Understanding Vacuum reading at idle

[Stkid93]

I'm trying to understand the fists vacuum reading at idle. Sometimes it will be -10.5 ish, sometimes it will be -10 ish,And sometimes it will be -9.5 ish. But it doesn't violently fluctuate. For example, if I'm at a light for a while and the reading is -10.5. It will stay at 10.5 for a long time. Then I'll start driving again, and next time I come to a stop I'll notice it's closer to -10 or -9.5. I've also noticed the vacuum reading is different depending on if the car is on a hill or incline, so this may have something to do with the slightly changing reading? Is it normal for the fists to do this? And if not what would cause this?

I'm also trying to figure out everything that could impact a vacuum reading. Can you guys tell me if any of this stuff could slightly impact my reading? And also if I'm missing something?

-being on an incline
-Vacuum leak
- different weight oil
-Too much oil or coolant
Different spark plugs (step colder etc)

Whatever else may affect vacuum at idle

 

[dhminer]

Step 1: Remove accessport and store in glove box
Step 2: Worry less

 

[cidsamuth]

Nothing like getting a smartass FB like response from other members.

I don’t have all the technical explanations you are looking for. I can tell you the readings and fluctuations you are seeing are normative. Higher idle produces less vacuum …. and there are 1001 other things that affect it. I doubt you have any leaks of consequence.

Fun fact, the numbers you see are negative PSI. Most gauges actually express vacuum as in.Hg, which is roughly twice the -PSI. However, the Accessport isn’t smart enough to change units when it transitions from positive to negative.

 

[Dialcaliper]

The car will self regulate the throttle to keep the idle RPM, there are a ton of things that will influence that and the vacuum reading:

- Engine/coolant temp and where in the warmup cycle it is.
- Idle load on the engine - this is everything from alternator charging the battery to running A/C, the hill holder function on the ABS, whether the power steering is activating, cranking up the stereo, etc etc etc.

Generally the lighter the load, the more the ecu will close the throttle, and the deeper the vacuum (-10.5 to -11). More load requires more throttle and so the vacuum will be on the lighter end (-9 to -9.5, or lower)

Don’t read too much into little fluctuations, unless if you’re not seeing much idle vacuum at all (near zero) in which case there could be an issue

 

[dhminer]

Nothing like getting a smartass FB like response from other members.

I don’t have all the technical explanations you are looking for. I can tell you the readings and fluctuations you are seeing are normative. Higher idle produces less vacuum …. and there are 1001 other things that affect it. I doubt you have any leaks of consequence.

Fun fact, the numbers you see are negative PSI. Most gauges actually express vacuum as in.Hg, which is roughly twice the -PSI. However, the Accessport isn’t smart enough to change units when it transitions from positive to negative.

Wasn’t trying to be too much of a smart ass. Accessport anxiety is real. I used to worry about things like this all the time and not once was my anxiety justified. Simply trying to help him find the same solace.

 

[cidsamuth]

Wasn’t trying to be too much of a smart ass. Accessport anxiety is real. I used to worry about things like this all the time and not once was my anxiety justified. Simply trying to help him find the same solace.

Fair. I was projecting a little . . . when I did my engine swap on a track dedicated car, I also made some changes that affect manifold air pressure/vacuum behavior . . . like headers. Further, I installed an adjustable BOV (and I moved it's location) and I was exploring the correct spring to install in it based on the vacuum readings I was seeing when the throttle slammed shut. That got me asking on FB about my vacuum readings, to include the negative PSI versus in.Hg.

Aaaaannnndddd, I got the same answer you gave, which was a poke in the eye. Not sure what this guy was asking, but his questions are legit to him.

 

[dhminer]

Fair. I was projecting a little . . . when I did my engine swap on a track dedicated car, I also made some changes that affect manifold air pressure/vacuum behavior . . . like headers. Further, I installed an adjustable BOV (and I moved it's location) and I was exploring the correct spring to install in it based on the vacuum readings I was seeing when the throttle slammed shut. That got me asking on FB about my vacuum readings, to include the negative PSI versus in.Hg.

Aaaaannnndddd, I got the same answer you gave, which was a poke in the eye. Not sure what this guy was asking, but his questions are legit to him.

For sure that sounds troublesome! I somewhat dismissed OP’s concern because I think his car is nearly stock and he’s posted several threads that sound worried about totally normal operating parameters. I want him to have as much fun with this car as we all do. Worrying about AP gauges led me to treat mine as though it were super fragile, making it no longer fun. Hate to see others fall into the same trap.