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Vacuum test?


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Hi, very specific vacuum pump question, have not been able to find the answer here or any other site / google. I've read a lot about failing vacuum pumps and re-sealing them, but what I need to know is this..

 

I do not have the vacuum brake booster, only thing my system feeds is cruise and hvac.

 

After my supply line fell off the pump the other day (rotted hose) I hooked it back up. I decided to replace some of the hoses and noticed there's a check valve right before the input to the hard line on the firewall... was not working as expected so replaced.

 

I also grabbed a vacuum gauge and tested. Several sources say the system should hold a vacuum well after the engine is shut down, mine drops fast.

 

My question is this:

 

Where should the system hold the vacuum? after this check valve? Or should it also hold a vacuum if i were to hook the gauge straight to the pump? If after the check valve, I'm good, if the pump itself should also hold a vacuum... well then time to move on to those re-seal topics.

 

Thanks in advance!

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Sorry I'm just guessing but I would say anything after check valve should hold vacuum, pump itself no. Pump got little plastic like paddles if you will rotate and create vacuum so as soon as it stops then it would probably bleed through what's in crank case, I'm not  100% sure if it's a completely positive displacement pump. So that would leave check valve to do the rest. Don't you also have CAD on your truck being 01.

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Thanks @Dieselfuture That's where I was leaning too but wasn't sure if there was a check valve that's supposed to be on/in the pump itself. Seems like a lot of hose before the valve.

 

Also, 2WD truck, so I don't think I have CAD?

 

Anyone have a known good pump with a gauge that could confirm?

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No you won't have cad, I don't know if it was a question. Sorry that's all l know on this subject, I guess one more thing is if you have a vacuum leak you'll have more blow by because vacuum pump pulls vacuum in to crankcase so if you have a leak instead of creating vacuum in lines you pump air in crancase.

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Funny you mention the blow by, thats what started me looking as one night while at a stop light I noticed lots of white smoke or vapor of some sort right in front of my headlights, pulled over to find my breather just spewing it as I have not extended that tube yet. Later while troubleshooting, found the disconnected vac supply. so now yeah, just trying to test everything to make sure its to spec, hence the question of where should it actually begin to hold pressure :)

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Thanks for the info / video KATOOM, had actually seen it, but again, this isn't a blow by discussion. lets re-focus on the question.

 

"Where should the system hold the vacuum? after this check valve? Or should it also hold a vacuum if i were to hook the gauge straight to the pump?"

 

Trying to verify nothing is wrong with the pump itself due to the fact the vac supply line doesnt hold a vacuum. even though its a brand new hose.

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I know its not a blowby discussion but you mentioned seeing "lots of white smoke or vapor...".  So I thought it worth showing how important it is to understand that a vacuum leak can increase venting.

 

That said, in answering your question, the complete vacuum system as it sits on the truck does not hold vacuum.  It will slowly bleed down releasing vacuum when the engine is off.  But if you're trying to test the vacuum lines/pods then you can connect a tester with a manual pump and check that way since everything post the main vacuum pump line should hold vacuum under testing.

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Quote from 2001 FSM Page 24-29
image.png.a8f01c839c79b8f8fc03b3e36bec9143.png
the check valve is not shown in the picture I attached at the end, but basically, the check valve goes in the "vacuum supply" line.  This means that when you turn off the vehicle, and the vacuum pump quits rotating, all of the components further down the system stay in the proper vacuum positions.  In gas engines it is very important, as when you open the throttle, manifold vacuum goes down.  So during acceleration, all the actuators would change position.

 

IF there was no check valve, in our trucks when the engine was stopped, the vane type vacuum pump will allow all the vacuum to escape backwards through the system, so all of your actuators would relax and move while the truck is off. 

 

It is by no means a killer problem if the check valve is broken (as long as it allows the vacuum to operate properly when the truck is running.)  Just don't put it in backwards.... you won't have any function of what you need. 

 

another interesting picture....  this shows(or leads one to believe) that the valve is  built into our vacuum pumps.image.png.32e536725121a95bea3a4be8e368edfc.png

 

 

Hth

Hag

Vacuum Lines.jpg

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They really don't hold vacuum at all. Not so much an issue as making sure it creating enough vacuum at the time of operation. In other words making sure there are no vacuum leaks like crack vacuum lines or bad rubber fittings. With age, you'll start to lose vacuum motors the diaphragms will crack or rip. The only thing the vacuum drives on my truck is HVAC controls and exhaust brake. No CAD axles (Thank Gawd!) Hate that damn thing on the 1996 Dodge Ram 1500. That truck has HVAC, cruise control, and CAD and more prone to vacuum issues. 

 

One thing to note. Make sure the HVAC is behind the check valve. The cruise control should be before the check valve. 

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Mike,

 

On a gas model, everything has to be behind the check valve.  (when manifold vacuum goes down, with no check valve, everything will change position as a function of manifold vacuum, not selected position) (if you ever want a fun feeling, hook your gasser cruise control directly to the manifold vacuum source without a check valve.... it will killer slow down uphill, and accelerate like a banshee on the other side of the hill.  or just surge like a son of a gun)

 

Notice in the FSM description for diesels..... The check valve prevents oil from being pulled up the vacuum line at shut off. (since our vacuum source is relatively constant, not manifold pressure dependent.)  So it really doesn't matter where the check valve is.  

 

Thinking through this.... I see why we have "blowby" issues with a vacuum leak.....   The vacuum pump normally reduces pressure in a very small known volume.  Lets say a gallon of volume.   So when we start the truck this volume is removed.  It stays constant unless we actuate something.   when we actuate, its a pint to a quart in volume at most.....

 

What is a vacuum pump?  simply Its a "fan" that can create negative pressure.  We know where the air comes from (the gallon or so in the actuators)  Where does the air go?  it goes right into the crank case.....  (there is no "exhaust"  port on the vacuum pump.....it would sling out oil due to the wet vane design of the pump....)  So when we have a vacuum leak, we have a "fan" taking in outside air and pumping it directly into the crankcase ventilation system.  (sorry if you guys already knew that.  It finally occurred to me why the vent system went wonky.)

 

Hag 

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15 minutes ago, Haggar said:

Mike,

 

On a gas model, everything has to be behind the check valve.

 

Sorry it not. The cruise control must be before the check valve. What happens is the cruise control wipes out the vacuum reservoir and then the HVAC defaults to DEF mode till you get out of hill climb then throttle drops and vacuum returns. So Cruise before the throttle just comes up short a bit but HVAC does not fail back to DEF mode. 

 

I've done the mod hoping to make the cruise last longer but it creates other problems being the cruise will continue to command throttle venting vacuum till it reaches cruise speed. This is why cruise vacuum motor has to be outside the vacuum reservoir. Not to mention it will UNLOCK the CAD unit when the vacuum runs out. Stupid CAD axle junk!

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Mike  tried to send you a message.  they are not going through.

 

H

Mike read 24-28 and 24-29 of the 2001 FSM.   On Gas models there is a second check valve that separate the HVAC from the cruise Servo. 

 

Our system in the FSM shows only the "main" one at the pump.  But I agree with you, we have both seen a second check in the dodge system, and that was probably on the cruise servo.  We also don't get the vacuum reservoir gassers do.  I think the engineers figured that the mechanical vacuum pump was strong enough at all engine speeds to always supply enough vacuum.   But since we have seen those check valves, maybe there was a running change help solve some issue, but never made it to the FSM

 

If I was going to have only one check valve, it would be between manifold and reservoir on a gasser.  ( I hate my air conditioning ducts swapping while climbing a hill.) 

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While continuing to track down leaks / bad hoses (I broke the one along the firewall) found a second check-valve right before it goes in to the HVAC. So I have one on the main line, and one on the HVAC. Cruise would be shared if I had any other accessories but I don't so I'm going to leave it how it is.

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