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Engine RPM slows down after removing dipstick

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BritCarFan

BritCarFan

Jedi Hopeful
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When the engine was still in the car, I noticed that if you removed the dipstick or the oil filler cap, the RPM would fall considerably. I have tried to diagnose this in my mind, but I never found a clear explanation.

Is there anybody out there who encountered this problem also and what was the cure?
 
I don't believe it's considered a problem. It is documented in the older manuals as a method to confirm that your crankcase ventilation (emissions controls) are working correctly. I'll look through one of my older manuals tonight and post a summary of what it says on the subject.
 
disregard my post above. I assume this is for the 1990 Mayfair? Emissions have changed a lot since my Mk1. Anyway, I found the following in my AutoBook service manual for the Mk1/2 Mini.

Page 24:
Testing the crankcase emission control.
To test for correct functioning, allow the engine to idle at normal running temperature and remove the oil filler cap. If the system is in order, the increase in engine speed will be audibly noticeable. (It goes on from there).

You'll notice that they expect the RPM to increase, not decrease as you report. Anyway, comparing the emissions controls on a 1990 and something prior to 1970 probably isn't possible. The only attempt at emissions control prior to 1970 was a PCV valve and breathers on the engine.

Sorry to have mislead you.

PM me and I'll give you some additional contacts.
 
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