Trying to use a Mityvac to find vacuum leaks
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Trying to use a Mityvac to find vacuum leaks
2000 b1500 5.2.
So I have had the common vents go to defrost under engine load problem. Check valve off the intake was bad. Swap that and the problem is much improved. However on bigger hills my vents still go into defrost and cruise control is still intermittent.
So I buy a mityvac that does pressure and vacuum. I pull the hose off the intake manifold. It looks like it goes:
intake manifold > rubber elbow > check valve > rubber elbow > Y fitting >
1) into the firewall through a rubber grommet and then back out with two lines. I'm guessing this is HVAC and cruise control.
2) I'm not sure yet where the other one goes. Is that evap?
Anyway, so my expectation is that if I connect the mityvac to #1, I should be able to pull vacuum unless there is a leak somewhere in one of the lines. Well it turns out I absolutely cannot pull vacuum. So I'm assuming I have a cracked line. So I hook up a smoke machine but not only do I not get smoke, but it builds pressure until the smoke machine fitting pops out. Hook up the mityvac and switch to pressure mode and it builds and holds pressure just fine. What gives?
The only thing I can think is that there is a secondary check valve somewhere in the dash that is functional but also reversed so that it allows pressure but not vacuum. Ok, but even if there is a backwards check valve, shouldn't I still be able to pull vacuum unless there is a leak somewhere further down the line?
Am I crazy?
If there is a backwards check valve then what? Find it, turn it around, then smoke to try to find the leak?
Edit: I might be thinking about that backwards. Maybe it is functioning correctly.
So I have had the common vents go to defrost under engine load problem. Check valve off the intake was bad. Swap that and the problem is much improved. However on bigger hills my vents still go into defrost and cruise control is still intermittent.
So I buy a mityvac that does pressure and vacuum. I pull the hose off the intake manifold. It looks like it goes:
intake manifold > rubber elbow > check valve > rubber elbow > Y fitting >
1) into the firewall through a rubber grommet and then back out with two lines. I'm guessing this is HVAC and cruise control.
2) I'm not sure yet where the other one goes. Is that evap?
Anyway, so my expectation is that if I connect the mityvac to #1, I should be able to pull vacuum unless there is a leak somewhere in one of the lines. Well it turns out I absolutely cannot pull vacuum. So I'm assuming I have a cracked line. So I hook up a smoke machine but not only do I not get smoke, but it builds pressure until the smoke machine fitting pops out. Hook up the mityvac and switch to pressure mode and it builds and holds pressure just fine. What gives?
The only thing I can think is that there is a secondary check valve somewhere in the dash that is functional but also reversed so that it allows pressure but not vacuum. Ok, but even if there is a backwards check valve, shouldn't I still be able to pull vacuum unless there is a leak somewhere further down the line?
Am I crazy?
If there is a backwards check valve then what? Find it, turn it around, then smoke to try to find the leak?
Edit: I might be thinking about that backwards. Maybe it is functioning correctly.
Last edited by OregonApe; Feb 19, 2026 at 02:01 AM.
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There is not another factory installed check valve, just the one at the manifold. The one-way action you're describing with the Mityvac leads me to believe the issue is with one of the vacuum actuators in the system, or perhaps the selector switch itself. If it were indeed a cracked vacuum hose, then it would not hold pressure like it is doing.