What are the symptoms of a leaking throttle body gasket?

Discussion in 'General Motoring' started by Jamie, Jun 7, 2006.

  1. Jamie

    Jamie Guest

    Just curious,
    If I cleaned my throttle body and reassembled it without changing the
    manifold gasket, what would the symptoms be, if the old gasket isn't
    holding its seal?

    To be fair, the engine performance changed after I cleaned the throttle
    body. I didn't change the gasket and would like to see if what happened
    matches up with what is expected to happen.

    Jamie, Jun 7, 2006
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  2. Jamie

    Jason Guest

    I cleaned the throttle body, and replaced the throttle body gasket on
    my 1988 Vovlo 740 GLE recently. Lumpy idle was cured (car "hunts" for
    an idle rpm).

    If the throttle body gasket is bad, you might have idle problems,
    because there is a vacuum present in the throttle body, and an
    electronic switch monitoring the vacuum in order to regulate idle. You
    should be able to tell by looking at the gasket. If there are voids or
    tears, it is bad. Price is only a few dollars, or you can cut your own
    from gasket material available at most auto parts stores.

    Its a good idea to do the gasket when you remove the throttle body,
    since you are in there anyway...
    Jason, Jun 7, 2006
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  3. Jamie

    Jason Guest

    oh. BTW, symptoms are probably rough idle, with other possible
    Jason, Jun 7, 2006
  4. Jamie

    Jamie Guest

    Thanks for the reply. I am kind of hinting at a pop-quiz to hear what
    people's answers are, then match that up to my case.

    Sort of like:
    "If a person removes and cleans the throttle body on a 1987 740 GLE,
    B230F engine, and reinstalls the throttle body without changing the
    gasket, the result of a leaking gasket will most probably be":

    A) A rough, erratic idle
    B) A lower idle speed
    C) A higher idle speed
    D) Car stall and die
    E) No change

    My result is one of the above. Which should it be?
    Jamie, Jun 7, 2006
  5. Jamie

    Jason Guest

    A) A rough, erratic idle

    ....assuming everything else is fine. Which means you have new spark
    plugs, new rotor, new distrubution cap, new wires, all electrical is
    good, engine compression is good, fuel filter isn't clogged, gas is
    fresh, there is no water in the gas, the air filter is clean...
    Jason, Jun 7, 2006
  6. Jamie

    Clay Guest

    you can add to your list:
    Air leak = lean running = Burned pistons...
    Clay, Jun 7, 2006
  7. Jamie

    Jamie Guest


    My answer was C - fast idle.

    Now we're getting somewhere. My idle shot up from around 1200rpm in
    park to about 2000rpm.

    It drops to about 900-1000 when I shift to drive. I thought maybe air
    was getting sucked in and the engine is getting too much air and
    running faster and hotter.

    This leak is past the throttle body, so I didn't think it would have
    the choking effect of a vacuum leak as much as it would appear to have
    the effect of added air fanning the fire of combustion.

    I'm getting a new gasket tomorrow.

    Jamie, Jun 7, 2006
  8. Jamie

    User Guest

    Maybe, if the throttle plate had been set correctly originally but was
    being held open by gum deposits and not allowing the throttle switch to
    Maybe, if the gasket had been damaged or the nuts left loose on
    reassembly or the wires to the throttle switch left unplugged,
    Not likely, unless the inlet hose to the idle air control valve were
    left off the bellows hose.
    Generally the gasket replacement or non-replacement has no effect. SOP
    would be to replace the gasket. But given the number of operations where
    the throttle housing is removed to gain access to other things smd it
    doesn't get replaced I'd have to say no change is the most common
    result. Also the most common on cars with regular throttle and pcv
    It depends on what it was doing before. One would remove it to clean it
    either as PM or to correct one of the other options given as answers by
    resetting a cleaned part to factory spec.

    With the motor warm and the idle air control valve plugged and the
    nipple to the belows hose plugged the motor should barely idle at around
    400-450rpm if the throttle plate is set correctly. Restoring the IAC to
    the system with bring the idle up to 850. Even with the throttle plate
    full shut the IAC will generally achieve an 850rpm idle. However if the
    the throttle plate is set too far open, say 950 rpm, and the throttle
    switch is reset to click in that position, then the idle air control
    valve cannot reduce the idle to 850rpm even if the control unit commands
    it full shut. Alternatively if the throttle switch contacts are open
    when the throttle plate is shut, the IAC will be commanded to open to an
    off idle position (around 20% open) and the motor will idle about
    1500rpm. The same thing occurs when the throttle switch is left
    unplugged. The control unit can see neither idle position or full
    throttle so the IAC is again commanded to the open throttle position and
    idle goes high.

    User, Jun 7, 2006
  9. Jamie

    Jamie Guest

    I think we're on to something here.

    Ever since I found my distributor was not seated correctly and was
    causing my severe loss of power, the repair I made to it restored full
    power, but now it idles at around 1200-1500 rpm in park and then around
    900-1000rpm in drive with the brakes applied.

    I've attempted turning the black idle knob all the way to the right,
    and it doesn't drop it under 1500.
    runs vertical from the throttle body to the throttle lever (with the 2
    nuts). I have this set so that I can hear the "click" and it engages
    the idle. If I turn it past the "click" it goes into fast idle.

    Are you suggesting I inspect the throttle plate to make sure it is in
    the proper position?

    I am guessing the throttle plate is the black device with the connector
    wire on the throttle body? I briefly read over this procedure, but
    didn't fully understand how to make the adjustment so I left it alone.
    There was something about loosening the 2 nuts with an allen wrench (or
    hex-head, I can't recall), and there was also another nut to loosen
    that I didn't understand.

    Do I understand correctly that this throttle plate could be out of
    adjustment, and even though I get the "click", it may be clicking at
    the wrong RPMs?

    Please advise. Also, I still don't see how cleaning the TB would have
    run the idle up higher if the gasket is sealed.

    Jamie, Jun 7, 2006
  10. Jamie

    KLB Guest

    Considering the age of your car I would also have a close look at the vacuum
    hoses off the Throttle Body. Best to make sure they are not leaking,
    cracked, and fitting properly with correct tubing and all spring clamps in
    place. Maybe somebody has changed things over the years or worse yet hasn't
    done anything at all.

    I put my IAC back on today and in the process noticed that one of the two
    small vacuum hoses off the top of the throttle body was loose. Not only
    that, the spring clamps were missing. It took the hose of and discovered
    that it had been replaced, not original Volvo either. Someone had put on a
    piece of 3/16th inch. Should really be 5/32nd or a bit smaller as the
    3/16th was pretty sloppy on the nipple. What looks good may not be quite as
    it appears.

    No question, my car seeks and finds idle quite nicely after cleaning IAC.
    Still have rough idle on cold start so next step is this little ditty below
    and if that doesn't work it's new distributor cap, rotor button, fuel filter
    and wires (going for the Bougicord wires, sounds like they are worth the

    Poor Cold Idle: B230F/T ECU E-Prom Needs Update. [Query] My wife's 95 945T
    (90k miles) starts fine on a cold (below 40F) start, but idles like crap for
    20 to 30 seconds. If I hold my foot on the gas and keep it at 1500 rpm for 8
    to 10 seconds, its fine. Car idles nicely when warm, and runs like a dream.
    When I pull the plugs, they look great. I've replaced ECT and other parts
    and cleaned the TB. [Response: Abe Crombie] Go to a dealer and have them
    look at Volvo Service Bulletin 28-102 "modified e-prom for cold start with
    low rvp fuel". This says the symptoms are: car starts and then immediately
    dies and requires re-start. Runs rough for the first 45 seconds and may
    hesitate on acceleration. Recent EPA regulations have necessitated changes
    in the formulations of gasolines (i.e. "oxygenated fuels"). The result of
    these reformulations has been a decrease in the relative vapor pressures
    (volatilities) of these fuels, which seems to be particularly problematic
    for cold starts/idling. The updated eprom chip to be installed in the ECU
    is the fix to make it have correct fuel mixture computations for cold start.
    You must have the number from your ECU in order to cross-reference the
    correct eprom update kit P/N. The change procedure described in 28-102
    requires careful attention to static discharge.

    I would think that most anybody would agree that an extra Throttle Body
    gasket or two should be sandwiched in the old glove box in amongst the
    insurance papers.

    Remember, don't be spraying silicone around intake anywhere if you might
    have leaks, goofs up the oxygen sensor. (and they are not cheap)
    KLB, Jun 7, 2006
  11. Jamie

    James Sweet Guest

    If the gasket is leaking, then unmetered air will be getting into the
    engine and it will run poorly.

    I've had reasonable luck reusing the gasket though so long as it didn't rip.
    James Sweet, Jun 7, 2006
  12. Jamie

    Jamie Guest

    I found this on brickboard and it matches my issue. I am going to
    replace the gasket today, remove and blast the knurled black idle knob,
    then follow this example:

    //Poor Idle; TB Cleaned; Now Idle is Too High. [Query:] Well my
    problem is certainly gone. It now idles at 1600 rpm, but that's a
    steady 1600. Did I do something wrong, or was the crud in the throttle
    body masking another problem? [Response: Evan] Nope, the crud WAS the
    problem. Crud makes the car idle lousy and slow. Lazy mechanics simply
    dial up the idle to mask the problem, rather than fixing it. You just
    need to dial the idle back to spec. On the end of the butterfly shaft,
    the end where the spring is, there's a stamped metal plate. It has a
    'leg' bent down that rests against the idle stop screw. The screw is
    held in place with a lock nut. loosen the nut and adjust the screw.
    [See Throttle Body and Throttle Position Switch Adjustment for more
    detail on 89+ cars and Adjustment of Pre-89 TB, TPS and Base Idle: for
    pre-89] Be careful, the screw head has a tendency to strip out. In
    retrospect, you should have taken a minute to make sure the screw was
    free while the TB was on the bench.
    Another thought: Before you do any of the above, make sure that the
    'leg' on the stamped metal plate actually touches the stop screw at
    idle. Some REALLY lazy mechanics just adjust the throttle cable length
    at the big obvious pulley, rather than adjust the stop screw//
    Jamie, Jun 7, 2006
  13. Jamie

    Jamie Guest

    In addition to this, I'll clean the IAC.

    Jamie, Jun 7, 2006
  14. Jamie

    Jamie Guest

    Problem Solved!

    Thanks All!

    1) Throttle body was dirty -- I cleaned it.
    2) I reset the idle screw
    3) I replaced the gasket
    4) I removed and cleaned the IAC

    Here's the kicker!

    I installed the IAC with the arrow facing in the PROPER direction
    (toward the intake). When the mechanic installed the engine, he had the
    IAC backwards.

    So, I have a nice idle at about 950rpms. I tried to use the black
    screw to come down to 750-850, but no luck.

    Still better than 2000 and it sounds so much more relaxed.

    Jamie, Jun 7, 2006
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