Finding vacuum leaks
From Crankshaft Coalition Wiki
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*Faulty vacuum devices like: | *Faulty vacuum devices like: | ||
**AT modulator | **AT modulator | ||
| + | **Brake booster quite commonly un or misdiagnosed | ||
| + | [little harder brake pedal remove booster vac line plug & idle will improve] | ||
**Vacuum advance canister | **Vacuum advance canister | ||
**Carb choke pull-off | **Carb choke pull-off | ||
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***PCV hose, valve or grommet | ***PCV hose, valve or grommet | ||
| − | == | + | ==Finding a vacuum leak== |
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. | 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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{{Warning}}Stay away from any moving parts like the belts and fan blade. the fan or fan belts. | {{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. | ||
==ATF method== | ==ATF method== | ||
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{{Warning}}Hold propane bottle UPRIGHT during testing as liquid propane may run out of an inverted bottle. | {{Warning}}Hold propane bottle UPRIGHT during testing as liquid propane may run out of an inverted bottle. | ||
| − | {{Warning}}This MUST in a well ventilated area! Only do this outside of a garage and away from dwellings. | + | {{Warning}}This MUST be done in a well ventilated area! Only do this outside of a garage and away from dwellings. |
==Carburetor cleaner method== | ==Carburetor cleaner method== | ||
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==Vacuum gauge method (internal)== | ==Vacuum gauge method (internal)== | ||
| − | + | {{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: | ||
#Disconnect/remove the primary wire between the coil and distributor. | #Disconnect/remove the primary wire between the coil and distributor. | ||
#Adjust the idle screw so the throttle plate in the carburetor is completely closed. | #Adjust the idle screw so the throttle plate in the carburetor is completely closed. | ||
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#Connect vacuum gauge to a manifold vacuum source. | #Connect vacuum gauge to a manifold vacuum source. | ||
#Have assistant crank engine while you read vacuum. If gauge reading is 0-2", you may have an internal leak between the manifold/head. | #Have assistant crank engine while you read vacuum. If gauge reading is 0-2", you may have an internal leak between the manifold/head. | ||
| + | {| | ||
| + | |-align="top" | ||
| + | |[[File:Vac2normal.gif|thumb|400px|Normal vacuum]] | ||
| + | |[[File:Vac4vac leak.gif|thumb|400px|Vacuum leak]] | ||
| + | |} | ||
=="Untraceable" vacuum leak== | =="Untraceable" vacuum leak== | ||
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). | 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). | ||
| − | 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. | + | 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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| + | 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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| + | ==Resources== | ||
| + | *[[Carb vacuum port ID]], including info on reading a vacuum gauge | ||
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[[Category:Engine]] | [[Category:Engine]] | ||
[[Category:Carburetors]] | [[Category:Carburetors]] | ||
| + | [[Category:Troubleshooting]] | ||
| + | [[Category:Good articles]] | ||
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