Newb question: normal fast idle on cold startup

Discussion in '80-Series Tech' started by gummycarbs, Mar 9, 2017.

  1. gummycarbs

    gummycarbs

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    Hi,

    I bought a well-worn 1994 Land Cruiser four days ago and I don't have a factory manual yet, so pardon the basic questions.

    What is a normal fast-idle RPM, starting up cold, on a moderately cold day? Say, 40°F. How quickly does it drop to the (I think) 650 RPM regular idle? And does it normally stay spot-on at 650 RPM with the vehicle stopped and the transmission in Drive?

    I've currently got a surprisingly high cold-start idle, like 1600 RPM, and it's sometimes bogging down a bit when I come to a stop.
     
  2. gummycarbs

    gummycarbs

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    Oh, and I should say this is with AC off. I'm pretty sure I had the climate control set to vent and floor, no defrost, too, so the AC compressor shouldn't be on. I'm assuming that the 'defrost' positions activate the AC compressor...
     
  3. IdahoDoug

    IdahoDoug

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    My 93 has done this since new. There is a trick on here somewhere involving adding 3 small O rings to the idle control shaft. I bought the O rings for it and they sat taped to my workbench for a decade. Then the tape rotted off and the last I saw them I'd tossed em in a large box of spares where some day my kids will puzzle over them. I don't think the 95+ years do this, at least mine doesn't. Anyhow the trick is likely in the FAQ.
     
  4. ConcreteCruiser

    ConcreteCruiser

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    My 96 does something similar until warm. No worries.
     
  5. ppc

    ppc M Go Blue

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    Totally normal. Most vehicles start with fast idle for faster warmups to control emissions. With the LC its just more noticeable because of the noise level and lack of sound deadening.
     
  6. gummycarbs

    gummycarbs

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    Okay, thanks, I won't worry about it. Though I will check the idle air control valve and throttle position sensors when I get my hands on a FSM...
     
  7. gummycarbs

    gummycarbs

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    Update:

    After finishing my power steering pump rebuild, installing a rebuilt alternator, and removing the old battery-draining DEI alarm, I finally got to drive the truck without having to disconnect the battery nightly. The cold-start situation got even worse, peaking at around 2,000 RPM. It would drop to maybe 1,800 RPM and stay close to that for probably a good five minutes. I wasn't happy with that, particularly given that it rocked to 2K on the first startup after changing the oil!

    I got to work replacing vacuum hoses. I've finished most of the tiny 3.5mm stuff, and all the larger (8mm?) hoses I could reach. That didn't have much effect. I noticed that the PCV hose had electrical tape near the valve. Looking closer, it was cracked, so I replaced both PCV hoses, plus the brake booster hose which looked nasty.

    That significantly improved the situation. The idle was more like 1,600 RPM instead of 1,800 - 2,000.

    My next step was to remove the IAC/ISC, intending to do the "O-ring trick". I found a lot of gunk in there. I disassembled it, cleaned everything (including the throttle body opening it goes into!), and put it back. I used a bit of Dow Corning high-vacuum silicone grease around the lip where it meets the throttle body. I decided not to resort to the O-ring hack yet.

    That didn't fix the peak cold-start RPM, but it greatly improved the overall cold-start behavior. Now the engine initially shoots to 1,600 - 1,800 peak RPM, but it immediately drops down to around 1,200 RPM. It gradually drops in increments from there, settling at around 900 - 1,000 RPM until it's fully warm.

    I will give it a few more days of daily driving before I look into it any further. If the situation doesn't improve, then I'll pull the IAC, test it according to the FSM, test the TPS, check the throttle cable, and then do the O-ring hack because it's vastly cheaper than replacing any of those parts. :)

    Oh, and here's an odd detail that makes me scratch my head: during the cold-start cycle, if I press on the brake, the engine RPM jumps up by around 200 RPM. Remove my foot from the brake and it drops back to whatever it was at. You'd think that means a bad brake booster, right? But the brakes operate flawlessly, and I can apply them quite a few times with the engine off before losing boost. And there is no change in RPM whatsoever from pressing the brake once the engine is warmed up.
     
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