How is one SUPPOSED to adjust idle speed?

[Supraman]

Hey all. My car is sporting a high idle (about 950RPM when warm), and I'm confused about how I'm supposed to adjust that. I figured the TPS wouldn't help, because it's sending IDL to the ECU, and as far as I know, the ISC is on its lowest setting when the engine is not under any auxiliary load (power steering, air conditioning, or auto transmission).

Anyway, on my '88T throttle body, there is a screw that seems to adjust air bypass and very effectively control the idle speed. However, I was told that this screw is factory-calibrated and controls vacuum for the BVSV and something else. Thus, it was actually eliminated on the later model design. I don't know what else to adjust!

Anyone?

 

[smagnotta]

I have an 87 and I used a flat head screwdriver to adjust my idle using that idle screw on the throttle body. It may not be the thing to do but it worked for me with no ill effects that I've noticed.

The idle is affected by many things, but check that your TPS and ISC (Idle Speed Control) are functioning correctly.

 

[SupraMario]

thats around normal idel speed. 750-950, and 650-900 for Ts(i think.)

 

[billspreston01]

Turbo idle speed is supposed to be 650, and n/a is 700 or 750

 

[smagnotta]

My 1987 manual says 650 RPMs for Turbo and 750 for N/A.

I was wondering why there is a difference and also what effects there would be if you have a Turbo at 750 RPMs at idle?

 

[SupraMario]

I dont notice a difference, my NA idels fine at 900. never had a problem.

 

[IHI-RHC7]

The problem is that at 900rpm your motor actually has oil pressure, and it won't self destruct like toyota wants it to. at 650rpm, you're lucky to maintain 9 psi... My car idles perfectly at 650rpm with 9 psi oil pressure...

 

[Supraman]

Strictly speaking, Toyota says 4.3PSI of oil pressure is nominal at idle. For those interested, I have an idea about why the N/A idle is higher than the Turbo...as far as I know, one of the main functions of a preset idle speed is to ensure that the crankshaft splashes enough oil around to lubricate the pistons. Just a bit too slow...and that doesn't happen. However, the 7MGTE has oil squirters trained on the back of the pistons, so its idle can theoretically be lower.

Anyway, I guess I'll just use the screw. But, how would you adjust the idle on an 89+? I suppose you don't!

 

[SupraMario]

The problem is that at 900rpm your motor actually has oil pressure, and it won't self destruct like toyota wants it to. at 650rpm, you're lucky to maintain 9 psi... My car idles perfectly at 650rpm with 9 psi oil pressure...

lol at start up mine hits 40 psi, and 1500rpm, then 1 min it evens to 19-20 psi and 850-900 RPM. engine runs strong as hell.:biglaugh: , im also using 5 Quarts of synthetic, 10w 30. Castrol.

 

[Shawndude]

Guys, the idle speed on North American models is NOT adjustable. The computer takes care of it.

If you're not idling around 650 RPM (turbo car, no lights or A/C on) you have other problems. Air leaks, ISC valve, throttle position sensor, coolant sensor, malfunctioning A/C signal or such.

Fix that shit and stop trying to repair the problem by monkeying with other stuff.

By the way, the oil squirters have a pressure valve built in, they will not be squirting at idle.

And just for the record, PSI does NOT equal flow. I can block your oil passage and you'll get full PSI on your gauge and your engine will blow up from lack of oil.

 

[CPT Furious]

^^^^ Nuff said...::wink::

 

[Supraman]

Interesting. I can't help but wonder how the heck the ECU manages to control the idle speed; it's almost certainly not the ISC. I'm nearly positive the ISC has four modes of operation, each of which are discrete levels of the bypass valve opening.

 

[Shawndude]

Supraman, here is a little secret between you and me, so check this out:
http://www.autoshop101.com/forms/h26.pdf

See also the other documents on that site.
http://www.autoshop101.com/

Most people refuse to read, so by spending an afternoon comprehending this stuff, you'll know 90% more than some of these numb-nuts around here.

 

[smagnotta]

Shawdude is very knowledgable. He was absent from the list for a while. I would take his word over almost anyone's.

I had a question as to why the 86.5 to 88 has the screw you can turn to increase or decrease the idle speed and the 89+ does not. I have an 87.

I was idling around 780 RPMs at operating temp so I adjusted it and am now idling at 650. You are saying I have some other problem --- which I think for me is a vacuum leak somewhere.

My vacuum at operating temp is around 17.5 PSI. Isn't that pretty low as it should be more like 20 PSI ?

I looked and double-checked my vacuum lines as per the diagram on the hood and also the TSRM. I could not find or hear any leaks.

I thought perhaps it was the timing so I reset the timing as per the TSRM but I am still at 17.5 PSI. This did stop the miss I had at idle, however.

 

[Shawndude]

I had a question as to why the 86.5 to 88 has the screw you can turn to increase or decrease the idle speed and the 89+ does not. I have an 87.

Just because one calls it an idle adjustment screw, doesn't mean that's what the darn thing is.

Are you guys talking about the dashpot system, which is not meant for adjusting your idle speed with?

http://www.cygnusx1.net/supra/library/TSRM/ec/EC_10.html

 

[Supraman]

Nope. The screw I'm referring to is pointed lengthwise along the car, and is right above the thottle body near the throttle plate. It is covered by a little rubber stopper. The screw itself is surrounded in the metal, exactly like the Lexus AFM air bypass screw, but smaller.

 

[Shawndude]

Is it a JDM throttle body? I have both a 87 and 88 N/A (ehm, North American turbo I meant) engines and none have such an animal, unless you drill out the brass plug or something. The overseas models do have it though.

 

[Jeff Lange]

Pretty sure mine has that adjustment screw too, with a rubber cover, I've never touched it though.

Auto and Manual cars have different TB P/N's listed in the EPC, maybe Autos didn't have it and that's what yours are Shawn?

I dunno.

 

[smagnotta]

I have an 87 5-speed Turbo and it is just like the Lexus AFM screw. Turn it clockwise/counter-clockwise with a flathead screwdriver and your RPMs increase/decrease. I messed with it and even looked at my S-AFC digital RPM display and it went from 845 RPMs to 650 RPMs at idle. It is not the dashpot. :icon_conf

So I have no idea why Toyota has it there on my car and blocked it off for later models.

 

[Shawndude]

I had a look yesterday, and the 87 engine does have it. My mistake on that. The one on my 88 doesn't though, but I can't remember where the hell the TB actually came from anymore.

At any rate, the ECU will adjust the idle speed, as long as it gets the proper IDL signal from the TPS sensor, and the rest of the stuff works properly, and the ISC doesn't run out of it's adjustment range.

One can fool it to make the idle higher of course, just like you stepping on the gas, or jerry rigging something to keep the throttle open more, but that's not the way it should be adjusted.