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Vacuum system '91 3.9

Discussion in 'Dakota, 1998-2013 Durango and Aspen' started by Peymack91, Aug 21, 2016.

  1. Peymack91

    Peymack91 Member

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    New to the board and need a little help.
    Just bought a '91 Dakota with 3.9 and I have a couple of vacuum issues. Does anyone have a clear photo (or two) of the driver's side engine compartment with vacuum hoses and connections?
    I have a couple of hoses that are not connected, but are capped off. I need to get the vacuum system back together to correct a high idle situation (and just get the truck running well).
     
  2. ImperialCrown

    Level III Supporter

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    Welcome to Allpar. Does the truck have cruise control and A/C? Is there a VECI label still under the hood with the vacuum diagram? The service manual had these diagrams in chapter 25.
    A fast idle could be a symptom of an intake manifold vacuum leak somewhere.
     
  3. Peymack91

    Peymack91 Member

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    It does have cruise and AC. The label is still under the hood and I have a service manual, but the diagram is somewhat vague. There are a few extra hoses under the hood and I would like to look at a picture of a 3.9 with everything hooked up correctly.
    The PO had some disconnected hoses and I'm not confident that they are where they should be.
     
  4. AllanC

    AllanC Well-Known Member

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    Here is a link to pictures of vacuum hose routing 1991 Dakota, 5.2 liter V8. I have owned this vehicle since 1993 so the underhood area is still OEM correct and unmolested. The factory service manual schematics indicate the V6 and V8 routings are identical. Hope this helps.

    Dropbox - Dakota 1991 Vacuum Routing
     
  5. Peymack91

    Peymack91 Member

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    AllanC, you hit a homer with this one. Great pics, clear as a bell, and they tell me exactly where I've gone wrong. Thanks so much.
     
  6. Peymack91

    Peymack91 Member

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    I think I have the vacuum all sorted out, but I do have one last question: AllanC, your throttle body looks like it has one less port than mine. I think this is cruise control, but I can't find where to plug in the other end. Here are two pictures. You can see the throttle body port with the small rigid plastic hose coming out. The other end is in the second picture.
    Any ideas? photo.JPG
     

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  7. ImperialCrown

    Level III Supporter

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    Generally the vacuum supply to the cruise control first goes to a vacuum reservoir with a one-way vacuum check valve. This may be part of the plastic battery tray?
    The supply may come from the brake vacuum booster tap.
    Then it goes to the vacuum servo.
     
  8. Peymack91

    Peymack91 Member

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    The problem then, I guess, is that I cannot find a single open vacuum hose connection that this would go to.
    If it helps with a diagnosis, there is definitely suction at the end of that hose. I can't imagine it connecting to another vacuum source.
     
  9. valiant67

    valiant67 Rich Corinthian Leather
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    It would not be unusual for a vacuum source to be capped off from the factory.
     
  10. Peymack91

    Peymack91 Member

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    Not at all. One of the Dakotas in the salvage yard had a cap on the brake master cylinder servo.
    What's odd is that there is a connector at the throttle body nipple, a plastic line, a connector at the end of the plastic line and then...nothing. Why wouldn't they just cap off the nipple on the throttle body?
    I would just like to know where this SHOULD go and if it is applicable to my truck. If not...I'll cap off the nipple on the throttle body.
     
  11. Peymack91

    Peymack91 Member

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    On the bright side, when I corrected one of the connections, I got my center vents blowing cold air again!!
     
  12. AllanC

    AllanC Well-Known Member

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    I had to look at the V8 throttle body and buried under the vacuum hoses is another nipple. It was taped closed; not used. I never have noticed that vacuum source before. So it appears the V6 and V8 used an identical throttle body but the V8 has one port unused.

    I checked the factory service manual and that port (V6 engines) is used for providing vacuum control to the AIR (air injection reactor) pump solenoid on the right side valve cover. The AIR pump was used to inject extra air near the exhaust valves to help reduce hydrocarbon emissions. During de-acceleration with high manifold vacuum, a vacuum signal was used to operate the diverter valve and cause the injection air to be introduced in the downstream exhaust and not near the exhaust valve. That helped prevent backfiring. The V8 engine gets this vacuum signal from other ports on the throttle body.

    That small, 1/8 inch relatively rigid vacuum line that you connected at the brake booster check valve and runs through the firewall provides the vacuum to operate the actuators in the HVAC system.

    Go back and look at the link that I provided for the vacuum routing. I added a picture of the AIR pump that is installed on the 5.2 liter V8. It is picture #5. V6 engines had a similar pump but it was mounted above the water pump and slightly closer to the driver side fender. Is this pump still installed on the vehicle? There was a vacuum control line that attached to the diverter valve on the pump. If the pump and / or diverter valve is missing, just cap the end of the line to eliminate a vacuum leak. I would not go back and remove the hose at the throttle body. There were multiple sensors tied into this line and you might cause problems if you remove that hose at the throttle body.
     
  13. AllanC

    AllanC Well-Known Member

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    Does this 1991 V6 Dakota have a 5 speed manual transmission or automatic transmission?
     
  14. Peymack91

    Peymack91 Member

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    OK. I went and checked and I do indeed have a pump installed, but...there is already rigid plastic line connected to the diverter valve. That line runs up along the TB and then runs to the passenger side of the TB along with two other vacuum lines.
    The line I'm trying to figure out doesn't run but about 6-8 inches from the TB, not far enough to reach that diverter valve, and I can't find anything that close for it to plug into.
    The truck is an automatic.
    BTW, thanks for all your help so far. This is the last piece to the puzzle.
     
  15. AllanC

    AllanC Well-Known Member

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    Unfortunately I made an error in interpreting the schematic diagram for vacuum hose routing and emission control devices. The picture of the AIR pump with attached vacuum controlled valve is NOT a diverter valve; it is an air switching valve.

    According to discussion on page 25 - 20 of the factory service manual, all V6 and V8 equipped 1991 Dakota have an AIR pump. All V6 equipped 5 speed manual vehicles and some 4 speed automatic vehicles have the additional diverter valve. My V8 equipped Dakota does NOT have the diverter valve but your V6 equipped truck came from the factory with the diverter valve. I believe that device is missing. from your truck.


    I changed the labels on the picture of the AIR pump that I posted. Now it shows the AIR pump with the switching valve. I have also added a picture of the vacuum schematic found on the underneath side of the hood. It is difficult to read but there is a notation that shows the diverter valve as optional. Go back to the link to the vacuum routing images.

    I think the solution is for you to cap this line at the port on the throttle body and the mystery is solved. I will assume you have the proper routing for the remaining vacuum lines.

    Upon sudden throttle closing the air- fuel mixture tends to go rich. If additional air is being injected at the exhaust manifold, there is a tendency to get a backfiring scenario. To eliminate this the high vacuum signal upon throttle closure opens the diverter valve (separate from the air pump) and vents air directly to the atmosphere instead of the upstream exhaust manifold.


    As the engine reaches operating temperature and the catalytic converter and EGR system begin to function to reduce exhaust emissions, the vacuum signal is removed from the switching valve. The valve closes and air is directed to the downstream or catalytic converter for best emission reduction.


    After some study I believe this is a correct synopsis of the AIR pump, diverter valve and associated parts. When the engine is cold and the catalytic converter is not at operating temperature to oxidize and react with exhaust gases, additional air is injected upstream to the exhaust manifold. A vacuum signal is sent to the switch valve and this directs air flow upstream
     
  16. Peymack91

    Peymack91 Member

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    If it is optional, and my truck seems to be idling fine now, I will take the advice of capping that port at the throttle body and calling it a day.

    Thanks for all the help.
     

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