What engine is it? If it is a 420 engine:

Best to open the valve cover, and check the cam too. If the cam is not scratched, do the finger test, mentioned earlier. Try to identify the offending valve, or cyclinder using a stethescope, while the engine is running. Replace the offending valve HLC, only if defective, and get the right shim as ctaylor738 mentioned. Even without the finger test, just put in the next size up, from the current size. To choose or order the right shims, I used a micrometer with a pointed tip. A flat surface one will give you a false reading since the rocker usually gouges the shim. Replacing the shim could work, and it is a cheap solution.

When changing the HLC, be sure to cleanout the small whole on the top edge of the head and get all of the gunk out of the bottom. I used wd40 with one of those slender plastic tubes on the nozzle to clean out the hole. Change the engine oil and filter, use synthetic like Mobile1.

I'm not a professional mechanic, but the above process worked for me on a 89 420 SEL.

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1989 420 SEL