Varying Vacuum Readings?

[DuoWing]

I've been going through my engine bay and fixing vacuum leaks, blocking off anything that might leak. From what I can tell the EGR may be leaking just a small amount, and I know my oil dipstick doesn't seal too well. Anyway I've noticed something odd.

 

Yesterday around 6pm or so I took a drive, my vacuum hovered right around 17-18inHG at idle. The car is on auto so my vacuum is a little lower at idle in Drive. Anyway then at night coming home from work I noticed my gauge was reading around 15-16inHG at idle. This morning I saw there was a little vacuum line that I must have disconnected by accident. I plugged it back in, went to the auto parts store. There and back vacuum at idle was 17-18inHG again. On my way to work and on my way back idle vacuum was down 15-16inHG again.

 

This isn't really a big problem or anything, but I'm just curious what would cause this. The only thing I can come up with would be at night with the lights on and such, that would increase the load on the alternator and the engine, which would cause engine speed to drop a little and vacuum to drop? Does this sound right? I'll have to take the car for another drive in the morning and see if vacuum is back up again.

[woldson]

Stabing into the dark, car gets nice a warm on a drive, and compression drops a bit?? Or a seal starts to leak a little?

[DuoWing]

I was thinking that at first, but yesterday being about 75 degrees out. I took the car for a nice long drive to make sure everything was warmed up all the way. Vacuum remained around 17-18inHG, but like clockwork at night it was lower. I'm thinking it may be definitely related to the alternator.

 

I need to run a check on my alternator to make sure it's working properly as I still have issues with my idiot lights staying on every so often. I measured the battery voltage across the terminals one day to find barely even 12 volts while holding the car steady at 2k rpm, no lights, stereo, etc turned on.

 

I've also heard a bad battery can cause this, but the battery is fairly new and not even a year old, plus I generally don't have problems on startup. It seems to turn over everytime. Although every now and again I will need to jump it, but that seems to be when I turn the car off before turning the headlights, stereo, etc back off and then the car off.

[woldson]

When the vacuume falls off is there any problem with performance?

I personally never have kept an eye on vacuume for a prolonged period of time, it might be normal (guessing).

Have you checked, (which car?) the atmosphere controls?

[naviathan]

Your vacuum sounds fine. The fall off at night is probably due to engine load with all the lights on. Keep an eye on that alt though, sounds like it might be (going) dead.

[woldson]

Thanks Naviathan:).

[DuoWing]

As far as performance there wasn't any difference. My Air Regulator apparently doesn't work too well. It seals, but it takes a long time to close. I noticed that when it's completely closed vacuum hovers right around 16, if not a little closer to 17 so the drop really isn't much different. I think the cooler temps at night just make everything take a bit longer to warm-up as my drive to work and back takes maybe 5 minutes.

 

I can't seem to find an air regulator that actually closes normally. One day it was about 75 degrees out. My O2 sensor came on within about a minute or so, but it was s till a ways down the street and a couple of stops later that the air regulator was finally closed and my idle went down and vacuum went to normal.

[naviathan]

The air regulators are nothing more than a heating element that expands a bimetal contact that closes the air opening. On a cold day it probably will take a lot longer for the regulator to close up. The O2 shouldn't kick on until after the 5 minute (roughly) warm up process completes.