Hello Again to ALL,

This is going to sound like the ultimate dumb a$$ question to all probably, but maybe a few others would get some insight so I will show you all why I ALONE have set the gene pool in my family back 100 years.........I am trying to understand the significance of engine vacuum and its effects on my car. I have an 85 automatic. Could someone please explain where, and how it is developed, measured, and how such measurement acted upon? My car does not have a MAP sensor as far as I can see. I guess it effects the egr in some way but is its presence actually measured? What would a total absence of vacuum cause to happen if a line were completely severed? And could a blocked line have the same effect? Is my ECM looking for vacuum somewhere? How does it verify its there? And what condition must the engine be in(running, idling, off ?) For me to be 100% sure the entire system is working? And does it effect the operation of my transmission in any way? I understand if no one wants to respond before they stop laughing and get up off the floor.......
Geoff

 

Don't be so hard on yourself, that's our job.


Your engine is a big air pump. The throttle body restricts, or "throttles" the inlet flow to directly control the amount of air (and indirectly, the amount of gas) that goes to the motor. Your engine trying to suck air through the throttle body, creates a low pressure area in the intake. This low pressure is vacuum. Vacuum measures how much lower it is than atmospheric pressure.

At lower throttle, or idle, you have more restriction, and therefore, more vacuum.

As you go towards wide open throttle, the vacuum gets closer to zero (atmospheric). It never gets to zero of course, because there's still some restriction.


If its older than 90, your car's computer doesn't measure vacuum, it doesn't care.

The vacuum in the plenum is used to control various things on the car, emissions items, timing advance (on carbureted cars), fuel pressure, EGR, power brakes, etc.

When you have a vacuum leak, typically it's because you broke one of those vacuum lines, which allows extra air to be sucked into the intake, bypassing the throttlebody. This extra air will cause the engine to speed up, no different than if you pressed the throttle a bit. Those lines are small though, and if one was cut, you'd still have vacuum, just not as much.

Vacuum won't directly affect your transmission operation.


Anyone, what's your real question, what's wrong with the car?

 

In addition, a couple small vacuum leaks probably won't make any difference to your engine because of the oxygen sensor in the exhaust pipe.

The Oxygen sensor is like a report card to the computer, it senses the amount of oxygen in the exhaust air, generates a small current, and voltage, and that's the ultimate signal the computer uses to alter the fuel mixture (in addition to all the other sensors).

so if your engine runs a little lean, (with a vacuum leak) the oxygen sensor reports back and tells the computer. The computer takes all of the other sensor's data, makes a correction, and changes the fuel injector's pulse width (time open) to enrich the mixture.

An 85 doesn't have a map sensor, it has a MAF sensor, the MAF sensor actually measures the amount of air entering the intake.

a map sensor is usually found on 1990-1991 corvettes. It measures vacuum, and the computer takes the map signal voltage and determines a fuel estimate in conjuntion with throttle position sensor, tach feedback, coolant sensor, knock sensor, vehicle speed sensor, etc.
a map sensor relays vacuum information to the computer. It uses that info to determine whether you are going up a hill or coasting down a hill or on flat and level.

more gas pedal position, no increase in vehicle speed with low vacuum means you gave it gas to maintain your speed going up an incline.

That means to the computer it must increase the pulse width and or increase the number of pulses to add the fuel called for by all the sensors.

The Maf sensor is also air related, it has a hot wire in it that is electronically temperature controlled. When the air rushes through it, the hotwire cools off, and the electronic circuit adds in more current to keep it at a specified temperature. THe circuit monitors that current, and interprets that value as an airflow equivalent. That equivalency voltage is interpreted by the computer as the engine digesting a certain CFM.(cubic feet of air per minute.) It is usually more accurate than using a MAP.

Older carbureted cars usually have a vacuum modulator to help the trans to know when to shift. when the vacuum gets high, it's time for another gear. when the vacuum reaches a low, it's time to go back wards and downshift.

get it?

 

I am beginning to understand it more....I read so much about vacuum...and assume it is one of the critical conditions in my car for proper engine control by the ecm....but begin to see it is secondary.....my car.....?????!!!! seems like 5 different cars. Lately if I get in start it up and take off, it is at its worst. I can go out in the morning start it up and give it 10 minutes to idle while I make a cup of coffee.....and it seems to help much. But either way it has the personality of the more you put your foot in it....the more it knocks...low throttle input its somewhat tame, and you may think there is nothing wrong...It can initally accelerate, but very soon after will knock and if you really put your foot into it at that point will knock very badly and even occasionally make the check engine light come on. When it drops into overdrive it seems something is cutting in and out. It does not feel like the transmission is slipping.....no, put you foot in and the engine takes off but the car slowly accelerates. As long as there is no rapid call for power it can move the car along.....But if I start from stop and ask for long term moderate acceleration It will get going initially ok...but as sepped builds begins to knock....and if I continue...will finally drop into overdrive knock worse....and slowly struggle on up cutting in and out. Again, (no slipping) like something is turning on and off. If I take my foot off the gas as it goes into overdrive and just ask for mild accelleration, It can respond with no knocking to most any speed. It just wont go there fast anymore. Not sure if anyone can understand all that. I have replaced the EGR, IAC, relocated the IAT to the front of my air box. Fuel pressure reg. wires, cap, rotor, ignition module, chip and thermostat (which are supposed to go together) when the egr and FPR and IAC went in had the intake cleaned of carbon. I have verified the burn off relay on the MAF makes the wire glow, which it does but not EVERY time it seems, but does work.....I just cant figure this out. I have an auto Xray scanner whickh I find does not produce as much useable information as they led me to think and am currently gearing up to make a small laptop run winaldl. Just wish I could understand the car more and make it the best it can be. Thanks for the Vacuum lessons. I was turning my hopes and wishes there and didnt understand its implications in all this.....
Geoff

 

You are catching on but I should mention that vacuum is really very necessary even though not directly monitored by your ECM. Strong vacuum at idle (18 to 20 in) is commonly regarded as a good indication of a sound engine mechanically since a low reading can be a symptom of incorrect valve or ignition timing, poor valve sealing, worn camshaft or piston rings.
As others have mentioned vacuum is also used to control other critical systems, in addition to A/C heater controls it may also control air injection which is necessary for proper emission/catalytic converter control and spark advance on some vehicles. The EGR valve needs a good vacuum supply to open, if it does not open that can cause pinging or spark knock. If supply lines to those systems leak it can effect performance even if total manifold vacuum is not substantially reduced by the small leak.
You mentioned you replaced the EGR valve but I would suggest checking that the supply lines to the valve are intact. If the tube or intake passages are restricted by carbon deposits that will reduce EGR function even if the valve and vacuum supply is good so that would be the next thing to check, A FSM will help tremendously when diagnosing these systems, best of luck.

PS: Should probably mention a high performance camshaft can cause a low vacuum reading as well, the above figures assume a stock engine.

 

Yep, in fact internal combustion engines are really no more than a vacumn machine...really! Any vacumn leaks will have a negative effect on idle or total performance and anything in between, brake booster, EGR valve, vac. lines or even a leak in the intake, and can play havoc with the ECM regarding fuel delivery.

Check out this site.

http://www.secondchancegarage.com/public/186.cfm

 

thanks guy's. In all these posts like this I end up learning something new, even if I understood the basics.

see ya on the road

 

As a related example, my 300K mile Astro minivan, V6, TBI induction system started having problems with a "hard" brake pedal effort. It gradually progressed to the point it got my attention and I finally took a look-see. Turns out the plastic 3/8" diameter elbow vacuum fitting on the brake booster had cracked and eventually broke completely off. That explained the gradual increase in brake effort. Meanwhile, the engine never lost a beat; no increased idle, no codes, just normal operation. I figure the oxy sensor and computer adjusted for the extra air and the IAC (idle air control) motor just kept reducing the idle air until the idle was correct. If this had been a carbureted engine, it would have run leaner and leaner until it quit. Aren't computers wonderful!

 

It couldn't be said better than that......I have a problem with my car, I do have a Chevrolet Corvette factory service manual, and I also bought Corvette fuel injection and electronic management by Charles O. Probst, and I also have motorbooks Chevrolet Fuel Injection by Ben Watson......and I am sure good money was spent to have writers, and editors put these books together...but these publications didn't allow me to understand vacuum principals and their application to "ME" like the information I just recieved here for the price of just saying hello...... and I'll I have to do to pay the price is say thank you.....?????
Only in America.......You guys rock.......Thanks Again

 

a quick check for a vacuum leak on a 85 corvette:
1. when you go up a hill, does your heater go full hot?
2. when you go up a hill with the cruise control on, does your car lose
speed?

answering yes means you have a vacuum leak that's not allowing your car to store enough vacuum long enough for you to get on straight and level highway, and restore the vacuum to the system.
(the ball or/and accumulated vacuum tubing tied to the engine)

What you are talking about is driveability. This requirement changes from the second you touch the starter to the time the key is turned off.
That's why you need so much equipment to control the modern engine.
In the years past, the carburetor in conjunction with the distributor were the 2 main controllers.
today, it's the computer with it's sensors. With good information going in to the computer, it can govern very well using fuel injectors, idle air controller, EGR valve, and ignition module.
The driveability part comes with getting your engine up to temperature, and staying in the normal temperature range so the computer can govern in a temperature range the engineers expected it to operate efficiently in.
For instance, the block and pistons and heads all expand with heat. The heat is transmitted to the fuel as it's blown through the intake ports in the head which in turn affects the ability of fuel to vaporize into a mist warm enough to promote better burning charicteristics in the combustion chamber. The fuel then is burned, and blown into the tail pipe, based upon the initial timing, valve timing, engine speed, ambient temperature, spark control, etc, your engine "runs fine".
take even one control, or feedback to the computer away, and your engine "runs like crap"
you must evaluate all the engine controls and functions, check past computer fail codes, check spark plug condition, check fuel pressure, check large vacuum detractors like power brakes, and PCV valve operation, then check subsystems to properly isolate the culprit.
chasing vacuum leaks is a means to an end, but after you finally check all the vacuum locations, you will probably have to check spark plugs, etc.
don't think "inside the box", there are many things that can affect your engine when it's cold.
Cold engines are what's said to be open loop, meaning the computer sensors are monitored, but not used to totally govern engine driveability until the engine sees a warmed up temperature and the coolant temp sensor says ok let's go to total computer control with all the sensors reporting in to control driveability ("engine runs fine")

 

Check out this site.
http://www.thehumansolution.com/forcegauges.html