Any leak will reduce pressure in your coolant system however generally a leak in your block heater bad enough to keep it from holding any pressure will cause considerable coolant loss. If your coolant loss isn't that much then you probably have a leak up high. I would try replacing the filler cap first. Of course fixing this leak will make your block heater leak worse so plan on fixing that too.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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