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Vacuum leak caused my car fail CA smog test

Tiny
ROB90230
  • MEMBER
  • 1989 FORD MUSTANG
  • 5.0L
  • V8
  • 2WD
  • MANUAL
  • 3,000 MILES
The check engine light is not on. Tester connects device to gas filler and told me that there is a vacuum leak. Where is the most likely place for the vacuum leak? What should the leak look like? Is there anything that I can put on the line to help me see the leak. I do not have a smoke machine.
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Monday, March 1st, 2021 AT 11:13 AM

11 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,640 POSTS
Hi,

A vacuum leak can be under the vehicle near the fuel tank or under the hood. First, here is a link that explains how to locate and repair one:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Now, often times a smoke machine is used to locate a leak. Basically, smoke is introduced into the EVAP system at a very low pressure. Where there is a leak, the smoke will come out.

I attached a pic below that should be the same as the label under the hood. It shows the EVAP system and the vacuum hoses.

Let me know what you find or if you have other questions.

Take care and God Bless,

Joe
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Monday, March 1st, 2021 AT 6:40 PM
Tiny
ROB90230
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I disconnected the line from the fuel vapor charcoal canister (the line leading to the fuel tank). I connected that line to a vacuum pump. Should I be able to draw a vacuum when everything is off (engine not running, etc.)? If I can not create a vacuum, does it imply a leak?
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Tuesday, March 9th, 2021 AT 10:54 AM
Tiny
ROB90230
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If I have a vacuum leak, it is on the line after the fuel vapor charcoal cannister. The line leading to the gas tank.
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Tuesday, March 9th, 2021 AT 10:57 AM
Tiny
ROB90230
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The image you included ends where I believe I have a vacuum leak (after the fuel vapor charcoal canister).
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Tuesday, March 9th, 2021 AT 10:59 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,640 POSTS
Hi,

Check to see if there is vacuum from the engine. If there is, chances are it is after. However, go to a local parts store and ask if they will lend you a vacuum pump. Most will at no charge. Attach the pump to the opposite side of the hose and see if it holds a vacuum. If it does, no leak. If it doesn't, then you know there is a leak.

Let me know.

Joe
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Tuesday, March 9th, 2021 AT 5:21 PM
Tiny
ROB90230
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  • 8 POSTS
I got a vacuum pump and have verified that the line that connects to the fuel vapor cannister to where the line is metal tubing (about half way to the car) holds a vacuum.
Where the line is metal tubing, it travels inward towards the driveshaft and then runs to the back where it come outward and connects to the gas tank. I connect the vacuum pump to the beginning of the metal tubing and there is no vacuum. I reconnect the vacuum pump to the pressure side and check if the line can hold pressure. It does. I know that the vacuum pump doesn't put out much pressure but the system hold pressure. I disconnect the line and I can hear the air escaping. I used a c-clamp on the rubber line at the gas tank and the same results occur with vacuum and pressure. Is the metal tubing all one piece? I.E. No connection running near the driveshaft? It seems that metal tubing has a vacuum leak but it should also not hold any air pressure.
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Tuesday, March 16th, 2021 AT 3:47 PM
Tiny
ROB90230
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Joe, thanks for your help. Does this vacuum line have a check valve somewhere in the system?
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Tuesday, March 16th, 2021 AT 3:49 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,640 POSTS
There shouldn't be a check valve. You mentioned the sound seems to come from the steel one-piece line. That shouldn't be. Is there any evidence of rust or corrosion?
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Tuesday, March 16th, 2021 AT 6:50 PM
Tiny
ROB90230
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  • 8 POSTS
The pumped air stays in the system. I have a long line connecting the vacuum/pressure pump. When I disconnect the pump, I hear air escaping from the tube, not the steel one-piece line.
What happens when the fuel tank vapor valve on the gas tank fails? Can it not seal and allow a vacuum test to fail?
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Wednesday, March 17th, 2021 AT 10:09 AM
Tiny
ROB90230
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I really don't think I have a leak in the line. I think there may be a problem at the top of the gas tank.
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Wednesday, March 17th, 2021 AT 11:07 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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It very possibly could be at the tank. I'll be honest. I know you don't have a smoke machine. However, you really should see if you can have it smoke tested. This kind of leak can drive you crazy. If it is holding a vacuum, there isn't a leak.

Is there any place you could have the smoke test done?

Joe
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Wednesday, March 17th, 2021 AT 6:29 PM

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