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Discussion Starter #1 (Edited)
Tips for Checking for vacuum leaks.
Some of the sensors and some of the actuators either read the amount of vacuum, or use vacuum to move controls, therefore it is important to the proper functioning of the engine that your vacuum system is fully intact. Otherwise one may get codes falsely indicating that there is a problem with a controller/actuator, or a sensor when in reality it is caused by a lack of vacuum.

Look at your rubber vacuum hoses (they are about 1/4 - 3/8 inches in diameter, if they are dry rotted or cracked, replace them. IF they slip on/off of their connectors too easily, they may have stretched out a little bit, cut the ends off, and put them back on.

Courtesy Miesk5 BroncoZone.com. You may listen for a leak. Sometimes using a cut off piece of garden hose is a good aid to listen with. Sometimes a cardboard tube (like from gift wrapping paper) is helpful. Just remember that whenever you stick your head inside a running engine compartment, that you are not wearing any loose clothing or jewelry that can get caught.

Another suggestion is to use a spray can of carb cleaner and spray around the bottom of the intake manifold, and the base of the throttle body assembly. IF there is a leak, you should hear a change in your engine.
Keep a fire extinguisher handy, or use water instead of carb cleaner (it works, but not as well.) Be aware that if you use water and the wires get wet and it runs worse... your wires may be bad, and the water is allowing them to arc instead of jump the gap of the spark plug.

Another suggestion is: to take a propane torch, turn it on (but don't light it) and move it over the vacuum hoses and connections. The engine RPM's will increase if there is a leak and the propane gets sucked in.

Courtesy of TheOldWizard ... Don't forget to remind folks to check the PCV valve, hose and grommet ! PCV = ( Positive Crankcase Ventilation ) valve.

Courtesy our friend Ramnasal... Clamp off the hose that runs to the vacuum booster (brake booster) or pull it and plug it. It is possible that the diaphram on the inside of the booster may go bad and cause a vacuum leak.

And one more place that is not always obvious is that the vacuum reserve canister in older vehicles sometimes rots out and leaks, but rarely gets checked. In older vehicles it looks like a metal juice can, in newer vehicles it is made of plastic.

EDIT: courtesy of riggs at fullsizebronco
You can do the smoke trick while the engine is off , basically plow a bunch of cigarette smoke through a vacuum line and you may be able to see smoke leaking out where the leak is.

Note:
there may be smaller vacuum leaks inside the passenger compartment. FYI the duct-work actuators are usually controlled by vacuum
This will be covered in the heating and cooling section of this forum
 

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Discussion Starter #2
Another method of checking for a vacuum leak is by using a scan tool capable of displaying short term fuel trim tables.

Observe the short term fuel trims while pinching off vacuum hoses. If the SHRTFT trim decreases by 15% or more you've located a vacuum leak. (allow at least 20 seconds for each pinch, for any change to register) Ofcourse don't try to pinch off hard plastic lines (you'll need to disconnect and cap them).
 

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Discussion Starter #4
It depends upon how big of a leak it has.
1. it could have a high idle
2. it could have a surging idle
3. it could have a very rough idle and stall
 

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tagging on to you answer.( 99 ford ranger 4.0 ohv 5 speed) my idle is fine sitting, driving it seems to be a bit weak in power, i have a CEL for both o2 sensors being lean. and when i back of the gas going slightly down hill. or barley on the gas in 4th gear at about 2000 rpm it runs really rough and jerky. could this be a vacuum leak problem?
 

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Discussion Starter #6
i have a CEL for both o2 sensors being lean
P0171 and P0174 codes are often caused by vacuum leaks. THere may be other reasons (such as low fuel pressure, or dirty/malfunctioning MAF)
Thanks for bringing that up.

Please start a new thread for your issue.
Thanks! :)
 

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Thanks for the VERY useful tips... I am trying to chase a Vacuum leak in my 2004 Econoline... Using your tips I am checking the connections first... If you have any knoledge of a common issue with the Econoline's please let me know... I can use all the help I can get!!!
 

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Bikeguy2011
I had a vacuum leak on my '02 Windstar that was giving me lean codes on both banks (3.8 L - V-6). The shop said they couldn't locate it. This was a couple of years ago and my engine light has been on since then. I recently decided to change the plugs and wires, which meant pulling off the upper part of the intake manifold (the plastic clamshell). When I did that, I found some build up of burned oil around where it meets the lower intake manifold. I scraped those clean (making sure not to get anything down into the manifold) and since I had it all apart, I changed the seals as well. When I put it all back together, voila! The lean codes were gone!

Hope this helps!
 

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Bikeguy2011
I had a vacuum leak on my '02 Windstar that was giving me lean codes on both banks (3.8 L - V-6). The shop said they couldn't locate it. This was a couple of years ago and my engine light has been on since then. I recently decided to change the plugs and wires, which meant pulling off the upper part of the intake manifold (the plastic clamshell). When I did that, I found some build up of burned oil around where it meets the lower intake manifold. I scraped those clean (making sure not to get anything down into the manifold) and since I had it all apart, I changed the seals as well. When I put it all back together, voila! The lean codes were gone!

Hope this helps!
Be careful in blaming vacuum leaks causing lean codes. Understanding of the lean codes is paramount to diagnosis. A lean code is caused by excess oxygen in the exhaust stream. This can also be caused by a misfire. Most people think that a misfire will cause a rich code since raw unburned fuel is going into the exhaust. BUT, unused oxygen is also going into the exhaust. O2 sensors by nature do not measure hydrocarbons. They measure oxygen, hence the name. If your car is misfiring because of bad plugs or wires, you will receive a lean O2 reading or code but when you look at the injector pulse width it will show rich.

Think like your ECU, if there is too much O2, then it is going to tell the ECU that it needs more fuel, thereby increasing the pulse width of the injectors.

While vacuum leaks are a prime reason for lean codes, they aren't the only reason.

I have actually seen conditions where the misfiring cylinder caused the rest of the plugs to become fuel fouled with too much gas. It is a very vicious cycle that these systems can lock into.

This can also cause major drivability issues if the system uses a mass air flow sensor. ANY leak after the sensor will cause the MAF to misregister the amount of air being fed into the engine causing all sorts of drivability issues.
 

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thanks for all the tips. i think i have a leak and this thread is helpful. As long as i don't catch on fire. thanks again i'll give it a go.
 

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Tips for Checking for vacuum leaks.
Some of the sensors and some of the actuators either read the amount of vacuum, or use vacuum to move controls, therefore it is important to the proper functioning of the engine that your vacuum system is fully intact. Otherwise one may get codes falsely indicating that there is a problem with a controller/actuator, or a sensor when in reality it is caused by a lack of vacuum.

Look at your rubber vacuum hoses (they are about 1/4 - 3/8 inches in diameter, if they are dry rotted or cracked, replace them. IF they slip on/off of their connectors too easily, they may have stretched out a little bit, cut the ends off, and put them back on.

Courtesy Miesk5 BroncoZone.com. You may listen for a leak. Sometimes using a cut off piece of garden hose is a good aid to listen with. Sometimes a cardboard tube (like from gift wrapping paper) is helpful. Just remember that whenever you stick your head inside a running engine compartment, that you are not wearing any loose clothing or jewelry that can get caught.

Another suggestion is to use a spray can of carb cleaner and spray around the bottom of the intake manifold, and the base of the throttle body assembly. IF there is a leak, you should hear a change in your engine.
Keep a fire extinguisher handy, or use water instead of carb cleaner (it works, but not as well.) Be aware that if you use water and the wires get wet and it runs worse... your wires may be bad, and the water is allowing them to arc instead of jump the gap of the spark plug.

Another suggestion is: to take a propane torch, turn it on (but don't light it) and move it over the vacuum hoses and connections. The engine RPM's will increase if there is a leak and the propane gets sucked in.

Courtesy of TheOldWizard ... Don't forget to remind folks to check the PCV valve, hose and grommet ! PCV = ( Positive Crankcase Ventilation ) valve.

Courtesy our friend Ramnasal... Clamp off the hose that runs to the vacuum booster (brake booster) or pull it and plug it. It is possible that the diaphram on the inside of the booster may go bad and cause a vacuum leak.

And one more place that is not always obvious is that the vacuum reserve canister in older vehicles sometimes rots out and leaks, but rarely gets checked. In older vehicles it looks like a metal juice can, in newer vehicles it is made of plastic.

Note:
there may be smaller vacuum leaks inside the passenger compartment. FYI the ductwork actuators are usually controlled by vacuum
This will be covered in the heating and cooling section of this forum
 

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I chased a vacuum leak for months in a 93 Aerostar. I finally found it in a cracked 1" long elbow hose UNDER the air cleaner box, which must be removed to replace the hose. Of course the hose was not available new, so I wound up removing 7 air cleaner boxes in Aeros in a u-pull-it yard before I found a good elbow hose.
 
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