Vacuum gauge not steady at idle

[Bluemustang]

After changing out my JLT filter from the oiled to the dry filter, I notice my vacuum gauge is not as steady at idle. It drops about an inch and then goes back up, drops and goes back up. I checked to make sure the filter was really tight. Any concern here or is this normal/par for the course? It fluctuates high 18s, low 19s and up to 20 at idle and in park.

No A/C on so that's not it. Lund tune on it. Car feels really good though so I am thinking it's just the tune adjusting load slightly for whatever reason. My light surging has improved or has gone away completely.

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[wildcatgoal]

When I put in a higher flow filter into my stock airbox my car did that when I first started it, then stopped after the first drive to the grocery store in the part of town where all the single girls go grocery shopping. Left empty handed, as usual.

 

[EFI]

It drops about an inch and then goes back up

You can really tell when the needle moves an inch of pressure each way. You must have some eagle eyes bro.

 

[Bluemustang]

You can really tell when the needle moves an inch of pressure each way. You must have some eagle eyes bro.

Mine is a non-PP so I have a digital boost/vacuum gauge. Probably the worst thing for an anal retentive like me.

 

[TexasRebel]

[MENTION=15869]EFI[/MENTION]
Maybe he's using the digital vacuum gauge?

 

[Bluemustang]

I don't think I have any of these issues. If anything maybe it's a small vacuum leak somewhere. Maybe I moved one of the hoses slightly when I put in the new filter.

 

[GTBOB]

Due to pumping efficiencies/losses of an engine, the ecu will do whatever it takes to keep it at the programmed idle rpm. Little fluctuations like this in vacuum and fueling will always happen, ultimately effecting engine load/ targeted tq. Hell, maybe the alternator was commanded to do some work??? I wouldn't worry about it.

 

[Bluemustang]

Due to pumping efficiencies/losses of an engine, the ecu will do whatever it takes to keep it at the programmed idle rpm. Little fluctuations like this in vacuum and fueling will always happen, ultimately effecting engine load/ targeted tq. Hell, maybe the alternator was commanded to do some work??? I wouldn't worry about it.

This I think is the answer. Plus the Lund tune keeps the idle at 740 RPM as opposed to stock which I think is 680?? But regardless, I think your answer makes the most sense and is probably correct.

 

[Grhumpy]

Hi Guys,

After installing a GT350 TB, I've noticed that my vacuum gauge drops to zero during acceleration, and this happens all the way to hi revs ... is it normal ?

At idle it's around 18-20inhg

I've got a airaid cai, GT350 TB and Bama tune

Thanks for your comments !

 

[TexasRebel]

Hi Guys,

After installing a GT350 TB, I've noticed that my vacuum gauge drops to zero during acceleration, and this happens all the way to hi revs ... is it normal ?

At idle it's around 18-20inhg

I've got a airaid cai, GT350 TB and Bama tune

Thanks for your comments !

It's not going all the way to zero, but yes that's normal. The butterfly valve in the throttle body is what determines the pressure differential. As the butterfly valve opens (during acceleration) the higher ambient pressure on the outside has a less restrictive path into the engine and the pressure difference between ambient and manifold reduce.

The larger GT350 throttle body provides an even less restrictive path than the GT throttle body. It's not much bigger in mm, but area depends on the square of the radius... adding a little to the radius adds a lot to the area.

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