Help With Mk Iv Supra Non-turbo High Idle (possible Iac Failure?)

September 10, 2006

Hi,

I need some help as this has been driving me nuts...

I Bought a 1994 Supra SZ about a week ago, got it for a fair price considering some of the issues the car has, mostly being cosmetic, such as cracked veilside skirts and a a ridiculously loud back box.

the car is a project car for me, and i plan to have it as tidy as possible, and to learn something about the cars inner workings...

the only modification to the engine appears to be a mesh airfilter.

With that background info laid out I have one underlying problem with the car in it has a high idle.

- Note: ECU was reset prior to all of this -

When the ignition is started, the engine sometime runs up to 1200rpm, other times 900rpm.

As the engine warms up the revs drop very slightly to around 1000rpm.

If the throttle is blipped to say, any value above 2000rpm, then released, the engine holds its rev's at 1500rpm.

If the Idle air control valve is tapped with the back of a screwdriver, the revs drop down to the correct values of around 700 rpm.

Following the diagnostics from the following page of the BGB found on this website:

http://www.max-boost.co.uk/max-boost/resou...ntrol_valve.pdf

I understand this is for a TT engine, but i found most of the directions applicable to the IAC unit.

-I can not identify a clicking sound from the unit when the engine is switched off.

-I have found the resistance on the terminals to be within tolerance

-Removal and IAC valve operation using a 12v Battery, showed some kind of operation but i am not sure of how much movement is supposed to be coming from the unit. At the moment there is only 1mm of movement. During removal the black plastic (?) center part was cleaned of carbon deposits, along with the groove it resides within the throttle body.

As a side test i located the ecu and verified the terminals on the socket connector to the IAC valve and verified each wires resistance to check for breaks, none were evident.

Voltage checks were carried out on terminals B1 and B2 on the wiring loom, with the ignition on, 12 v stable was noted.

If the car was started with the IAC unplugged the revs would blip between 900-1500 rpm repeatedly, i note this in case this provides any clues i am unawre of... incidentally this was done accidentally after a long day...

I had not noticed this during purchase, and only when i started to give the car the once over, reviewing:

http://www.mkiv.com/manual/index.html

Toyota BGB maintenance sections did i note the idle problem.

Its annoying me as i cannot with complete certanty identify a definate fault with the IAC, and the cost of a replacement is horrendous. Toyota UK seem to want ~ £300 for a replacement, which for what i can gather is a 4 stage solenoid unit (forgive me if im wrong im only going with what i see)

What i ask is if any Supra boffins out there can throw me some ideas for other tests to carry out on the engine, or have i checked everything? I just wish i could be 100% certain before i Shell out god knows how much for a replacement part.

Its frustrating as the car handles great, drives great, the engine runs well whilst driving, and the paint job is in good condition. Short of an oil and coolant change the car is good to go :)

Any help would be greatly appeciated, and i thank anyone who contributes for there time.

September 11, 2006

Hi,
I need some help as this has been driving me nuts...
I Bought a 1994 Supra SZ about a week ago, got it for a fair price considering some of the issues the car has, mostly being cosmetic, such as cracked veilside skirts and a a ridiculously loud back box.
the car is a project car for me, and i plan to have it as tidy as possible, and to learn something about the cars inner workings...
the only modification to the engine appears to be a mesh airfilter.
With that background info laid out I have one underlying problem with the car in it has a high idle.
- Note: ECU was reset prior to all of this -
When the ignition is started, the engine sometime runs up to 1200rpm, other times 900rpm.
As the engine warms up the revs drop very slightly to around 1000rpm.
If the throttle is blipped to say, any value above 2000rpm, then released, the engine holds its rev's at 1500rpm.
If the Idle air control valve is tapped with the back of a screwdriver, the revs drop down to the correct values of around 700 rpm.
Following the diagnostics from the following page of the BGB found on this website:
http://www.max-boost.co.uk/max-boost/resou...ntrol_valve.pdf
I understand this is for a TT engine, but i found most of the directions applicable to the IAC unit.
-I can not identify a clicking sound from the unit when the engine is switched off.
-I have found the resistance on the terminals to be within tolerance
-Removal and IAC valve operation using a 12v battery, showed some kind of operation but i am not sure of how much movement is supposed to be coming from the unit. At the moment there is only 1mm of movement. During removal the black plastic (?) center part was cleaned of carbon deposits, along with the groove it resides within the throttle body.
As a side test i located the ecu and verified the terminals on the socket connector to the IAC valve and verified each wires resistance to check for breaks, none were evident.
Voltage checks were carried out on terminals B1 and B2 on the wiring loom, with the ignition on, 12 v stable was noted.
If the car was started with the IAC unplugged the revs would blip between 900-1500 rpm repeatedly, i note this in case this provides any clues i am unawre of... incidentally this was done accidentally after a long day...
I had not noticed this during purchase, and only when i started to give the car the once over, reviewing:
http://www.mkiv.com/manual/index.html
Toyota BGB maintenance sections did i note the idle problem.
Its annoying me as i cannot with complete certanty identify a definate fault with the IAC, and the cost of a replacement is horrendous. Toyota UK seem to want ~ £300 for a replacement, which for what i can gather is a 4 stage solenoid unit (forgive me if im wrong im only going with what i see)
What i ask is if any Supra boffins out there can throw me some ideas for other tests to carry out on the engine, or have i checked everything? I just wish i could be 100% certain before i shell out god knows how much for a replacement part.
Its frustrating as the car handles great, drives great, the engine runs well whilst driving, and the paint job is in good condition. Short of an oil and coolant change the car is good to go :)
Any help would be greatly appeciated, and i thank anyone who contributes for there time.

ive been a toyota tech for about 6 years now and what you say in youre thread points to a dodgy idle control valve(the one youve been playing with)

steep it in some petrol over nite then try refitting it (not the best but it was the only way to clear it out properly.

it is also worth giving the throttle body a good clean out too (wd40 and emmry cloth) remember clear it out when its on full throttle (the butterfly gets stuck on the deposit and holds the revs up and when youre giving it a knock its returning

i recon get both them done and she should be ok.

if any more trouble get a link wire and bridge out te1 ane t1 on the diagnostic socket under the hood this will give you any trouble codes if there are any

hope this helps!

alex

September 11, 2006

te1 ane t1 ?? weird.

most toyotas are TE1 AND E1 ? forgive me if i'm wrong.

September 11, 2006

I agree...seeing as the revs drop to normal with a tap id say that the valve is dirty. Give it a go with the petrol trick and that should sort it out :thumbsup:

September 11, 2006

te1 ane t1 ?? weird.
most toyotas are TE1 AND E1 ? forgive me if i'm wrong.

whoops well spoted

thats what i ment

i should really know better but we have a diag tester now so the link wire got binned.

alex

September 11, 2006

Hi again,

Thanks for all the info.

The IAC valve has had a good clean out already, just started working on the car an hour ago.

The comment regarding the butterfly in the throttle body being dirty has rung true...

As a test i tried tapping the top of the throttle body (as a side note it takes 3-5 taps to return to proper idle), and got the same result, this got me thinking about the butterfly. I tried pulling the silver cylyndrical part of the throttle up the way, as in closed (sorry for my description) The part i beleive to be directly attached to the butterfly, and low and behold the revs dropped.

Me dad, whos an old school mechanic, who thinks modern engines are the work of the devil is currently giving me a hand to clean out the throttle body which is caked in a greyish substance, which turns to black surrounding the butterfly.

yeah and the comment about TE1 and E1 im no stranger to that from my Gt Four... uhh error codes about intercooler levels *shudder* :)

Hopefully that will have it cracked, will post back when i know more....

September 12, 2006

Hi again another update:

Stripped the entire throttle body out and cleaned it out with Injector/Carb cleaner, which to me seemed like WD40 under high pressure but never mind...

Having tried a clean out i thought i might be worth removing the IAC, plugging the center hole the IAC plunger goes into and starting the engine and seeing what it idled at. With the hole plugged with the end of a rubber handled screwdriver the revs were as previous at start, on increasing the revs the same problem was evident. Using this method, however dangerous it may seem allowed for visual verification of the IAC operating, including full range of movement and return to open on engine shutdown.

So the same problem was still evident, then we noticed somthing on the back of the throttle body.

According to the TPC its a "Dash Pot Sub-assembly (for throttle body)"

If this unit was pushed the revs returned to the correct value. With this is mind while the engine was being reved i would push gently on the part that hold the throttle. Low and behold the revs dropped. Tofurhter test, using a screwdriver i held the Dash pot "closed" and the engine was revved. The engined reved correctly and returned to correct idle.

So by the looks of it the diaphram in the dash pot is shot or sticking, so im looking for a replacement in the morning. But before that im gonna triple check it is indded this unit... was late tonight we tested it...

Can someone possibly confirm exactly what this unit does? As from inspection its not attached to the engine by any vacum pipes or anything, as in the diagram of it. It appears to be just a spring loaded diaphram that softens therev drop if you let of the throttle. I read somewere this has somthing to do with improving emissions but that was on a ford.