Weird Vacuum test results?


I hooked a vacuum guage up to the intake manifold of my beluigered little truck, a 1994 Mazda b3000 3.0l v6 with 54,xxx miles and I don’t know what to make of the results. Idle, the vacuum is 15, and when I rev it, it instantly drops to zero then back up to around 20 or higher depending on how high I rev it, then slowly goes back down to 15.

Clogged cat? What?


Video of the gauge


I suppose it could be due to a partially blocked CAT but I would suspect a vacuum leak. Have you checked for any intake leaks?


A few questions first.
Is the needle ROCK steady at 15" while at idle?
When the throttle is snapped closed, does the needle also snap from zero to 20 instantly with no hesitation?
Any quivering movements in the needle while at a steady idle?


It has a tiny quiver at idle, barley noticable, and when I open the throttle it takes a split second longer to get to 20, and it slowly goes down to 15 from 20. See the video too…


Your readings are perfectly normal. You always get a vacuum drop to zero when the throttle is opened wide. You always get higher vacuum when you close the throttle with the engine wound up. Always. The throttle, open at idle will let the air in, reducing the vacuum. Close it at high RPM and the vacuum goes way up because you are preventing the air from going into the manifold. The less air, the more vacuum. The more air that goes in, the less the vacuum. Then you idle it and it stabilizes. Always.


What is weird about it? Sounds normal to me…


Your readings are perfectly normal. When the throttle is opened vaccuum drops when the throttle closes at high RPM vaccuum increases and then as the IAC reduce the RPM’s back to idle the gauge drops back to normal.


Guess I’m going to be the odd man out again go figure. :slight_smile:

15" of manifold vacuum at idle is too low. The only way it’s normal is if you live on a mountain at 10,000 feet elevation.
Low manifold vacuum means incorrect valve timing (not likely), low compression (you’ve said it’s good), vacuum leak (possible), or retarded ignition timing (which I’ve mentioned.)

A slight quiver means either a small vacuum leak or an ignition miss (see retarded timing maybe?)

The needle jumping up to 20 and then falling slowly back points to a clogged exhaust system, probably the converter.

If the truck has been running for a while with retarded ignition timing then this has the effect of richening the air/fuel mix up quite a bit. This could be the reason the converter is clogged.

JMHO anyway to do with as you see fit. :slight_smile:


Thanks ok!

A few months ago I took my truck into a shop and they said they cleaned the distributer, so I’m thinking maybe they accidentally messed it up. Anyway I’m going to have my timing checked soon.


If they removed this distributor for cleaning (no clue why they would do that) then it’s possible they could have just dropped it back in and not set the timing properly.

If the timing is checked without use of the test connector the timing will be retarded and this will cause loss of power, bad fuel mileage, etc. and can lead to converter clogging over time.

If timing is the problem and is behind any converter clogging, then resetting the timing properly MAY help clear any converter clogging out after some driving.
I would not bet any money on it but it’s possible.

Hope that helps, and again, JMHO. :slight_smile: