Finding vacuum leaks

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(→"Untraceable" vacuum leak)
 
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Here's a method for diagnosing a bad running engine that may have a vacuum leak:
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{{youcanedit}}
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==Overview==
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An engine having a vacuum leak will tend to have an uneven, "hunting" idle speed. If the vacuum leak is sufficient, it may even run poorly under light throttle settings.
  
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#Get a soldering Propane tank and valve attachment.
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Unless the vacuum leak is very large, the symptoms will disappear with higher engine speeds due to the relatively small amount of unmetered air entering the engine under those conditions.
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#Unscrew the end of it so you have a threaded pipe without the nozzle.
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#Attach a 2 foot piece of hose to the end.
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==Causes==
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#Start the engine and let it warm up a bit.
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*Split, cracked or broken vacuum lines
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#Turn on the propane and run the hose all aroung the vacuum lines and the bottom of the carb.
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*Leaking intake manifold or carburetor gaskets
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*Leaking carb gasket or loose carb-to-intake fasteners
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*Open carb or intake manifold vacuum ports
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*Loose/missing carb body/throttle plate screws
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*Faulty vacuum devices like:
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**AT modulator
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**Brake booster quite commonly un or misdiagnosed
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[little harder brake pedal remove booster vac line plug & idle will improve]
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**Vacuum advance canister
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**Carb choke pull-off
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**HVAC controls
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**Emissions equipment
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***Check valves
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***Vacuum switches
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***Charcoal canister and hoses
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***PCV hose, valve or grommet
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==Finding a vacuum leak==
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There are several methods for locating vacuum leaks. If you suspect a vacuum leak on a carbureted or TBI engine, you may be able to verify your suspicion by performing a simple check.  With the air cleaner removed and the engine at idle speed, use the choke blade to ''partially'' block the airflow into the engine. Or you can place a wadded-up rag ''lightly'' over the throttle body throat to partially choke off the air supply.  If this artificial richening of the air/fuel mixture causes the engine rpm to increase, you may well have a vacuum leak.
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Often a vacuum leak will cause a whistling or hissing noise. Use an automotive stethoscope or even a length of vacuum or fuel hose to listen for the vacuum leak with the engine idling.
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A stethoscope is self-explanatory. To use a piece of hose, put one end of the hose by your ear and use the other end to probe around for the leak.
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{{Warning}}Stay away from any moving parts like the belts and fan blade. the fan or fan belts. 
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{{Warning}}NEVER use a long screwdriver or other stiff object to locate engine sounds! This type of thing could kill a person if it was jammed into the head.
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==ATF method==
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Using ATF mixed with a flammable solvent in a squirt can, apply to the suspected areas. Listen for a change in the idle speed/quality, AND watch for the tell-tail white smoke caused by the ATF. There should be a distinctive odor associated with the burning ATF as well.
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==Propane method==
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#Get a propane bottle and a soldering torch and valve attachment.
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#Unscrew the end of torch so that you have a threaded pipe without the nozzle.
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#Attach a 2-foot piece of hose to the end.
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#Start the engine and let it warm up.  
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#Turn on the propane and run the hose all around the vacuum lines and the bottom of the carb.
 
#If the engine smooths out or revs up a bit, you have found your leak.
 
#If the engine smooths out or revs up a bit, you have found your leak.
  
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PLEASE do this in a well ventilated area due to propane gas and exhaust fumes!!
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{{Warning}}Hold propane bottle UPRIGHT during testing as liquid propane may run out of an inverted bottle.
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{{Warning}}This MUST be done in a well ventilated area! Only do this outside of a garage and away from dwellings.
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==Carburetor cleaner method==
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#With the engine running, use a can of carburetor cleaner to spray down the hoses and vacuum connections.
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#If there's a change in the idle speed or if the engine runs smoother, you might have found the vacuum leak.
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'''Note:''' You may also use engine starting fluid instead, as it does not leave residue.
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{{Warning}}Carb cleaner and starting fluid are extremely flammable! If this is attempted, have a fire extinguisher at hand. Keep away from sparks and any other sources of ignition. Even a cross firing plug wire could ignite such materials.
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{{Warning}}This MUST in a well ventilated area! Only do this outside of a garage and away from dwellings.
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==Vacuum gauge method (internal)== 
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{{Note1}} See [http://www.crankshaftcoalition.com/wiki/Vacuum_gauge_engine_diagnosis '''Vacuum gauge engine diagnosis'''].
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The following procedure may be used to determine an internal leak:
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#Disconnect/remove the primary wire between the coil and distributor.
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#Adjust the idle screw so the throttle plate in the carburetor is completely closed.
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#Remove PCV, breathers, and all vacuum ports on carburetor/manifold and seal openings with plugs/tape.
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#Connect vacuum gauge to a manifold vacuum source.
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#Have assistant crank engine while you read vacuum. If gauge reading is 0-2", you may have an internal leak between the manifold/head.
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{|
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|-align="top"
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|[[File:Vac2normal.gif|thumb|400px|Normal vacuum]]
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|[[File:Vac4vac leak.gif|thumb|400px|Vacuum leak]]
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|}
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=="Untraceable" vacuum leak==
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Occasionally on engines that use the intake manifold to seal the lifter valley, you may experience a vacuum leak from the lifter valley area into the intake ports. This may be due to a bad gasket seal or because the intake manifold-to-cylinder head interface was not machined parallel (usually done to correct for [[milling cylinder heads]] and/or block).
  
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No amount of propane or carburetor cleaner will find such leaks. The best way to insure there's no such problem is to use quality intake gaskets installed correctly, and to double check any machining before final assembly of the engine.
  
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Another method:
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These can be found by removing pcv and plugging pcv hole and attaching vac gauge to pcv inlet, then seeing if crankcase develops vacuum!  GM 3.1, 3.4, and 3,8 engines have these problems often.
  
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#With the car running, using a can of carburetor cleaner spray down the hoses and vacuum connections.
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==Resources==
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#Notice a change in the running of the engine and you might have found your leak.
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*[[Carb vacuum port ID]], including info on reading a vacuum gauge
  
  
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Occasionally, you may experience a vacuum leak into the intake ports from the crankcase of the motor due to the intake manifold / cylinder head interface not being machined parallel. No amount of propane or carburetor cleaner will find such a leak. The best way is to insure the elimination of such a problem while building the motor.
 
  
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Here's how I set up the intake manifold/cylinder head interface to prevent vacuum leaks from the crankcase to the head ports....
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[[Category:Engine]]
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Measure the thickness of a new intake manifold gasket. Get flat washers or shims that will measure that thickness. With the manifold off and the mating surface on the cylinder heads de-greased, put a dab of RTV on the washers/shims and stick them on each corner bolt hole on the cylinder heads. Let the RTV set up. Stuff paper towels into the ports to keep debris out. Make up 16 pea-sized balls of modeling clay. De-grease the intake manifold at the ports. Place the balls of clay on the top and bottom of each port of the manifold, squishing them down well so they stay in place. You want them to be thicker than the shims/washers that are RTV'd to the heads. With your fingers, coat a little oil on the heads where the clay will meet the heads to keep it from sticking to the heads. Now carefully place the manifold into place on the heads and use bolts on the four corners to just snug the manifold down until you feel resistance against the shims/washers. Remove the manifold carefully and measure the thickness of the clay at all 16 positions with the depth function end of your 6" dial caliper. You'll know pretty quickly if the manifold/head interface is square. Record the measurements on the manifold with a permanent marker like a Sharpie. The widest measurement will be the standard to which you will want your machinist to cut the other positions on the manifold to make it square with the heads, thusly sealing up the motor.
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[[Category:Carburetors]]
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techinspector1
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[[Category:Troubleshooting]]
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__________________
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[[Category:Good articles]]

Latest revision as of 01:07, 11 July 2014

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