forums. Most of them seemed to be sending people down crazy paths of connecting oscilloscopes to the fuel pump or replacing the heads. (I have found faulty alternators causing computer problems with an oscilloscope, but a fuel pump having this problem seems unlikely.)

It seems that everyone is willing to post the problem on a forum, but when they find an answer, they forget to let the rest of us in on it.

Thanks,

[old_master]

Originally Posted by marcatq

....The first distributor gear lasted 135,000 miles, so I don't think I have an alignment problem. The position of the distributor is set by the hold down and doesn't appear to be adjustable at all.

When the distributor is installed during engine assembly at the factory, it is aligned correctly. When the distributor is removed is when the problems start. The hold down will allow for small adjustments to camshaft retard. If care is not taken when installing and adjusting, the camshaft gear and the distributor driven gear may not be aligned correctly. That's what causes the driven gear to wear quickly. Most people blame it on a poor quality driven gear, when actually the poor quality is in the installation.