valve stem guides? you need to take the head off to do that. I think you mean the valve stem seals. Try it, can't hurt. However I have not had it help any engine I tryed it on. Always ended up being rings. If the car has not been run on the highway much it may just be carboned up. Italian tuneup will help this. However I found it takes quite a few highway miles to clean up. My Dodge pickup(with cummins) smoked a lot on startup. It took nearly 10,000 miles of trailer pulling , and highway miles to finally start without smoke. Oil usage went down too. Now a reasonable quart every 2000 miles.

A compression test will tell a lot.

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1977 240D turbo