AE101 low idle, HELP!

[snetsov]

I have a 1995 Toyota Sprinter Trueno AE101. I bought it recently and it came with this "issue" so i am wondering what may cause it. Any ideas will be greatly appreciated.

122 000 km on the clock (not sure if genuine)
4A-FE, injection
Automatic Gearbox
A/C

Just serviced, oil+filter, air filter, fuel filter, spark plugs.

Problem was there before the service, now still there...ive read somewhere that old fuel filter may cause this but not in my case.

So, what is the deal.. the car always starts from first turn of the key, cold idle speed is good, the car drives fine.
While the engine gets hotter, the idle rpm are going down to normal 800.
The problem comes when the engine get to normal working temp. (when the fan turns on every now and then), when i stop still the idles are going down to 500, this is both on neutral and direct. When i turn the A/C ON the rpm are going back up to 650-700(A/C sensor seems to work but still low rpm).
As far as i know, at normal temperature with A/C on and Direct gear the idle must be 800rpm.
All this rpm issue is shaking all the plastics and steering wheel and stuff, so its very annoying.
Other than that the car drives fine, it has plenty of power etc...
Anyone with similar problem? What about the idle control valve? What else could be wrong?

[ganda1f]

Idle air control (IAC) valve gives trouble mostly while the engine is cold, since this is when all the oily residue from Positive Crankcase Ventilation thickens and prevents the valve from opening properly. Cleaning the IAC and the throttle body with a toothbrush and some carburetor or MAF sensor cleaner is not a bad idea though.
Injectors seem to be good, since they clearly are able to deliver the extra fuel while engine is cold and/or running under load.

While at idle, can you tell whether the mixture is overly rich or lean?
I suppose you could try to remove the electromagnet that controls the IAC and see how things behave when you open the IAC valve by hand. Remove the 2x2 inch green-ish rectangular platic box that is held by 2 bolts. Just under the throttle body. You will see a small metal knob kind of thing. It should rotate freely about 1/2 to 3/4 of a revolution, closing and opening the idle air passage in the throttle body. It must not stick in any position.

A faulty coolant temp sensor (for the ECU, not the temp gauge) could throw the air-fuel ratio off, but since the car drives fine and has good power, the temp sensor is unlikely to be a problem I suppose.

[snetsov]

Obviously the IAC will be the first thing to check in the weekend because my research is also leading to it. Unfortunately all the info on the internet is for low when cold idle, or sticking high idles. I couldn't find anyone with similar to mine problem. That makes me think that the Temperature sensor as you said might be messing with mixture, making too rich while the engine is hot enough to run on much less fuel.

[ganda1f]

You could also check the 'learned value' correction factor. Yours being a '95 it should have OBD1 ECU, which means you can check the voltage across VF1 and E1 terminals in the diagnostic connector in the engine bay. Nominally it should read 2.5V. Just start the engine, let it warm up and measure. The voltage can change in 1.25V steps between 0 and 5V.
Possibly the ECU is correcting for some false long-term *lean* condition and erroneously adds too much fuel.
A higher voltage means that the ECU is increasing fuel to correct for some long-term lean condition.

Another thing that crossed my mind, but that is a long shot and could be tricky to diagnose, is that the MAP sensor gives false information during high vacuum of idle. With cold engine the IAC will let more air in and it might not matter that there is a bit too much fuel. But, once the engine gets warm and IAC reduces air, the excess fuel starts causing problems at idle.
Not sure though, exactly how much these things can throw the ratio, or how much of an error the engine can take and still keep running. It is just a theory, I have not actually seen this particular scenario happen.

[godanigo]

Did you ever figure this out? I'm having a similar problem.

Idle is really low when the engine is warm, but fine at all other times. I noticed that the idle goes up slightly when I pump the brake, but will then fall again.

[Hiro Protagonist]

Originally Posted by godanigo

I noticed that the idle goes up slightly when I pump the brake, but will then fall again.

That is normal, when you pump the brake you are using up the vacuum in the booster which will cause the engine idle to increase to replenish it