Please someone help!!! (idle)

[A.k.]

My small port ae86 4age won't stop hunting for an idle it just goes between 1.5k and 2.5k, up and down, up and down!!
It's driving me nuts, please help, how do I fix it??

[70XIN]

unplug TPS, see if hunting goes away. if it does, TPS needs adjusting or replacing

also check your coolant level, and try and bleed the system if you can, any air bubbles can make it chuck a hissy fit

[A.k.]

Ok sweet, I'll try bleed it.
Sorry what does TPS stand for??
Did this happen to you aswell?

[70XIN]

Throttle position sensor. It's the black thing with a plug going into it on your throttle body.

http://i45.tinypic.com/244u178.jpg

^^ That's a TPS

[Nikkojoe]

If the hunting goes away after unplugging, it will probably still idle at 2k. An adjustment wont fix it, you need to lower your idle (adjust idle screw all the way down if need be). If your idle is at its lowest, you will need to see for any post throttle body leaks. This also means checking to see if your cold start idle up valve (one under the throttle body) is working correctly, you may need to remove it and plate it off if you haven't hooked up the cold start coolant lines. Also remember that the ports on that valve must be blocked off individually is not in use, do not loop vac hoses on them to block them off as it creates a bypass.

[A.k.]

Oh ok I have a look after work, if I unplug the TPS and the hunting goes away do I replace the TPS sensor before driving again?

[timbo]

if it was the cold start idler (bit under the throttle as nikkojoe said) it wouldn't hunt it would just rev at 2k all the time, this happened to me.

[A.k.]

Just unplugged the TPS and it stopped hunting, plugged it back in and it started hunting again.
It sits at 3k when TPS is unplugged, do I leave it unplugged, what now?

[17zon]

undo the screws and move it till it stops hunting and revs freely the smallest adjustments make a big difference(you wont break anything playing with it)

[A.k.]

Ok I'll do that tomorrow morning and post how it goes.

[Stain]

TPS don't fail that often. And you shouldn't leave it unplugged as it is what tells your ECU how much throttle is being applied... it part of the system that is used to calculate how much fuel and ignition timing is required.
If the ECU doesn't know where the throttle position is you won't get the best performance or fuel economy. Just do a search on Google how to adjust a Throttle Position Sensor.

But by far the easiest is to undo the two screws holding it on just enough that you can move it, but it won't flop around if you let go. This way you can nudge it back n forth ever so slightly until the idle stops hunting.
Once you've stopped the hunting at idle, slowly (very slowly) and steadily increase the revs to check if it begins to hunt at a higher rpm. If its okay tighten it up again and your set.
But be warned, the TPS is VERY sensitive to adjustment, the tiniest bit will throw it out again. Its one of the more frustrating sensors to calibrate.

[Nikkojoe]

if it was the cold start idler (bit under the throttle as nikkojoe said) it wouldn't hunt it would just rev at 2k all the time, this happened to me.

Dont think you understand what im saying.

In depth, the reason why it's hunting is because the ecu is trying to knock down the idle level. It basically cuts the engine for a split second if the idle is too high. The ECU is getting reports from the tps that the engine is in idle mode, yet the idle is very high (usually around 2k is the threshold, in conjunction with map reading). It activates the safety measure to cut the engine to stop possible damage.

You can somewhat simulate the hunting by increasing a vac leak. I.e. winding your throttle screw all the way out. If your car does not hunt in this test and your idle is 2500rpm, your tps probably isn't working or it needs adjusting.

A.k. -
A TPS adjustment WILL NOT FIX YOUR IDLE OF 3k rpm!!! You need to find your vac leaks! If i unplug my tps, idle increases to about 1k from 800rpm with it connected.

As a test measure, remove your air filter and place your finger over the air ports in the throttle body (the ones in front of the butterfly) and see what happens to the idle/if it stops hunting.

[timbo]

ok i get what you're saying, but the cold start idle up is mechanically controlled (water temp) not electrically or ecu controlled. the idle adjustment screw works by allowing more air to pass into the motor without affecting the position of the butterfly valve right? the computer doesn't know when either of these are working or adjusted so surely there is quite a bit of rpm headroom for adujstment before the ecu would think that there is a high rpm problem? a.k. said it it revs to 2.5k?

i've had my motor hunt after some poor work from a mechanic. he failed to tighten one of the TPS screws. the TPS came loose and the rotating back piece in the TPS actually rotated past the two prongs off the throttle. the car just rev'd from 900 to 1.5k~ like someone was stabbing the throttle with their foot.

but yes i agree, look for vacuum leaks, do as nikkojoe says with the throttle body test. also get some contact cleaner or something flammable and spray around the intake manifold to check for leaks, cracks, split hoses etc.

[Nikkojoe]

These valves dont actually work that great anymore (they are 20 or more years old). The valve inside which expands with water temp can get stuck and stay open. You need to pull it appart and clean it out, usually gets built up with crap from the intake and oil breather. There is a tollerance of about 2krpm because when the engine is cold it is normal to idle up to 2000rpm.


Just unplugged the TPS and it stopped hunting, plugged it back in and it started hunting again.
It sits at 3k when TPS is unplugged, do I leave it unplugged, what now?

I agree that if you adjust the tps incorrectly with a correct base idle level, it will cause it to hunt (as you said between 900-1500rpm). Usually though if the base idle is over 2k, the tps setting usually isn't the problem (there is one combination where it is, that is if the throttle is not returning fully and the tps is not set correctly). Although if it didn't hunt with the tps plugged in, who would want it to idle at 2k all the time?

Sure it may not be the cold start valve itself, but it is the one that usually causes the most drama's of high idle (normally the thing to stuff up without any input). The thing is I don't know if this engine is:

-A fresh conversion, someone converted to rwd using remote thermostat and cut the pipes removing the cold start water lines, and has not blocked off the cold start valve (aka auxilary air valve)
-A fresh conversion, someone converted to rwd properly retaining cold start lines, but left all the ports on the valve open to atmo
-An untouched smallport in an ae92/3

I had a problem when I first did the 4age conversion on my old white 86. Was a proper rwd 4age setup, cold start appeared to work fine until a few km after having it running idle started getting higher and higher. Got into the drive way and it began to hunt between 2000-2500rpm. I whipped off my intake pipe and put my finger on the auxilary port in the throttle body (i could hear the large woosh of air being sucked in through it) and it died back down to 1000rpm. Turned out there was a blockage in one of the cold start lines to the valve.