1969 Deville - Vacuum readings

[69Deville]

Hello friends,

When I take vacuum readings directly from the intake, the needle fluctuates by a degree. If I connected to the tee that connects the smog pump to the vacuum break, the needle is steady. Where should I be connecting the vacuum to get the most accurate reading?

Thanks,
Dave

[James Landi]

Usually, the vacuum reading is taken directly from the intake manifold with all of the other vacuum lines blocked in order to assess first the engine's condition without vacuum devices that may leak.  If you've read the directions for assessing vacuum readings, there is a "steady needle" result that indicates an engine in good running order. Because there are many vacuum actuated devices, one or two on the Carb, the vacuum advance on the distributor, the brake booster, the vacuum emergency brake release, hvac, etc. etc., it's important to know that they are all out of the loop in order to perform an accurate test.  Usually, a rapidly fluctuating needle indicates a possible intake valve leak... and that can be checked out with a compression test.  Another way to analyze the rapidly fluctuating needle is to note if the engine is running rough, i.e. shaking in synchrony with the gauge reading. More information from you would be helpful in assessing an answer toyour question, as there may be nothing wrong with your engine. James

[69Deville]

Thank you James.

I plan on doing another compression test next weekend. When I do, I'll hook up the gauge as advised. Overall, the engine runs very good, I am just a perfectionist. I'll report back with my findings.