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Throttle Body Shaft Vacuum Leak

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  • Throttle Body Shaft Vacuum Leak

    I have been trying to track down some very particular information for a while now and all of my internet searches, including on this site, have come up empty handed. It has been many years since I was on this site (under a different name), but one thing I remember about it here is that if there’s anyone at all who would be able to give me a good answer it would probably be you guys on the sphere, so here goes.

    I decided to do a vacuum test on my 94 DOHC and I have actually done this twice now just to be sure of what I was seeing. I know vacuum lines and other seals and gaskets get old eventually, and this car is now 25 years old with mostly original vacuum lines so I figured I would check, given some issues I have been having.

    After using my vacuum tester on two different days and placing the smoke tip of the tester in two different locations, it is obvious that the one and only vacuum leak, and it’s a rather large one, is coming from the throttle body shaft.

    During the vacuum test, a large amount of smoke from the tester comes out of the bottom and sides of the throttle body shaft on the spring side of the shaft and like I have said, I have done this twice and in two different ways just to be sure.

    I have done the research on this issue and it seems that this is a well-known issue among other makes of cars, but I have yet to see anyone talking about it in the Saturn S Series community. The issue is that the seals within the throttle body shaft area get old and degrade over time and eventually lead to a vacuum leak through the throttle body shaft.

    I have had my throttle body out to look at it and see if it could be taken apart and repaired, but to me (and I could be wrong and I hope I am), it looks as though the inner shaft area that the throttle body spring fits around on these first generation DOHC throttle bodies is all factory sealed and not accessible from the outside. It looks like where the throttle cable retainer is mounted to the throttle shaft that it’s somehow pressed on from the factory, although it looks like maybe there’s some kind of metal retainer on the very end of the shaft that could be pried out. I wasn’t sure if this could be removed and I just didn’t want to try to get in there only to find out that I just destroyed the throttle body for my daily driver. I am considering buying the cheapest first generation DOHC throttle body I can find and trying to tear it open just to see if I can get to the inner part of the throttle shaft.

    Now, just out of curiosity, I looked all over the internet to see if there is a kit to repair the DOHC throttle bodies, and as far as I could tell, there isn’t. I know the SOHC throttle body seems to have one repair kit still out there, but there’s nothing that I can see for the DOHC throttle body, unless the SOHC repair kit seals will also work with the DOHC throttle body.

    So, my question is two-fold. One, has this been an issue for anyone else (that anyone is aware of) or could this just be a defective throttle body? If it’s defective, then I might be alright just buying another used one and cleaning it up (since you can’t get them new anymore) and hopefully it would be fine.

    And two, given that my throttle body is in otherwise great condition, I am trying to find out if it really is possible to get inside the throttle body shaft area on a first generation DOHC throttle body and replace any seals that may be in there. Is this possible? If so, does anyone know which kind or size of seals to use for replacements?

    I apologize for the more lengthy post, but I did want to explain everything as clearly as I could because I just don't see this topic discussed anywhere in the Saturn S Series community.

    Thanks for your help guys!

  • #2
    First I've heard of it and I've owned MANY S series cars. Owner
    2016 Honda Fit
    2002 Saturn SL1 - RHD
    2002 GMC Sierra 2500HD Duramax


    • #3
      They all leak a little bit. The worst one I heard was on a 400whp DSM; it would whistle at idle.
      This issue shouldn't matter too much on a saturn as the fueling strategy is based off of the MAP sensor reading and not a MAF sensor. Any little bit of leaking around the throttle blade shaft will be picked up as a change in pressure by the MAP sensor and the PCM will adjust accordingly. The vacuum leaks that are detrimental are the ones that leak by head and intake manifold junction. Those leaks cause one cylinder to be leaner than the others. Also, grossly large leaks that cause high idle, for obvious reasons, are bad too.

      Is there an undesirable running condition that you are trying to fix? Maybe we can help with that.
      -6S Resident Mechanical Forensics member #001.
      1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
      1998 F-250 5.4L triton...stock.


      • #4
        Thanks for the input, guys. Since you asked, there is an undesirable running condition that I have been trying to solve for many years now actually (I’ve had this issue for probably 7 seven years or so), and over the past several years I have done several things to try to fix the issue, but to no avail. Given everything I’ve done to the car to try to fix the issue, it’s likely that I’ve narrowed down what the issue could be to just a few things, and one of them is a vacuum leak somewhere, which is why I did the vacuum test in the first place.

        Just a warning, though. This post will be a deep dive into what’s going on, which is why I posted in the technical section and I will try to be as clear as possible. I was debating whether or not to add all of this to this thread, but if the throttle body leak is not the cause of the issue, then I guess I’ll have to keep trying to narrow it down. There is quite a bit to cover, so thanks a lot to anyone who gets through it all and tries to offer up any thoughts.

        It’s still concerning to me how much the throttle body leaks, but if that is somewhat normal and the MAP sensor is just fine compensating for that particular vacuum leak, then I suppose I will have to look elsewhere to find the issue. (As a quick side note question – is it really not possible to take apart the throttle body shaft area to get inside on a first generation DOHC throttle body? Does it not use any seals that are accessible?)

        Basically, I have an idle issue that also seemingly causes other issues to occur. It feels like a vacuum leak issue and from everything else I’ve tested, repaired, or replaced on the car over the years, my thought was that I had narrowed down the issue to a vacuum leak somewhere, although it could be one or two other things, as well, that I’ll get to in this post.

        The idle issue doesn’t cause the car to idle high, it’s actually the opposite, which I think means that the car is letting too much air in for some reason. The RPM run too low at normal idle and I don’t think the idle screw has been adjusted at all, and it also gets worse when it’s in gear. In fact, this car has an automatic transmission and I can kill the engine sometimes just by shifting from park to drive or reverse, because the RPM drop so far. And if shifting into gear doesn’t kill the engine, the RPM drop to somewhere around 500 and then they will come back up a little, but not to where they should be. Of course, this also causes the car to run slightly rough all the time.

        Then, every time I take off, I can actually let off the brakes and without touching the throttle, the car will start to slowly speed up and even shift into second gear eventually if I let it go far enough, like it’s being pushed or something. It’s a strange feeling, and it also happens on a level surface and even if I am on an incline. It’s almost like the throttle is stuck open just a little bit or the idle air control valve isn’t working right, but as you’ll see on my list of things I’ve worked on already, neither of these are probably the case.

        Next, whenever I have been driving for any distance, and I come to a stop and the car is in drive, and even right before I have come to a complete stop, the car’s RPM drop way down to almost nothing and then come back up again to idle speed. This takes place in about a second, where the idle will bounce down quickly to almost nothing and then back up again quickly to idle speed. It only happens once after coming to a stop, but it happens every time I do come to a stop. And even when the RPM come back up again, they aren’t ever quite where they should be.

        Now, the car is also running really rich all the time and because of everything I have done to try and fix the issue above, I feel like this issue of running really rich is directly related to the issue of the car taking in too much air somewhere. It doesn’t matter if the car is cold or if it’s been running for an hour. No matter how hot the car gets, it never stops running really rich. This is real apparent in two different situations.

        One, whenever I park the car and turn it off after driving for a while, it smells very rich all around the car. And two, when I try to smog test the car now, it’s on the verge of failing because when they do the test where they bring up the idle speed for a couple minutes to make sure the engine is at operating temperature and then let the RPM back down and then test for emissions, the car never stops running rich. It never reduces the fuel mixture to compensate for the car being warmed up. It just keeps pumping in more fuel, which of course, has also hurt my fuel economy some.

        My thinking is that either the car is taking in too much air for some reason (a possible vacuum leak, the type where unmetered air actually gets allowed in and not pushed out) and always trying to compensate by adding too much fuel, or it’s the opposite, and the car is getting too much fuel (a possible fuel system issue) and is always letting in more air to compensate and causing what seems like a vacuum leak during idle.

        The car is not setting any hard codes that cause the check engine light to come on, but even if it did, it may not tell the whole story anyway because the car is OBD1 and sometimes the codes it sets are only symptoms of the real problem. I do have an old scan tool (not the Tech 2, though) that tells me there’s a flag in the system that keeps saying there’s an O2 sensor circuit problem somewhere, which would actually make some sense given the symptoms, but as you’ll see, I have tried to address that issue already.

        Without fully explaining everything item by item, I will just list everything I can think of in no particular order that I have tried to do over the past few years to address this issue to give you an idea of what the issue may or may not be.

        - Throttle body (removed, thoroughly cleaned out and checked over with no signs of excessive wear)
        - Throttle body gasket (replaced)
        - Idle air control valve and o-ring (replaced, twice)
        - Idle air control connector (replaced)
        - Throttle position sensor and connector (replaced)
        - Leaky evaporative vacuum line on top of throttle body (replaced)
        - Fuel pressure regulator and vacuum line (checked and they’re both good)
        - PCV valve and hose (replaced)
        - CCV hose (replaced)
        - EGR valve and gasket (replaced, then tested with block-off plate which didn’t change anything)
        - California car so California EGR valve
        - Engine coolant temperature sensor (replaced with brass-tipped one, and the connector was checked and it’s good)
        - Intake air temperature sensor (replaced with brass-tipped one)
        - O2 sensor and connector (replaced)
        - Brake booster vacuum check valve and grommet (replaced)
        - One old and degraded fuel vapor line that’s near brake booster that was leaking vapors (spliced and repaired)
        - MAP sensor (checked using richpin06a’s Youtube video as a guide – it checked out and is good)
        - Spark plugs and wires (replaced just a few years ago)
        - Coil packs (removed and cleaned up)
        - Fuses (all checked and intact)
        - Vacuum tested all but one area of car (with no manifold leaks and no other vacuum leaks except at the throttle body shaft)

        Now, I realize that any one of the parts that I replaced could still be faulty, so I tried my best to research and put the best parts on the car that I could find. Hopefully this isn’t the issue. Hopefully this isn’t an electrical gremlin, either. That last point I mentioned above on the list was vacuum testing all but one area of the car. That leads me to my final thoughts before I kick it back to you guys.

        Given the symptoms and everything I have done so far, the two main systems of the car that would make sense to me for the issue to be in are the evaporative emissions system and the fuel system. I have not vacuum tested the evap system, including the canister or the hoses that go to and from the canister, and they are getting old (as I mentioned, I did test and replace the evap vacuum line that goes up to the throttle body, though). I also have not tested the evap canister purge solenoid valve or its connector, and the connector seems difficult to get to (it’s attached to the underside of the starter, correct?). One thing I should probably do is the purge solenoid vacuum test that richpin06a shows on his Youtube video since I haven’t done that. Otherwise, any of the evap system could be the issue.

        The other issue, the fuel system, could also be the culprit because the car has 159k miles on it (these are verified miles, because I replaced the odometer gears right after they stopped working several years ago) and the car still has its original fuel injectors. I know it’s probably well past time to replace them, but could faulty fuel injectors be what’s causing the idle issue and rich fuel mixture (makes sense)?

        The fuel filter was replaced maybe 30k-40k miles ago and the fuel pump has never been replaced. Could it possibly be a failing fuel pump?

        Otherwise, I’m at a loss. The car has been in my family its entire life since it was brand new, and I can verify that it has always been well maintained and babied, so I would tend to doubt that there are any serious internal issues with the engine, especially with only 159k miles on the clock, but I guess it could happen.

        Maybe after everything, I’ve finally narrowed it down enough to figure it out myself, but again, thanks a lot to anyone who reads through all of this and offers any thoughts as to what the issue could be based on your experiences.


        • #5
          It sounds like the IAC may not being doing what it is supposed to do.

          The EGR may be bad. If it is hanging open, it will cause a low idle. It took my OBD1 95 a year to set a code for a bad EGR valve that cause a low idle. I would temporarily remedy it by clutching to neutral and reving the engine a few times. The high vacuum when the engine speed was coming down would suck the EGR pintle shut and the issue would go away.

          If that is not the case:
          I would start with resetting the "base idle" and then play with the TPS to get it you were it needs to be (and clean out around the IAC port if need be).
          To do this you need the IAC all the way closed, or you can put you finger over the IAC hole while you set the idle. If you don't have a scan tool that will reach into the engine bay and read out RPMS then you will need a helper to read the tachometer.
          Get the engine up to operating temp (but not smoking hot...your finger will hate you). Put you finger in the throttle body over the IAC hole and the RPM should drop down to about 450-500RPM. If the engine stalls, then crank the idle screw open a few turns and retry. You want the engine to idle at about 450-500RPM with the IAC hole plugged.
          Once that is done, you will need a reading from TPS at the idle postion. It should be between 0.45 and 0.50v in the closed position. If it is not in that range, the PCM will do weird things with the IAC valve pintle. The TPS technically cannot be adjusted. But if you loosen the TPS screws a couple of turns and twist the TPS sensor then you can adjust the reading a little bit. If you need to adjust it more, you can try to take the TPS off and elongating holes a little to get you to were you need to be... 0.45v - 0.50v in the "closed" position.

          When I was a tech I did see a few of the 00-02's that had a "bent" throttle stop. The only thing I can think of is that over the years of the throttle slapping closed it slowly bent back and caused idle issues. Reseting the base idle and the TPS fixed the issue.

          PS: sorry for any typos... I have had a few Yuenglings.
          -6S Resident Mechanical Forensics member #001.
          1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
          1998 F-250 5.4L triton...stock.


          • #6
            Have you done a compression check? Also, check your timing chain. If it is loose, it can cause a rich condition, which makes the pcm pull fuel. It can get to the point that it can't pull enough fuel out. This also causes strange idle especially letting off throttle due to the slack not keeping the cams in sync. The chain doesn't need to make noise to be a problem. Another item to look into is exhaust backpressure. Most of what you're mentioning, I have dealt with recently. I actually have scanning software to view sensor and pcm response, and went through the ringer trying to find my problem.
            Ian---The Fat Ass

            1995 SL2-High compression engine FOR SALE
            2010 Chevy Cobalt SS- tuned with plenty of goodness
            2002 SC2-New daily
            2017 Kawasaki Ninja 650 KRT- got a stupid deal
            2003 Ford Taurus SES- free daily beater


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