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Discussion Starter #1
Trying to fix a major vacuum leak. In order to get A/C air out of the panel vents I have to find a way to get some compression braking from the engine, either on a downhill or dropping into second gear or a combination of both. It can take several tries to build enough vacuum. I'm sure that much leakage isn't helping the engine either, so need to fix this leak.

Does anyone have a copy of the vacuum diagram p/n 4306879?

Any "usual suspects" that I should look for?

There are two large diameter hoses going from the carb to under the right side of the cab, and it looks like there's a place for a third hose to connect to a metal tube next to the other two. The metal tube comes up from under the cab and ends where the firewall curves under. The open end is packed w/ road debris. Is there supposed to be a vacuum hose connected to this, and if so where would the other end connect?

Thanks as always!

Matt
 

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Those sound like fuel lines, and the missing hose would be to the rollover vent valve at the gas tank. Just a guess.

I don't believe those are vacuum lines, unless you have 4WD and those are the controls for it. The vacuum leak may be a separate problem.

My Dakota is fuel-injected, so it won't be like yours.
 

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Discussion Starter #3
I left out that once there's enough vacuum to keep the air flowing through the vents, it seems to maintain the vacuum ok. But if I shut off the engine, even for a short time, I have to start the whole process over again.

Those sound like fuel lines, and the missing hose would be to the rollover vent valve at the gas tank. Just a guess.

I don't believe those are vacuum lines, unless you have 4WD and those are the controls for it. The vacuum leak may be a separate problem.

My Dakota is fuel-injected, so it won't be like yours.
The two lines that are connected to the carb have some cracks in them. Wouldn't anything connected to the carb that's not air tight be a possible vacuum leak?
 

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Possibly a vacuum leak, but they may be for venting float bowl fumes also. Replace any cracked or rotted rubber hoses.
There may be a fuel return line from the fuel pump back to the tank. This is necessary to establish correct fuel pressure.
You should have 17-19" of vacuum at idle. Low vacuum will cause HVAC and power brake assist issues.
There should be a check valve to keep the HVAC vacuum switching stable under low vacuum conditions. All cars that I know of will default to defrost (for safety) in case of vacuum supply loss. These check valves are failure-prone after a few years.
When checking for vacuum leaks around the base of the carburetor or manifold runners, a spray bottle of water can help locate them. Listen for changes in engine running or hiss noises.
 

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Measure intake manifold vacuum. This should be available at the base of the carb or a tapped fitting in the intake runners or at the power brake booster.
If the idle speed is too low where the engine isn't running efficiently, intake manifold vacuum will be low and probably uneven. Bringing the idle up to a faster, steady idle should give a higher vacuum.
An idle speed of 700-800 rpm should be close to spec. No vacuum leaks and correct idle mixture and correct ignition timing are important settings to improve idle quality.
Is the underhood VECI label still there? If your vacuum routing label is missing, you may find one printed in a factory or other high quality service manual. The main local public library may have one in the stacks for loan if you ask. Some main libraries may even have an internet subscription to Alldata.
 

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Discussion Starter #8
I have a vacuum gage, but nothing to measure RPM. I used to have an automotive multimeter, but it bit the dust. Any suggestions for a homemade RPM gage? I have a regular household multimeter if that can be used somehow.

The vacuum routing label is there, just upside down and very awkward to read. Tried taking a few pics to reprint, but they were all out of focus and couldn't read the wording.

Looked for some replacement check valves, but only found "universal" check valves that are not nearly as thick as the OEM... where can I find the big OEM check valves?
 

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A generic check valve should be OK. Size isn't important.
If the distrubutor/oil pump driveshaft bushing is worn, this is the TSB on it. Note that it doesn't go back to 1987, but yours could still be worn at high mileage: http://dodgeram.info/tsb/1993/18-08-93.htm
The bushing part # 1737725 was used until 2003 and is still available. The gear driveshaft is almost unobtainable. Saluda Motors in South Carolina shows one.
Diagnose this first before condemning it.
Someone must have an old dwell/tachometer somewhere you can borrow or buy. I've seen them at flea markets and garage sales for $2.
A regular DVOM won't do rpm.
 

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Discussion Starter #10
If the distrubutor/oil pump driveshaft bushing is worn, this is the TSB on it. Note that it doesn't go back to 1987, but yours could still be worn at high mileage: http://dodgeram.info...93/18-08-93.htm
The bushing part # 1737725 was used until 2003 and is still available. The gear driveshaft is almost unobtainable. Saluda Motors in South Carolina shows one.
Diagnose this first before condemning it.
How does that affect vacuum?

I'll check around for a used dwell/tachometer. I need to do a tune-up as well, so might as well start looking.

Where is the "damping canister" located? Is it under the right side of the cab?
 

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Anything that affects spark timing will affect vacuum and therefore the way the truck runs. A symptom would be a timing mark or idle speed that jumps around and won't hold still. I'm not saying that this bushing is a problem, but was common on older 3.9L's and might as well be checked as part of a 'tune-up'.
If by 'damping canister' you mean the emissions charcoal canister, that should be on the other end of the purge hose if you follow it on the vacuum diagram. The canister is a black, plastic assembly with maybe 3 hoses on it. It maybe where that underbody tube-to-nowhere goes.
 

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Discussion Starter #12
Anything that affects spark timing will affect vacuum and therefore the way the truck runs. A symptom would be a timing mark or idle speed that jumps around and won't hold still. I'm not saying that this bushing is a problem, but was common on older 3.9L's and might as well be checked as part of a 'tune-up'.
If by 'damping canister' you mean the emissions charcoal canister, that should be on the other end of the purge hose if you follow it on the vacuum diagram. The canister is a black, plastic assembly with maybe 3 hoses on it. It maybe where that underbody tube-to-nowhere goes.
Ok, I'll check for that bushing problem when I do the tune-up.

I don't think the damping canister is the same as the charcoal vapor canister. The vacuum diagram shows the damping canister as a separate item inline on one of the heavy lines going to the vapor canister. I'll keep looking.
 

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By its name, a damping canister sounds like either an empty or charcoal-filled chamber that would 'dampen' the evaporative fuel fumes? Like a secondary canister?
How important it is to a well-running vehicle, I don't know but probably not much as long as the rest of the fuel vapor emissions system is closed to the atmosphere and intact.
 

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Discussion Starter #14
Yeah, some further reading in the Haynes told me it's basically a secondary chamber on the back end of the vapor canister. Both of those lines are cracked so I'll just go ahead and replace them. I can't check the vacuum coming off the carb since I seem to have misplaced the adapter for my vacuum gage. Par for the course... a lot of things have gone missing since I moved from a house to an apartment.
 

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The typical setup for Chrysler AC climate control systems was to take a vacuum feed for the pushbutton actuated climate control system from the check valve on the power brake booster. There would be a 1/2 inch hose between the intake manifold (not carburetor) that connects to a check valve on the brake booster diaphragm outer case. The check valve would have other, smaller hoses connected to it. If equipped another smaller hose would connect the brake booster check valve to the curise control servo. The smallest vacuum line at the check valve would route through the firewall and into the climate control pushbutton mechanism.

Maybe the brake booster check valve was replaced at some point in time and the replacment did not have the proper number of ports and the vacuum line to the climate control system got lost or attached to an incorrect vacuum source.
 

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Discussion Starter #16
The 1/2" hose splits and goes to the booster and to a plastic manifold on the firewall above the booster. Each leg of the 1/2" hose has a filter on it. The check valve on the booster has a large port where the 1/2" hose is connected and 2 smaller ports that are capped. That was the condition of the OEM booster when I replaced it.

Connected to the manifold is 2 1/8" plastic lines and a 1/2" line going to the cruise control diaphragm. I can't tell where the 2 1/8" lines go after they leave the manifold.

I don't think the line to the HVAC controls is open to atmosphere since the air does eventually come out of the panel vents.

I bought some carb cleaner at A/Z and once the truck is out of direct sunlight I'll do some leak hunting.

Does the PCV valve control the flow of vacuum or just prevent oil from going up the vacuum tube? I ask because I was thinking that any blow-by coming from the crankcase would reduce the amount of vacuum.
 

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The vacuum diagram is in a Haynes manual, should not be difficult to locate. There should be a single vacuum line that goes into the cab located just to the right of the gas pedal, inside the cab then is a controller and the lines are then clear and color coded, more like fuel lines for a lawnmower, clear/colored plastic lines vice black rubber hoses. There may also be a black plastic ball vacuum canister to increase vacuum volume, so that is something else to check, might be a good time to just simply replace all the vacuum lines, small cracks and loose/hardened ends all start leaking over time. I don't think the distributor has a vacuum advance canister, so you should be able to alleviate that as a location for vacuum leaks.
 

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1987 is the only year the 3.9 used a carb in the Dakota. Haynes may not be that specific on their digram.
 

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Does the PCV valve control the flow of vacuum or just prevent oil from going up the vacuum tube? I ask because I was thinking that any blow-by coming from the crankcase would reduce the amount of vacuum.
The PCV (positive crankcase ventilation) valve limits the amount of vacuum applied to the crankcase. During engine operation gases from the pistons leak past the rings and into the crankcase. There are unburned hydrocarbons in these crankcase gases and they are metered back into the throttle body for combustion so as to prevent them from escaping into the atmosphere.

The PCV valve limits vacuum applied to the crankcase. There should be no vacuum operated accessories attached to the PCV hose.

On my 1991 Dakota with 5.2L V8 there is a vacuum line 1/8 inch in diameter that connects to one of the ports on the brake booster check valve. The line enters the firewall immediately underneath the lower heater hose connection at the firewall.
 
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