Can't bring engine revs down!

Discussion in 'Workshop Forum' started by dan in new mexico, Oct 11, 2018.

  1. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    while driving a couple days ago noticed engine continued to rev at about 3,000 rpm, even when should have just been idling. First thought i had was had to be linkage or cable problem. cable operation seemed ok..not stuck or anything. opened up area around foot pedal cable hookup attached ok, moved freely...but did not seem to impact engine revs...now stablizes at 4 k rpms. cable action does not seem to impact engine rpms at all! i though sure it must be cable related..but does not seem to be the case! ideas please. thanks
     
  2. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    ooops..sorry..its an 81 fuel injected model.
     
  3. JimD

    JimD Waiting for Godot... Moderator

    Location:
    Missouri, USA
    You could disconnect the accel cable from the linkage and see if that changes anything.
     
  4. lidox19

    lidox19 Daily Driver

    Location:
    Cheltenham UK
    Idle control valve stuck open or leaking ?
     
  5. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    tried that jim...no difference
     
  6. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    could you tell me where that's located? as i mentioned, my x has the bosch fuel injection system. thanks.
     
  7. carl

    carl True Classic

    Location:
    Virginia
    Have you checked the throttle plate to make sure it's closing all the way? Your throttle linkage could be binding up on the cam cover.
     
  8. Dan Sarandrea (Phila)

    Dan Sarandrea (Phila) Waitin' On Parts...

    Location:
    Philadelphia, PA
    It's so easy to take vids and snap pix these days, post up some of your throttle linkage area.
     
  9. Cratecruncher

    Cratecruncher Daily Driver

    Location:
    Austin
    If the engine is running smoothly that would rule out a manifold leak because the air/fuel mix would run lean and the engine would pop and likely quit- been there.

    The idle control valve or auxiliary air regulator as some call it is located down low on the forward side of the engine and is best found by tracing hoses - one from the big ribbed rubber intake hose and another from the cold start valve area on the left end of the cast manifold. You can test the auxillary air valve is open with a cold then closed with a warmed up engine by removing the hose at the manifold and sucking on it. If it's open you can pull air past the metering body pretty easily. It should close about 4 minutes into warm up and be impossible to suck any air through. If you listen closely you can hear air "hissing" through it during initial warm-up, then the hissing subsides as it closes and idle drops from 1300 to 900 rpm.

    There is a large idle adjustment screw with a jamb nut on top of the manifold that might have backed off due to vibration. That would increase your idle a lot. You might check to see that it's not loose. There is also an idle adjustment screw at the throttle lever itself that might have come loose. Since you are seeing 3000 to 4000 rpm my vote is a binding or gummed up throttle plate that is somehow not seating correctly as Carl mentioned. You could check by simply forcing the throttle shaft closed with your hand and see if you can get the wayward idle down.
     
  10. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    will check for the large idle adjustment screw tomorrow
    as well the throttle plate issue. could the throttle position sensor be bad? thanks all for input.
     
  11. Dan Sarandrea (Phila)

    Dan Sarandrea (Phila) Waitin' On Parts...

    Location:
    Philadelphia, PA
    On Bosch L-Jetronic systems such as installed on our cars, the TPS is a Throttle Position Switch, not Sensor. It only knows three positions: A=Throttle plate fully closed; B=Throttle plate somewhere in between fully closed and fully open (aka WOT [wide open throttle]; C=Throttle plate wide open [WOT].

    Yes, if this switch is hung up, broken, or mis-adjusted, it could be holding the throttle plate open enough to hold 4000rpm when in neutral.
     
  12. lookforjoe

    lookforjoe True Classic

    You do need to look at the air slide hoses that Karl mentioned. They harden , crack and then let much air bypass the throttle plate which will cause very high idle. The top one going to the intake would be the one to check, where it is clamped to the air valve. Not easy to see, so some disassembly may be required. If this is a sudden change, it seems more likely than a miss-adjustment.
     
  13. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    raining today..but more good advice..thanks. i have a donor 1980 x..... might swap out that tps to see what happens. will advise
     
  14. lookforjoe

    lookforjoe True Classic

    Not gonna be the TPS.
     
    kmead and aarpcard like this.
  15. kmead

    kmead True Classic

    Location:
    Michigan
    The TPS sensor has two switches in it. Only one is really used as it is only adjustable for the one direction. The idle part of the switch tells the ECU when the throttle plate is closed the other switch is for wide open throttle but it really only indicates when the engine is near WOT. It would be odd for it to suddenly become loose and bind up the throttle plate as it has two screws holding it down.

    A TPS that is not working wouldn’t have any effect on idle speed, it basically tells the ECU the throttle is closed and when the engine is above 1500 rpm, the ECU shuts off the fuel injectors, if the throttle plate is still closed below that rpm as indicated at the switch, it brings the injectors back online. If the switch mechanically failed internally (unlikely but anything is possible) it could hold the throttle open but unlikely. Removing it if the throttle is being held open without any cable attached to it would answer that question. Either way with the engine off, you should hear the internal switch clicking as you open and close the throttle.

    The idle adjustment screw can allow unmetered air to sneak past its threads and what is likely a very hard, crusty O ring. The seat and the tip of the screw are also likely covered with oil deposits from blowby coming into the intake and should be removed and cleaned. The O ring of course needs to be replaced.

    There are other areas where you could have an air leak, two of which could be providing metered air. If you have AC, there is a electro bypass valve that allows additional metered air into the intake intended to raise the idle when the engine is under AC load. Without a load it could keep the unloaded engine revving higher but it is unlikely that it would hold it at 4K. The other is also designed to provide more metered air when the engine is cold, the auxiliary air valve, which has a bimetallic spring inside with a heating element that turns a disk to close off an air opening. The air opening provides metered air and raises the idle. As the heating element runs it closes off the opening and reduces the bypass air, it will close completely. Even if the unit doesn’t receive electricity, due to its position near the head, the engine’s heat will cause the bimetallic spring to close the opening.

    There are a variety of hoses from the bypasses covered above that could be leaking air into the system but being unmetered air the engine would lean out and likely wouldn’t be able to run at much more than a high idle. The main hose between the air flow meter could be cracked but if so the engine wouldn’t respond properly to the throttle being opened as the AFM wouldn’t ‘see’ that air. The AFM flap could be being help open due to warpage, backfire damage or something being lodged in the opening but that would tend to cause it to be horribly rich and again likely wouldn’t run properly.

    With the throttle cable disconnected at the valve cover to the pedal, the throttle plate should be closed. If it isn’t then you need to find what is holding it open. Disconnect the cable from the throttle plate itself, looking into the opening after removing the main hose to see if something is lodged between the throat and the plate and possibly by removing the TPS.

    My vote is for something holding the throttle open: the cable to the accelerator pedal, the crank arm across the valve cover that transfers the motion to the second cable or something at the throttle plate mechanism or in the throat of the throttle plate jamming it open.
     
    Cratecruncher likes this.
  16. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    excellent info..will concentrate on your last two paragraphs which seem to offer best bet for now..appreciate your knowledge.
     
  17. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    well..opened up throttle plate..not stuck..moving ok...so here are a few pics..notice the one hose..where does it need to go? think i need check auxillary air valve next.. 006.JPG 008.JPG 007.JPG partially visible at top of pic 3?..also checked idle adjustment...it is seeded all the way. tips about accessing the auxillary aire valve?
     
  18. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    think i might be on to something..tried disconnecting accelator cable completely from throttle plate..engine then would start and idle at about 1500..manipulating plate rod by hand..was able to vary rpms...i think the assembly on the right side of the throttle plate is hanging up..on my way to get to carb cleaner..and further tinkering..hoping this might be the prob..will advise.
     
  19. dan in new mexico

    dan in new mexico Daily Driver

    Location:
    silver city nm
    cleaned that mechanism..also sprayed with wd40...when reconnected cable still reving high..as that mechanism still was not returning throttle all the way closed..could bring back by hand...but would not return all the way when hooked...up ..so...then i noticed one of the two return springs on second cable mechanism was missing...so not getting adequate return pull when i let up on accellerator. i moved the top spring to the bottom...and used several thick rubber bands on top till i can get a spring with definite pull. so...the high rev is sort of under control..just gotta make sure i get that throttle to return properly.
    Like many suggested, it seems the problem was the the throttle plate movement all along. so for now, looks like i won't have extract auxilliary air valve. thanks to all
     
  20. lookforjoe

    lookforjoe True Classic

    That line is currently a vacuum leak. It needs to be attached to the crankcase breather tube - which is connected to the underside of the air intake pipe. Look on the backside, close to and above the oil dipstick, that's where the nipple it attaches to is located.

    Good catch on the missing throttle return spring. It is a fiddly design.
     

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