1986 GMC Vandura needs help!

Discussion in 'Shop Talk' started by Columbia,SC., Dec 9, 2005.

  1. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    Hey,
    I just got a 1986 GMC Vandura (66,000 miles) from my dad the original owner. This is a U-haul type truck with a 14 foot box on it. It has a 350 V8 and 4.10 rear end set up to pull a trailer. My problem is this, I am not sure about what kind of carb. is on the truck, pretty sure it is a 4 barrel. The choke sticks and it idles very high. This truck has been sitting for a couple of years (5 or so) but it starts great and runs fine but only gets 7 miles to the gallon. I know it has to do with the choke, I changed a vacume hose that was closed coming from a black ball thing mounted on the hood. That seemed to help for a couple weeks but now it is running bad and the choke is sticking again. Real bad backfires and run-on problems.
    When I start it up it idles high and stays that way, If I take the breather off and stick my finger in the carb and reset the choke it idles nice and quiet, as soon as I give it the gas the choke slaps shut after about 1/4 throttle and stays that way. Any ideas, I am not a mechanic but I do pretend to be one from time to time and I am not totaly helpless.
    I use this truck everyday as my work truck and drive about 50 miles a day, the gas is killing me! I would LOVE to get 12 MPG or better if it was possable.
    Thaks in advance, Columbia,SC.
     
  2. Esteban29304

    Esteban29304 Well-Known Member

    Messages:
    772
    Joined:
    Apr 29, 2003
    Sounds like a GOOD tune-up & probably a carb rebuild are in order. Sitting for a long time makes the gas in the carb &&& tank, turn to GUNK!! A couple of cans of " Seafoam" to a full tank of gas will help the tank. Replace fuel filter at tune up & again after the Seafoam treatment.
    BUT, 12 mpg is stretching reality!
     

  3. BigBoy

    BigBoy No attitude here...

    Messages:
    169
    Joined:
    Oct 24, 2005
    Location:
    Central & South Mississippi
    '86 probably has the Q-Jet carb. I'm going by memory here, but the choke is adjusted on the right (passenger) side of the carb. You should see a housing that's about 1 1/2" in diameter with a black plastic cover. Here is a pic (below) of where, it's 336 on the bottom left of the diagram below. To adjust it, loosen the 3 small screws holding the choke cover and, with the throttle held open, turn the choke cover till you just see the choke plate opening. That should have it right. If the choke still isn't working you may want to turn it until it is wide open. You won't have any choke then but it will not be restricting your air. Where you are you may need it to start. I'll check back later to see if that helped and advise you further if needed. Gotta Go!

    [​IMG]
     
  4. Beeman

    Beeman Well-Known Member

    Messages:
    6,977
    Joined:
    Dec 28, 2002
    Location:
    East TN
    Carb should be a Rochester Quadrajet as mentioned and the picture is correct, if it is original. There are a couple of different styles of choke coils used on this carb. It could be hot air or electric. Electric will have a wire going to part 336 in the diagram. First and simplest thing is to check for the wire, if it is connected, and if it has power with the engine running.
    If this carb hasn't been touched since new choke coil is probably riveted on so adjustment would be hard and might not be a problem. If it has screws it has probably been rebuilt or replaced, enter the human error factor.
    If the choke is a hot air choke it will have a small tube attached to the choke housing. This tube will run to just below the choke into the intake manifold. There will be another tube running from that spot to the back of the carb. The choke housing will draw vacum thru the tubes and the drawn air will be heated in the exhaust crossover built in the intake manifold. Start the engine and remove the tube at the back of the carb. You should feel vacum being drawn thru. If no vacum then remove the tube from the choke housing and feel for vacum at the housing. If you have vacum at the housing but not at the rear of the carb tube you must have a bad tube, broken hose where the tubes connect to the carb or a plugged tube.
    Now for the human error part. If it has an electric choke and has been touched many times people install a gasket under the choke coil which stops the current flow to ground. This will prevent the coil from heating up and cause your condition. There is also the possibility if it has been touched that they installed a part wrong causing your problem. Of course if it has been touched there is also the possibility the adjustment is just way out of the range and will require adjustment.
    The black ball thing on the hood is a reserve vacum tank. Good possibility you have many bad vacum hoses on that van, those years were a vacum nightmare. Many can be eliminated if you know what to do and you live in a state with no or little vehicle emmission laws.
     
  5. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    It is an electric choke, I saw that this morning. The thing is rivited on like you 'Beeman' said. How much voltage should I get at the plug?

    I was wrong when I said it closes all the way, it does close about half way and idles really high till I move the choke butterfy and tap the gas to reset it again. But as soon as I give it any gas there it goes again.

    What would you all think great milege for this truck would be? just couriouse I know it is not much.
    350- v8- 4.10 rear- auto trans/ no overdrive.
    Thanks again Columbia,SC.
     
  6. Beeman

    Beeman Well-Known Member

    Messages:
    6,977
    Joined:
    Dec 28, 2002
    Location:
    East TN
    If you get 12mpg celebrate.

    Voltage should be 12v. Run a jumper wire from the battery to the choke coil and see if it opens the choke.
     
  7. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    Beeman, I tested it today and I got 14.36 volts.

    This might be crazy but it does seem to run better when I add STP fuel treatment to the gas. I put in a large bottle and a small one, enough to treat 35 gallons. I have a 33 gallon tank.
    The choke butterfly still sticks after 1/3 gas pedal is engaged but can be manualy reset and idle real low and correct untill I step on the gas again. Then it goes back to the high idle.
    Any ideas?
    Thanks on advance, Columbia,SC.
     
  8. cfabe

    cfabe Well-Known Member

    Messages:
    1,526
    Joined:
    Feb 27, 2005
    Location:
    Mid-Michigan
    If it's a van you're planning on keeping it might be worth a trip to a carb shop to have them look at it and tune up the carb.
     
  9. BigBoy

    BigBoy No attitude here...

    Messages:
    169
    Joined:
    Oct 24, 2005
    Location:
    Central & South Mississippi
    cfabe has the best idea, but...

    You are getting correct voltage to the choke. If it is still only opening halfway even after having voltage to it for 5-10 mins then the coil is bad. The way it works... behind that plastic cover is a bi-metal coil that contracts when it is heated from the voltage that is applied. As it contracts it turns and the lever fully pulls open the choke plate. If this isn't happening then the coil is bad.
    Carb rebuild kits come with screws to replace those rivets. I would think that a replacement choke coil would have them also. To remove the screws all you do is drill them with a bit that is just a bit larger than the head of the rivet. They are soft aluminum and they come out pretty easily.
    Another option is to replace the coil with a manual choke kit if you can still get them... or replace the screws and manually turn it wide open so it isn't choking at all. Neither of these is the best choice.

    ...I used to rebuild these all the time and even tried it blindfolded once and almost got all the way through BUT that was MANY years ago and my brain has filed a lot of this away.
    Anyway I just thought... look at number 320 on the diagram. That's the choke pulloff. Make sure it has vacuam to it when you are running. If it does then when you start the engine, vacuam should pull the linkage (number 322) toward it. If it doesn't,replace it (320)
     
  10. Esteban29304

    Esteban29304 Well-Known Member

    Messages:
    772
    Joined:
    Apr 29, 2003
    I would estimate that if it ran well, you would get 8-9 mpg.
     
  11. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    That 320 part does move when the engine starts and seems to operate correctly. I changed a couple vacume hoses as well. I am thinking that it may not be so much a choke proble anymore after playing around with it. Is there any way just to idle it down? I looked all over the carb (rockchester quadrajet) just like the one in the picture. I do see the stop screw down on the side by the throtal linkage. It does not rest on it, it is about 3/16" away from touching the screw. I can twist the thing and slow down the idle but I don't know how to do it other than that.
    Thanks again, Columbia,SC.
     
  12. BigBoy

    BigBoy No attitude here...

    Messages:
    169
    Joined:
    Oct 24, 2005
    Location:
    Central & South Mississippi
    It's possible that the coil is working but not as well as it used to. If that's the case then you just need to replace the rivets with screws and adjust (turn) the coil just enough to drop the idle. Part 360 in the diagram is what is keeping the idle up. When the coil is cold it sits on a stop that keeps the throttle open some. Once the coil heats up and turns the shaft, it puts tension on the part and when you tap the throttle it moves enough to fall off of the stop and idles down to wherever the idle is set with the idle screw you mentioned. And there is no adjustment on the high idle. It's either on or off.
    Being down here in Miss'sipi I always used to just adjust the choke to where it was just barely on or I would adjust it to wide open. Even with temps into the teens I was always able to start up without much problem.
     
  13. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    I have called Auto Zone and they have a carb for my truck, it costs $100. lifetime warranty. It is a Rochester quadrajet same as mine. I can swap them out my self 'free labor'. Should I do that?
    The carb only resets itself if I take off the breather and stick my finger in and set the choke straight up and down, tap the gas. That is a real pain in the @.. sometimes it will not start either (this just started happening) the butterly is totaly shut and it floods the engine unless I hold the pedal clear down to the floor, then is starts but smokes like crazy and runs rough for 10 - 15 secs.

    The new choke controler is $30. the new carb is $100. I do not have the money now at Christmas to try parts if you know what I mean. Do you think I should drill out the rivits and adjust the choke then rescrew it? That would be cheaper. What size screws should I buy in advance, this is my only ride and I work out of this truck so I can not afford to be "down" very long.

    Hope I do not sound needy or whinney, just desperate.
    Thanks Columbia,SC.
     
  14. BigBoy

    BigBoy No attitude here...

    Messages:
    169
    Joined:
    Oct 24, 2005
    Location:
    Central & South Mississippi
    It has to be your choice but I would hate to spend $100 on something that I don't really need but for the sake of time you may need to.
    Personally, I would adjust the choke first. If there is a shop in your area that rebuilds carbs, you may be able to get the 3 screws from them for free. Sometimes they will have them as leftovers from a carb rebuild. There are also 3 'washers' that go under the screws that are shaped like a crescent moon. The washers actually hold the choke cover in place.
    You can make do with some tiny and short metal screws and round washers. About 1/8" long and #4 maybe... not sure of the number size.

    Good luck
     
  15. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    Ok today I filled her up $52.00 after 170 miles, when I shut her down I had all of the same old diesling and backfire problems along with the high idle. When I took off the gas cap off there was a huge pressure let off. I put the cap back on but not tight. After that it ran nice (low idle, no backfires, no run on)for 40 miles and 5 or 6 starts and shut offs. Could this be a fuel line pressure or vacume canister thing? I am still scared of the drilling out the electric choke ajustment and have not done it yet.
    Could it be a vacume canister that is clogged or blocked? I loosened the gas cap so the tank can breathe and today it is doing better.

    Just wondering and struggling. Columbia,SC.
     
  16. Beeman

    Beeman Well-Known Member

    Messages:
    6,977
    Joined:
    Dec 28, 2002
    Location:
    East TN
    First of all there is a chance the $100 carb doesn't come with the electric choke coil and you might have to use your old one, check first. There is also a chance the choke coil is not easily adjustable as some are indexed and only fit in one position. With gas at the price it is it won't take a lot to recoup the money you will spend if you drive this van much.
    Personally I would rather rebuild what I have then get a rebuilt carb, but that's me.
    Let's get back to your high idle/dieseling problem. When the choke blade moves it also moves a cam and follower behind the choke coil housing (parts 352 and 360). This is where the fast idle screw contacts (fast idle screw part 425) it has steps on it that changes the fast idle depending on how open the choke is. When you push the choke butterfly open you are allowing the cam to drop and the idle to return to base idle. When the butterfly is partially closed the fast idle screw is contacting the cam and won't allow the base idle screw to contact it's stop. The run on and dieseling is caused by the too high idle, temporary fix is to shut the truck off while in drive and then move the shifter to park.

    I don't see how the fuel tank pressure could cause any idle problems but I've never said I've seen everything. Most fuel tanks will make the whooshing sound especially when run low. If you think it does help try running with no gas cap and see what happens. If what you described happened it was probably from the heat buildup under the hood after driving over 100 miles which caused the choke to open and stay open.
     
  17. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    That #360 part seems to be the problem. I don't know what to do about it so I bought a manual choke cable, I want to try and put it on in the morning with help from my dad. I would rather keep it auto but this is starting to get lame with the backfires and run on problem. If I reset the choke butterfly (by hand) to full open then let it run for a few seconds it idles and shuts of nice and quiet. The gas has gone down to about 6 MPG!
    Thanks again for everyones help! Columbia,SC.
     
  18. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

    Messages:
    831
    Joined:
    Nov 24, 2005
    Well the whole manual choke thing did not work out. The area is WAY to tight to get a drill in there to get the rivets out, I got one out but the other two are too hard to reach with the drill! I would like to remove the carb but not sure how to. I think I am against the wall here.
    This is just an update of my ongoing struggles.
    Columbia,SC. :grump:
     
  19. fixer1958

    fixer1958 None of the Above

    Messages:
    1,741
    Joined:
    Dec 11, 2005
    Location:
    NE Kansas
    Put a new choke stat in and you should be fine.The old one is trashed and not heating up properly and is keeping the flap shut, as to why it is on fast idle all the time. Reset the new one to the original setting ( a notch in the stat housing and on the new stat) and screw or rivet it down with or without a gasket depending on what was OE.
    Spray some slickup (WD40 or something like it) down the choke flap rod hole because the rubber gasket in there gets dried out and makes it stick.
    It the carb is not trashed it should run like a stripped assed ape. Change your fuel filter and change your oil. The loading up from the carb washes down the cylinders and trashes your bearings.

    I Love Quads. It's to bad they are all gone. It used to be easy money because nobody liked to work on them.

    Just my thoughts! Take it or leave it.