How can I find a vacuum leak??

[Chuck1986]

My car is throwing codes for running lean in both banks (P171 & P174). I checked the long term fuel trim and it's +12% in both banks - should be +10% at the most, so it's running slightly lean. Car runs fine so I doubt it's anything serious - guessing just a vacuum leak.

My question is, is there a simple way to look for a vacuum leak without buying on of those expensive tools? Thanks!

 

[mpvue]

spray starting fluid around some vaccum hoses and the throttle body and see where the revs climb.

 

[tig1]

Use brake cleaner and spray around each vac cconnection. If there is a leak the engine will increase in RPM as it draws in the cleaner. Also your cats my be failing which can throw those codes.

 

[Chuck1986]

Originally Posted By: tig1
Use brake cleaner and spray around each vac cconnection. If there is a leak the engine will increase in RPM as it draws in the cleaner. Also your cats my be failing which can throw those codes.


I got rid of all the cats so I could get better gas mileage.

 

[severach]

Did you get a measurable increase worth breaking the law for?

Import Tuner Beat the Law: The truth about going catless

 

[JHZR2]

I believe pros use a smoke machine of some sort.

 

[mpvue]

Originally Posted By: Chuck1986
Originally Posted By: tig1
Use brake cleaner and spray around each vac cconnection. If there is a leak the engine will increase in RPM as it draws in the cleaner. Also your cats my be failing which can throw those codes.


I got rid of all the cats so I could get better gas mileage.

and how much did your mpg improve? you're in Erie; you don't have the same emissions testing as we do here in Easton? if you remove the cats here, you fail the test.

 

[Chuck1986]

Originally Posted By: mpvue
Originally Posted By: Chuck1986
Originally Posted By: tig1
Use brake cleaner and spray around each vac cconnection. If there is a leak the engine will increase in RPM as it draws in the cleaner. Also your cats my be failing which can throw those codes.


I got rid of all the cats so I could get better gas mileage.

and how much did your mpg improve? you're in Erie; you don't have the same emissions testing as we do here in Easton? if you remove the cats here, you fail the test.


I live in Erie but I'm a grad student from out of state living here for just a couple years...car is registered in another state.

 

[Chuck1986]

Been saving about 2 miles/gallon

 

[hsd]

Originally Posted By: Chuck1986
My car is throwing codes for running lean in both banks (P171 & P174). I checked the long term fuel trim and it's +12% in both banks - should be +10% at the most, so it's running slightly lean. Car runs fine so I doubt it's anything serious - guessing just a vacuum leak.

My question is, is there a simple way to look for a vacuum leak without buying on of those expensive tools? Thanks!


If you have a propane torch sitting around, remove the burner tip including the orfice plug, slip a 2 - 3 foot rubber hose over the remaining torch tube and crack the valve open till you hear it hissing. Point the open end of the hose where you think the vacuum leaks are and listen for a change in engine rpm. Propane is not messy like carb cleaner and cheaper too. You need the area to be ventilated for the running engine, so that will also take care of any excess propane vapors if you are worried. I've been using this propane leak testing for years and it works great.

 

[mechtech2]

Carb cleaner blasts in short bursts, or propane [not lit!] will cause the idle to change, and you can find a leak by using these.
You can't beat a good visual inspection - ALL the lines and their connections. Also any intake connections or fitting.
Don't forget the brake booster system, and the vacuum line to the interior that controls the vents.

A 2 MPG increase can only be achieved if the cat were previously plugged. Cats open up to 5-10 times the area of the pipe, and are not a significant restriction, especially at part throttle.
I am saying this for others. So that they will not dump their cats in vain hopes of MPG increases.