Vacuum System

[drallan]

Just a general question here. 1999 3.4 v6 Limited 4wd about 200k.

I was tracking down a P0420 followed by a P0171 code and decided to give the MAF a good cleaning. It looks like that cleared everything up and hopefully I don't have to change out any O2 sensors. However, while I had the airbox out I noticed that a few of the vacuum hoses are starting to show their age. I wrapped the ends (that I could find) up w/ some electrical tape and made sure everything still fits snuggly.

But I am wondering, what should the normal vacuum range be? Truck runs fine, no noticeable issues. 4wd works relatively quickly and mileage is about 16 mpg. I have a Bluetooth OBDII connector and Torque Pro. I looked at the Vacuum reading and it shows an average of about 20, although that changes depending on the throttle.

One thing that I found odd was that the vacuum drops when I step on the gas. So if I gun it to say merge into traffic it drops to 5-10 and climbs when I let off 25-30. How does the vacuum system work? I would assume it would be the other way around. The more gas you give it the more air is sucked into the plenum and the more vacuum you get.

Is my logic wrong or something w/ the vehicle?

Thanks,

 

[alanflies]

Vacuum is basically just negative air pressure. -20 at idle, That's normal...It's a function of the throttle blade opening and closing as you move the accelerator. When you open the throttle blade, more air is able to move into the plenum, you get decreasing vacuum. When you close the blade rapidly, the higher engine rpm is trying to suck more air against a closed throttle blade, you create more vacuum behind the throttle blade. nowiamsane?

The real fun starts when you can get your numbers to climb up to +25 when you have the throttle blade open...My daily driver idles around -20. When I mash the go pedal, it climbs to +20..."Oh what a feeling"...Toyota!

 

[phattyduck]

Bingo. The engine (valves and pistons) are a constant air pump, sucking air in. The throttle plate is a limiter that keeps air from getting to that pump. The less air it lets through, the more vacuum (air pressure below atmospheric pressure) you have. More air pressure (less vacuum) means the air pump (engine) gets more air and can make more power.

-20 at sea level is fine. Your engine will make less vacuum at higher altitudes, as it is a relative measurement to atmospheric pressure. And yes, the real fun comes above 0 vacuum - yay for boost!

-Charlie

 

[cbr900boy]

Just a general question here. 1999 3.4 v6 Limited 4wd about 200k.

I was tracking down a P0420 followed by a P0171 code and decided to give the MAF a good cleaning. It looks like that cleared everything up and hopefully I don't have to change out any O2 sensors. However, while I had the airbox out I noticed that a few of the vacuum hoses are starting to show their age. I wrapped the ends (that I could find) up w/ some electrical tape and made sure everything still fits snuggly.

But I am wondering, what should the normal vacuum range be? Truck runs fine, no noticeable issues. 4wd works relatively quickly and mileage is about 16 mpg. I have a Bluetooth OBDII connector and Torque Pro. I looked at the Vacuum reading and it shows an average of about 20, although that changes depending on the throttle.

One thing that I found odd was that the vacuum drops when I step on the gas. So if I gun it to say merge into traffic it drops to 5-10 and climbs when I let off 25-30. How does the vacuum system work? I would assume it would be the other way around. The more gas you give it the more air is sucked into the plenum and the more vacuum you get.

Is my logic wrong or something w/ the vehicle?

Thanks,

I replaced every single vacuum line under the hood with silicone vacuum hoses. This company makes awesome hoses, I saved a ton of money by just cutting to length and using zipties for good measure. This "shop kit" comes with all the various sizes to outfit our motors with extra to spare.

Do not rely on the Torque app to detect a vacuum leak, it is useless. For shit and giggles I actually unplugged a vacuum line and the app didn't even detect it. The only true way to test vacuum is to buy a basic vacuum meter and plug it in the front of the plenum. It should be at around 20 in Hg while at idle. Check out this video. I was able to address all of my vacuum leaks using this method and replacing the hoses.

 

[TheDurk]

I replaced every single vacuum line under the hood with silicone vacuum hoses. This company makes awesome hoses, I saved a ton of money by just cutting to length and using zipties for good measure. This "shop kit" comes with all the various sizes to outfit our motors with extra to spare.

Do not rely on the Torque app to detect a vacuum leak, it is useless. For shit and giggles I actually unplugged a vacuum line and the app didn't even detect it. The only true way to test vacuum is to buy a basic vacuum meter and plug it in the front of the plenum. It should be at around 20 in Hg while at idle. Check out this video. I was able to address all of my vacuum leaks using this method and replacing the hoses.

Don't blame Torque. It only reports what the ECU tells it. Blame the Toyota OBDII system.