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Low idle after disconnected battery

Discussion in '80-Series Tech' started by aim, Jul 17, 2006.

  1. aim

    aim New Member

    Messages:
    450
    Location:
    Western MA
    ... cured by disconnecting the TPS. Why?

    While replacing the starter contacts, I disconnected the battery. Upon first startup, the idle was 600, and started dropping as the engine started warming up slightly. After a few minutes, the idle was 0. Many restarts and a dozen miles of driving have no effect. The IAC occasionally bumps the idle, but to no avail. AC will raise idle, but it will still stall after dropping to 150-200. What consistently solves the problem (I experimented...) is:

    1. turn engine off.
    2. unplug TPS
    3. turn engine on and keep RPMs at 600-1000 for 10 seconds or so (the CEL obviously comes on)
    4. turn engine off
    5. plug TPS back in
    6. turn engine on and enjoy reasonable idle.

    The LC is a '94. The air hose is new. No obvious vacuum leaks. New throttle cable, but similar behavior observed with old, rotten cable. Plugs, wires, distributor cap all new. TPS not checked (next step). The normal idle is 550, and occasionally erratic. I'm still catching up on maintenance.

    Question: What exactly is going on? How can unplugging the TPS possibly cure these symptoms? What should I look for when I take it off in the next couple of days and start following the FSM. And how do I apply vacuum to the TPS?

    I did search, I did read the 170+ posts found, I got inspired to do more AR maintenance and I apologize for not finding an answer in the collective repository of mud wisdom

    Thank you in advance!
  2. dfmorse

    dfmorse New Member

    Messages:
    1,518
    Location:
    EVERGREEN, CO, USA
    Aim;

    Well, just off the top of my head in the early morning here, I would start with the FSM, pg EG-8. The TPS has a mechanical switch that activates when throttle is closed; if this is damaged or dirty, the ECM will not see the closed contacts for the idle position and that may screw up the idle control to the IAC system.

    In other words, the IAC can't see the ECM control voltages to its stepper motor [and sometimes it can] giving erratic idle symptoms.

    That's all I can think off the cuff,

    Keep us posted

    ...And welcome...
  3. aim

    aim New Member

    Messages:
    450
    Location:
    Western MA
    You know, I forgot all about that table this time around, but what confuses me is that letting the engine run with the TPS unplugged fixes the problem permanently. I'll be either eyeballing closely or replacing the TPS RSN. The switch is vacuum activated, right?

    Thanks...
  4. dfmorse

    dfmorse New Member

    Messages:
    1,518
    Location:
    EVERGREEN, CO, USA
    Aim;

    Based on my short look at the books, I'm not sure if its vacuum or not.

    I do remember, a long time ago, some posts on MUD about being really careful with the TPS and MAF systems so that one does not damage the connectors and electrical connections.

    I hope someone with more knowledge jumps in here.

    I'm going to have to study this more closely. Of course, if u go thru all the tests in the FSM, u may come up with some ideas. Leaving the TPS disconnected implies no control of the stepper motor for idle control adjustment [maybe]. See how dumb I'm getting. I don't think the TPS has a direct vacuum connection. I'll break out the FSM and start reading more closely.

    Some thing just crossed my mind, how about using some carb cleaner and spray the hell out the throttle body.

    ...

    Also, see EG-280, which covers the throttle body and TP Sensor; somewhere back in my soggy brain I remember that this a delecate device...
    Last edited: Jul 18, 2006
  5. dwainf

    dwainf Regular Member

    Messages:
    66
    Location:
    Winnipeg, Manitoba
    Canuck Newbie idle problems

    Greetings from the great white north! Newbie to the forum...just purchased a 94 FZJ80 after losing my '85 60 to rust. The 80 is a beautiful rig, but it has developed a bit of an attitude. It is displaying the same idle problems as "aim" has described. A disco of the TPS did nothing. The kicker is that it doesn't idle eratically all the time. There seems to be no rhyme nor reason to the problem. It will start fine, the idle will slowly creep down from the start high idle, then it will "bounce" between 900 and 200, eventually dying. Does this sound like a TPS problem as well?
  6. MoJ

    MoJ Moderator

    Messages:
    3,254


    Dwain -
    Although there are many things that can cause this, first very thoroughly check the large rubber intake hose running off the air filter box for cracks between the ribs --- a very common problem that causes idle issues similar to yours. The cracks are often on the bottom side.

    Welcome to the forum!
    Jason
    Last edited: Oct 7, 2006
  7. RavenTai

    RavenTai New Member

    Messages:
    6,085
    Location:
    Dixie co. Florida
    The TPS is mechanically connected to the throttle plate shaft, it has two electrical parts to it, one section changes resistance as the throttle moves, the second is an on/off switch that closes when at idle, there is an adjustment procedure in the FSM using an ohm meter and feeler gauges on the throttle stop

    You may want to take the TB off and clean it and do the adjustment; it is hard to adjust on the truck and also all the deposits may be keeping the throttle plate from fully closing and making the switch in the TPS. I like 3M throttle body cleaner, works the best at dissolving the crud of any I have tried

    The TPS may also be bad, there is a test procedure in the FSM


    as for why it works with it disconnected here is a WAG, for some sensors if the ECU thinks the sensor is unreliable or giving no input the ECU will throw a code and start using a fixed standard value inplace of that input, I am not sure if this limp home mode applies to the TPS or not.
  8. dwainf

    dwainf Regular Member

    Messages:
    66
    Location:
    Winnipeg, Manitoba
    Checked the intake hose...PO has some silicone sealant in the first rib crack that appears to have cracks in it. Is this a readily available part or something that should/could be repaired?
  9. RavenTai

    RavenTai New Member

    Messages:
    6,085
    Location:
    Dixie co. Florida
    it is readilly available from our parts Guru CruiserDan of American Toyota, very few parts are not readily available from Dan

    It can be temporairily fixed with sealant or tape but should be replaced if cracked,
  10. dwainf

    dwainf Regular Member

    Messages:
    66
    Location:
    Winnipeg, Manitoba
    Great! I'll try re-sealing it first. If that fixes it, I'll replace the hose.

    Here's hoping that it works! The stalling part is getting a little annoying.

    Thanks for the fast replies and the welcome.
  11. dwainf

    dwainf Regular Member

    Messages:
    66
    Location:
    Winnipeg, Manitoba
    Happy cruiser once more

    A story with a happy ending!:) Turns out the intake hose had been epoxied together in the past, then sealed with black silicone. In the course of cleaning my new toy, I must have disturbed the joint (It was probably broken already). I resealed the intake hose and she purrs like a kitten. I'll be ordering a new hose monday!

    Thanks for your help. Hope I can return the favor.

  12. Beowulf

    Beowulf New Member

    Good call Jason!

    -B-
  13. GeoRoss

    GeoRoss New Member

    Messages:
    1,295
    Location:
    Tucson AZ
    Just a note about my experience with low RPM at idle after disconnecting the battery.

    I did some EGR rehab this summer, I replaced the modulator, EGR valve, all vacuum lines were replaced or cleaned, temp sensor was cleaned and all vacuum line ports were reamed out. I also replaced the intake hose. I had a new one and tarbe asked why have the old one that was as hard as PVC on there when you have a new one ready to go in? So I replaced it as well.

    I pulled the ECU fuse (same as battery) to clear my codes. It took far longer than 3 trips to get everything reset. I think it was about 5 days before the RPM's settled down (or up :) ). Now everything is fine 3,000 miles and two months later.

    Ross
  14. AfricanToy

    AfricanToy New Member

    Messages:
    526
    Location:
    USA
    CAN YOU PROVIDE A PIC OF THIS FIX?:idea:
  15. dwainf

    dwainf Regular Member

    Messages:
    66
    Location:
    Winnipeg, Manitoba
    I'll go and take a picture if you like...It's mostly epoxy (Loctite Weld) covered with electrical tape. Just a quick fix until the new hose arrives. Does anyone use automotive "Goop" for temp fixes? I have seen this stuff, but I'm somewhat leery of silicone-like sealants.
  16. dwainf

    dwainf Regular Member

    Messages:
    66
    Location:
    Winnipeg, Manitoba
    Walked out and snapped this...doesn't show much. The hose was split all the way through at the first rib next to the throttle body. There was a black sealant in there that hid any cracking, but the sealant itself had broken down allowing leaks.

    Attached Files:

  17. aim

    aim New Member

    Messages:
    450
    Location:
    Western MA
    Time for an update...

    The engine is fully rebuilt; everything was fully cleaned (and I mean everything), many things (including the TPS) were replaced. The low idle problem is the same, with the same solution. I am about to replace my battery, so I will try to match GeoRoss' patience. In the meantime, if anyone has any new insight...

  18. aim

    aim New Member

    Messages:
    450
    Location:
    Western MA
    GeoRoss FTW. A few days and over 300 miles and the idle is 625. Of course, the engine now revs much higher at startup, but one can't have everything I guess.

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