kushiel

Hello all,

I've been slowly restoring this old 4Runner that I came across. It's been up a running for a few years now, like three. I am stumped by this high idle problem. I poked around the boards and then made a checklist do go through. Oh, it's running on a new-ish engine that now has ~8000 miles on it.

Fuel pump died last summer. I replaced it.
Things ran fine for a few months then I noticed a high idle ~1100 and in town MPG around 14 (was 19 before).

So I went through the check list:
AFM - passed
Cold Start Injector - passed
Temp switch - passed

TPS - failed, replaced
Main relay - was ify so I replaced that.

The TPS adjustment is hellish. But I bought some of 4Crawlers hexes to make it easier. Just today I finally got it seated right. So now I have a high start idle of 1100 which then drops to 950 after warmup. The idle is actually stable now but I need to drive around a bit to verify that. Right now I am getting 16.9 MPG in town and it should be just up over 19.

Routed around the power steering to check for a vac leak there. Nope all was good.

I wasn't sure what I should check next. Thanks for any pointers.

Co_94_PU

iTrader: (-1)

Idle speed is dictated by the amount of air entering the combustion chambers.

Vacuum lines secure, they should have resistance when you try to rotate them on the barbs.

Vacuum lines routed properly, the FPU vsv switches to atmosphere make sure the line hooked to the intake isn't passing air.

Brake booster diaphragm intact, disconnect and plug the large vacuum hose to the brake booster and observe for idle speed changes.

Intake gaskets not properly seated, did you use thick (quality) gaskets or fipg.

Power steering or AC idle up active?

Throttle plate loose on its support, this gives static idle when it doesn't return to the same position when you let the throttle snap closed.

Faulty/plugged dash pot, on manual transmission vehicles this diaphragm slows down the throttle plate return when the accelerator is released.

Idle adjustment screw leaking from an old dry o-ring, slather some fipg over top of it it keeps it from moving and insures a good seal.

And lastly..
Timing in spec, advancing the ignition timing adds to the idle speed. Idle speed and timing are interlinked. The procedure is to set the idle speed with the idle screw, set the timing, then check the idle speed again, if the idle goes out of range you have to check the timing again after bringing the idle speed back into spec, then repeat as needed until both stay in spec.

kushiel

Great! Thank you. I'll start working my way through this list.

Griswald

Look at the dashpot first

Is it a manual transmission?

If so,
When it idles high, what happens if you rev it up to 3000 RPM, and then release?
Will it fall back to where it needs to be? around 800 RPM?

You can also just pop the hood (while running) and see if the dash pot is sticking. Just flick the linkage back, and see if throttle goes back down.
If it does, you can just disengage your dashpot by unscrewing the little nut on the allen screw.

You dont need the dashpot. Its only purpose is to provide smoother transition after you let off the throttle.

Try this first. Wont cost you anything, easy, etc.

kushiel

So when I run it up to 3000 RPM and then let of the gas the idle drops to 800.

I'll go fiddle with the dashpot here in a few minutes.

kushiel

Now my dumb questions is where are the screws that hols the dashpot on? Are the under the throttle body near the front?

I can see a little L-bracket coming off and disappearing under the throttle body.

Griswald

kushiel

Thanks! Lemma go try that.

Griswald

So crack that nut, and loosen it just enough so it no longer touches the dashpot plunger when the throttle is engaged.
Or you could just remove it all together and set it aside.
This way you'll bypass the dashpot - which most likely is sticking (unless you rev it high)
From what I understand, I dont believe the dashpot is a part you can order anymore.
But again, you dont need it.

I would also hit your throttle cable with a pinch of WD40 lithium grease (clean up the excess since its flamable). then repeat once or twice if needed.

let us know if that fixes it - worked for me.

kushiel

Ok I removed the nut for the dashpot which should disable it. Over the past four days with in town and highway driving my idle is either 850 or 950 when I come up to a stop. It doesn't creep up to 1100 like is was doing before. So it looks like part of the problem was the dashpot system.

gsp4life

Not at all surprised to hear that helped, sticking dashpot seems to be the most common erratic-idle problem on our engines.

Also, instead of using sealant on the throttle adjust screw, simply replace the o-ring! Same goes for the plastic adjuster screw for the a/c idle-up valve. I don't remember the sizes offhand, but it's standard metric. Any o-ring kit or decent hardware store bin will have them.

Another common vac leak spot is the rubber accordion hose that connects the long plastic intake tube to tb. If it looks old, remove it and gently flex to check for cracks along the ribs, especially the underside.

Co_94_PU

iTrader: (-1)

Sticking dashpot is sometimes a clogged filter, the bit at the end, needs cleaning.

Everyone has sealant, not everybody has orings laying around. In my particular case the engine I pulled my idle screw from used a rubber cap so there is no room or provisions to put an oring on it.

Thommo Thompson

air is not the only player in determining idle speed, but it is the most significant.
A rich idle mixture can make an engine idle up to about about 200 rpm faster than it should for a 4 cylinder.
The correct idle mixture (just a little bit richer than stoichiometric) would result in a lower idle speed, but a more economical, hotter combustion, lower Cylinder head temp, hotter Exhaust gas temp, faster flame front, but with more requirement for perfect compression, and good fuel atomisation.
What im saying, is you cannot jsut say PHANTOM AIR is the only thing that can make the idle higher. It may be getting a bit too much fuel.
THose possible fuel related culprits, are:
fuel pressure regulator.
injectors dirty
Air leak in exhaust before O2 sensor
tired spark plugs or coil, (leaves oxygen in exhasut and tricks ECU)

Griswald

That idle sounds about right. 800-900

Also, hit the throttle cable with a bit lithium grease or something similar.

You'll notice that when you take your foot off the gas while in gear, it may jerk a bit - the dashpot was there to make that a smooth transition.
But you dont need it...