P300 code while driving

[GrayKnight]

I have a 1999 Yukon with just under 310,000 miles on it. The other day I was driving it and it was running fine. I stopped at a red light. When the light turned green I stepped on the gas and the engine began running very roughly. I managed to get it home. When I started it the next day the check engine light was on. The code reader read P300. I did some research on the web, and it appears that to repair everything that might be causing a P300 code could cost upwards of $2K.

Because of the high mileage I've been thinking about upgrading to a newer model anyway, but had planned to wait until spring. My problem is that I can deal with a lot of basic mechanical things but electricity pretty much baffles me. I installed new plugs and wires back in the summer and didn't have any problems after doing so until now.

I'm hoping some of the real experts here can recommend relatively simple things I can things I can look at to see, for example, if something has come loose or broken that would cause the P300 code. I've seen recommendations for replacing the distributor and/or the alternator, both of which I can do, but I'd like to check the simple stuff before putting more money into parts.

Any and all suggestions gratefully accepted.

 

[exp500]

Test individual cylinders for miss using scanner. test all sensors. Fuel pressure test, key on engine off, leak down time to 10# loss, engine running pressure, leak down. Book is 62-66#
Cap and rotor known weak spot on this motor. Any coolant loss?

 

[dnt1010]

If this 99 model still utilizes the old crossover Distributor cap design that is a very common failure point and high probability for your P300 code. It is so common that I would probably swap that cap out first and THEN start my diagnosis if it was still missing.