I was just out running the Chevelle and on the way back she started to run a bit rough. By the time I got home the engine was running really rough and knocking loudly from the passenger side.

The engine is a 396/325 with some mild performance modifications by a previous owner. These include cam, intake manifold, carb, a fuel pressure regulator, and headers.

I have not had the car long. Stuff I've done is an oil change about two weeks ago, a fill-up with premium unleaded yesterday, and I tightened the terminal nuts on the coil a small bit today. Other than than I have done alot of polishing.

All of the vitals (oil pressure, water temp, amps) were normal throughout the run and when I shut her down. There is no water in the oil or steam from the tail pipe. No smell of coolant. When it cools off I will see what the inside of the radiator looks like.

I'm wondering a couple of things. One is whether it would be wise to start the engine, in order to narrow down the source of the noise. The other is what should I check out first? Is there a most common offender in cases such as these?

Any help would be appreciated.

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69 SS 396/325

It's possible that a peice of carbon got loose and that's what's causing your knock. You would be suprised how loud it can sound. If you can narrow down what cylinder is misfiring, pull the plug out and inspect it. It could have a piece of carbon trapped in the plug gap. Now if this is a faily fresh engine, carbon is not the problem. But if it's been together for a long time, it's definietly a posibility. You can buy a can of GM top engine cleaner and just follow the directions. It could be that run knocked some carbon loose, especially if the previous owner really didn't get on it much. Hope it works out. good luck.

Well, if it was internal damage, you can't do much more damage idling it(hopefully it's not). If it were a bearing, your oil would become super black pretty fast. You can get a small squirt bottle and lightly shoot water on each hearder as it idles to check to see if any cylinders are misfiring. Water should evaporate immediately on the good cylinders. A long screw driver can be used as a make shift stethoscope. Just put the end of the handle up to your ear and probe the different parts of the engine, heads, block, water pump, etc

Thanks for the help. Once the engine cooled down I was able to do a closer imspection and I tracked the problem down to a blown hedder gasket on the rear cylinder, the rear most bolt of which was loose. I can see why it was loose, it's going to be hell getting a wrench on it when I replace it. The A/C evaporator is in the way.