duowing

Alright, quick question about my vacuum readings. On the autos idle vacuum in drive will be lower than idle vacuum in neutral.

Anyway I've noticed that in the day time, I can go for a good drive, get everything completely warm and my vacuum readings will rest at 17-18inHG, this has occurred 3 days in a row. At night my vacuum readings seem to sit more around 15-16inHG, again about 2 days in a row, tonight would be third day. Two things have crossed my mind on this. Maybe my drive to work isn't quite long enough, so maybe the Air Regulator hasn't fully closed? Or is it possible that at night with headlights and everything on, that the alternator puts more load on the engine which brings vacuum down more, similar to the way that idling in Drive is putting more load on the engine and bringing vacuum down to a lower reading than in Neutral? It's not a big deal, but I've been reading alot of sites that show you how vacuum readings can tell you all about the condition of your engine.

As for the Air Regulator closing, I really do think I just can't find a good one or something. Temperature is about 75 today, I went for a drive and not even halfway down the street my A/F gauge was bouncing around showing that the O2 sensor was warmed up, but it took me at least a good couple of minutes before my idle finally came down to about 700 and my vacuum readings went to about normal, indicating that the Air Regulator finally closed.

280zx2by2

remember im a noob, plus i dont understand half the vaccum lines and their purpose but could it be the seal on the EGR valve?

NismoPick

Because the EGR only works when the sun is out?

280zx2by2

well the erg is made of a selophane compiste and when it has a constant convective heat source then it expands... i dont know thats why i said i was guessing

NismoPick

Here's some advice: DOWNLOAD AND USE FIREFOX AS YOUR INTERNET BROWSER. It has spell check for you... which you REALLY need.

More advice: Look at the words on the screen while you type (instead of your fingers). That way you can (hopefully) spell words correctly.

280zx2by2

i acctully hvae firefox, i just dont use it... ill check it out now

Ratfink

More importantly.. What is your idle at the given vacuum? I think you were correct when you were talking about load. If it is lowering your engines rpm's just a little, it can make a big jump in the inches of mercury read on the vacuum gauge.

Your car is running fine right? I wouldn't worry about it at all. I thought I was picky, but I didn't think to monitor the difference in vacuum between day and night. hahaha

Just giving you a hard time. Valid question. Seems like a question you would see in a automotive tech class.

Did you try turning all your lights off, letting your battery get back up to charge, and seeing if your vacuum increased? It would take a little while fir the alternator to catch up to the deficit in the battery. But, it shouldn't take long. I have manual control over my alternator through a switch. I like being in control of everything. But, I can tell a pretty fair jump in idle speed when turning on the alt. And if your car makes up for the loss in idle speed by adjusting the IAC (inlet air control) in a fuel injected engine, it would let more air in, increase idle, or maintain it in this case. And thus decrease the vacuum.

Also. Because of the position of the 02 sensor, it would be one of the first parts of the engine to heat up, and get to operating temps. The rest of the engine has much more mass, and cooling, and takes longer to heat up. Again, I don't think I would worry about the Air regulator. It sounds to me like it's doing it's job. Better to let the idle down a little late, then a little soon and have a car that dies on you all the time.

Niku-Sama

cooler more dense air at night?

Ratfink

Vacuum would be based on ambient pressure vs. the pressure absence caused by the pull of the engine. Lower temperature actually creates lower pressure. Pressure and heat are directly related. Almost one in the same.
If anything, I think that more dense air would actually create a higher vacuum. But, I am not the best when it comes to chemistry. Still planning on taking classes.

duowing

My vacuum yeah I played around and noticed it would vary by what I had running and stuff. Then I found that the little line going to the factory boost gauge had started to split open. So I just removed it and plugged off the line. I noticed another slight increase in vacuum. My vacuum will sit in Neutral or Park right around 18-19 inHG, this is with a 750-800 RPM idle. It used to sit right around 20inHg in Neutral, but this would have been when I had the AAC and VCV hooked up. After I removed those my car idled super low, and I had to adjust the throttle stop to bring the idle up, so I figured by doing that, it would cause vacuum to lower a little.

Yeah, the car runs really well though, so I figured it's nothing to worry too much about. Oh and by the way if anyone knows, how much variation can you see by doing a compression test without holding the throttle at WOT? Last time I did a compression test I held the throttle at WOT and got 140 across the board. About 5 months later I did a test, and was down to 130 across I was kind of depressed to see a drop like that only about 1000 or so miles later. Later on I realized I didn't hold the throttle open, just left it closed. This would also be with plugged off Air Regulator, VCV, AAC, etc at the time with the other 5 plugs still in.

Ratfink

Not holding the throttle wide open could make a very big difference.

When doing a compression test, you want to remove as many variables as possible.

Make sure the engine is at normal operating temperature. This is something that everyone forgets. Go ahead and remove all the spark plugs at once. This will let you do the test easier, and faster. Disconnect the power going to your distributor, or coil. You don't want to end up on the ground pissing yourself. haha

Hold the throttle wide open, and crank it until the needle will not move any more. I only use a compression tester with a check valve. That way it will hold the reading. Do this on all cylinders, and make sure that the tester is sealed well. I got 190psi on all cylinders with like 1psi difference. I was shocked to see those numbers when I did mine, and did it a couple times to make sure. Even did my friends car at the same time. That was a much higher reading than I was used to on everything except for my motorcycle. But, you have a turbo, and lower compression pistons. So I think that 130 should not be to bad. You should be more concerned with the difference between cylinders.

duowing

Well that's what I wanted to hear. Anyway yeah my compression when I did it without holding the throttle open was 130 across. Factory Spec at least according to the Haynes is 142 psi. So I figured it's in pretty good shape. I did it at operating temp, I also made sure to pull the coil wire and the injector connectors off so I wouldn't flood out the cylinders or anything. Like I said at first I was bummed to see this 10 psi drop across the board, then later on I was like hmm...I forgot to hold the throttle open on all of those.

Ratfink

I don't even worry about factory spec too much. That's great at the right ambient temperature, at sea level with good humidity...

When rings wear, they don't normally lose a perfect even amount of compression across the board. You see one or two cylinders with excessive blow-by, and the rest hold up until things get bad.

That's why I say to watch for consistency more than the correct pressure.