diagnose engine miss

They sprayed so-called "Windex" on te distributor cap, told me it was a bad cap, showed me a spark, and charges $90.00 for the "diagnosis. ' After replacing the cap, I found the miss was inchanged. Three weeks later i learned it was a problem with the computer. The cap had nothing to do with it. I looked at the old cap recently and it IS STILL MOIST - like it waws covered with salt or another dessicant. I think they sprayed something other than Windex. Don't waste your money with that con.