M30 intake vacuum and removing the oil cap

[Martin in Bellevue]

Pulling the dispstick on my m30 will cause a stumble. The vacuum gauge on the map sensor reflects a big dip in vacuum.

Martin:

At idle, if you pull the oil filler cap and put your hand over the hole, do you have vacuum or not?

How does it run if you pull the filler cap? Same or funky?

Jeff

[Jeff N.]

We should check 'em side by side. Do you recall home many inches yours pulls at idle?

I'm having a hard time figuring out why just my cam change would impact the vacuum in the engine case that much. If I had like 5 inches of manifold vacuum at idle, that'd be one thing. 17 to 18 seems to be another.

...and your setup with no AFM flapper AND vacuum in the crankcase pretty much puts cold water on my hypothosis.

Jeff

[Bill R.]

cap, if its lean as in set to emissions lean then opening the cap and creating a vacum leak would lean it out to the point where the idle would be more affected... can you set the idle mixture with your mass air flow setup?

We should check 'em side by side. Do you recall home many inches yours pulls at idle?

I'm having a hard time figuring out why just my cam change would impact the vacuum in the engine case that much. If I had like 5 inches of manifold vacuum at idle, that'd be one thing. 17 to 18 seems to be another.

...and your setup with no AFM flapper AND vacuum in the crankcase pretty much puts cold water on my hypothosis.

Jeff

[Jeff N.]

The first pot on the pro-m unit I think sets the idle flow. I know twisting it changes the cars idle behavior.

On the other hand, conversations with Mark D. have suggested (but I can't recall for sure) that the idle is mapped similar to the WOT circuit. Basically a static fuel table based on RPM and likely engine coolant temp.

A simple experiment is to lean out the "idle" pot, pull the cap and see what happens.

Jeff

cap, if its lean as in set to emissions lean then opening the cap and creating a vacum leak would lean it out to the point where the idle would be more affected... can you set the idle mixture with your mass air flow setup?

[632 Regal]

hey Jeff try laying a piece of paper over the oil fill hole to see if there is any vacuum at the opening.

[Jeff N.]

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hey Jeff try laying a piece of paper over the oil fill hole to see if there is any vacuum at the opening.