Black Oil on a 72 350/4

[brown7373]

My 350 starts, idles and runs perfectly. The problem is the oil turns BLACK after only a few miles. I recently added SeaFoam to the oil, drove it for 2 days, then changed the oil. The PCV shares a vacuum line with the gas vapor canister. I question whether the canister is any good, and may cause low vacuum to the PCV. How do I check out the canister? If I block vacuum to the canister, will it adversely alter the function of the PCV?

[Alex V.]

I don't think low PCV pressure would affect oil contamination that much. First, just to be sure, I'd check your spark plugs to see if there's evidence of it running overly rich. If not, and it runs good, I'd just suspect the engine is very dirty inside or has a big gob of sludge in the oil pan. I wouldn't use Seafoam too often (maybe every third or fourth change) but use a good high-mileage oil like Valvoline MaxLife (more deposit control additives, detergents, etc.) and good filter, changing it on short intervals if you like. Once an engine is sludged up inside it takes a while (like, multiple multiple changes) to get it all cleaned out, but as long as you know your oil and filter are changed often enough I wouldn't be too concerned about it unless the problem persists after several 1,500-2,500 mile oil changes. And I don't think blocking off the charcoal canister hose will hurt the PCV at all - my guess is that the line you're looking at is simply the return line for vapors that wind up in the canister, to feed them into the vacuum side of the PCV system that gets drawn into the engine and burned.

[ChuckLee]

If it's a real concern (and it should be if there is that much gunk running through your bearing journals) then pull the pan and clean it the best you can. while the pan is off, check out your rotating assembly and clean her up a good bit before putting the pan back in.

[Alex V.]

X2. All the better, if pulling the pan is an option.

[brown7373]

I have owned the truck since 2003, driven it about 50K during that time, including numerous 8-900 mile trips. Oil is always changed at 3000 miles with quality oil and filter. Plugs look fine, no sign of rich and no smoke from exhaust. Total shown miles is less than 100K and I am fairly certain that it is correct. It just has very dark oil compared to other old cars I own.

I am not sure exactly how the vapor canister works. Is there supposed to be any resistance to the vacuum to the canister, or is it almost like an open vacuum line? If no resistance, wouldn't it be almost like a vacuum leak, and the PCV not getting enough vacuum to work properly? I'm not looking for a problem where none exists, but I have been driving and changing oil often enough for it to clear up, if it was just gunked up.

[geezer#99]

Pull the pcv out and block the end.
If the rpm drops about 100 then it's working ok.

[BugzC10]

Here's a schematic of the hose routing to and from the canister. There is a small diameter hose that runs from your carb to the canister. The larger hose is taking the fumes from your tank via the canister to burn off in through the PVC system....I think I got that right. If not someone will correct me.

[ChuckLee]

Here's a really cheap way to go....before having to pull stuff apart that's working good....

http://www.blackstone-labs.com/

Send in for their free lab kit and send it off and they will analyze your oil down and tell you EXACTLY what is happening in there. They do some amazing work.