Aircon on = high idle?

[RomanV]

Okay, so ever since I've had my new engine in the MR2, it has always idled high. It definitely doesnt have any leaks in the intake manifold, I've checked this thoroughly, several times!
I've only had a tacho since recently, so I havent known how high exactly it does idle. It idles between 1500 and 1900rpm, or there abouts.

One thought that has crossed my mind as to why, is that the ECU thinks that the aircon is on, and so makes the car idle higher to compensate for the extra load?
I removed my aircon while the engine was out, and since then, both of my radiator fans run 100% of the time. (I've unplugged one though) Which is what happens when the car thinks that the aircon is running. Does the ECU get a signal regarding the status of airconditioning, and adjust idle etc to suit? If so, is there a simple way that I can make it think that the aircon is 'off' to make my idle go back to what it should be? Also, is there an easy way to make my rad fans go back to normal, AKA only coming on when it gets hot?

[big_boy]

it douse sound like your ka thinks the air con is on & yes the ecu gets a signal to say its on theres a pin on the ECU that is conected to the switch witch turns on the air con procoutions like high idle & a fan but you must have also removed the temp sensour for the air con radatot witch is why the fan is running as the sensour is closed normaly but opens when it gets hot turning the fan on

[fivebob]

A/C idle up valve is not controlled by the ECU, the A/C computer controls it. Sounds like you may have a problem with the wirng loom from the cabin not being patched correctly with the ECU loom, or a problem with the A/C loom.

Easiest fix if you have no A/C would be to disconnect and plug the tube going from the A/C idle up valve, which is located on the rear firewall. IIRC it's got an adjustment screw, but I don't know if it would fully close the valve. Also this won't stop the ECU advancing the idle timing, or the deceleration fuel cut parameters. So your idle may still be a bit high, and fuel economy may suffer.

The left fan is controlled by the A/C pressure switch (and possibly the ECU). Disconnecting it is probably the easiest answer.

The right fan is controlled by the radiator temp sensor (lower right rear of the radiator), if the sensor is disconnected then the fan will run all the time, so I'd check that one first.

[RomanV]

How is the idle speed control valve controlled?

Is it via pulse width modulation?

If so, if I unplugged it, would the idle speed valve go back to the 'shut' position? as I'd imagine PWM in this case would require some sort of spring/force that it works against?

I just want to see if the problem persists with the ISCV disconnected, so I can narrow it down to that.. then work from there.

Also, where is the AC computer located? up the front?

[big_boy]

you relise the ISCV & air con idle up are different dont you

air con idle up is ECU controled

ISCV is automatic it has power going to it on ON & water running thro it & as soon as it reaches 55`c or 100 seconds (bi metlic strip) it shuts off

[RomanV]

Okay, so how exactly does aircon idle up work then?

The only way that air could get in, excluding the throttle plate, is via a vaccuum hose that leads to the charcoal cannister, and the air bypass around the throttle plate. Which appears to be controlled via a stepper motor or something, as it's got a plug going to it with 4 pins on it. (And no, I'm not talking about TPS, which is seperate)

Bear in mind that I've got a gen 4 3SGE engine, which may work a little different to earlier 3S's perhaps?

[fivebob]

air con idle up is ECU controled

ISCV is automatic it has power going to it on ON & water running thro it & as soon as it reaches 55`c or 100 seconds (bi metlic strip) it shuts off

Not on the 3S-G(T)E. Please don't confuse the situation by spouting BS when you don't know the answer.

The A/C idle up is controlled by the A/C computer, The ISCV is controlled by the ECU, it does not use a simple wax or bimetal strip control.

I know this becase I spent ages trying to get my MoTeC ECU to control the (electronic) ISCV, only to realise that the A/C was controlling the A/C idle up on it's own without any ECU interface whatsoever.

[big_boy]

just relised its a 3S MR2 not a AW11 4A

so different

[fivebob]

How is the idle speed control valve controlled?

Is it via pulse width modulation?

Not excactly, but you can get it to work by PWM. It's a bit more complicated than that, can't find my notes at the moment, but IIRC it's a matter of balancing voltages to get the valve to open/close.

If so, if I unplugged it, would the idle speed valve go back to the 'shut' position? as I'd imagine PWM in this case would require some sort of spring/force that it works against?

Depends on which position you unplug it in, if it's open then it may stay open. Although I've found it eventually returns to the closed position.

I just want to see if the problem persists with the ISCV disconnected, so I can narrow it down to that.. then work from there.

The ISCV is different to the A/C idle up valve. the ISC is part of the throttle body assembly, the A/C idle up is on the firewall.

Also, where is the AC computer located? up the front?

yes it's part of the A/C controls box.

[fivebob]

http://www.autoshop101.com/forms/h26.pdf has details of the ISC system. Most 3S-G(T)Es uses the rotary solenoid system, but don't seem to have the bimetal backup

The later model 3S-GTEs (ST215) seem to use the Duty controlled ISC system, so it may be that your engine uses the same system.

[RomanV]

If so, if I unplugged it, would the idle speed valve go back to the 'shut' position? as I'd imagine PWM in this case would require some sort of spring/force that it works against?

Depends on which position you unplug it in, if it's open then it may stay open. Although I've found it eventually returns to the closed position.

Okay I just tried unplugging it, to see what would happen. No change, idle stays exactly the same. Although I only unplugged it for a second or two.
There's no way that idle could be adjusted, apart from via this plug...

I just want to see if the problem persists with the ISCV disconnected, so I can narrow it down to that.. then work from there.

The ISCV is different to the A/C idle up valve. the ISC is part of the throttle body assembly, the A/C idle up is on the firewall.

Okay, this is the part that I dont understand. I dont actually have anything on the firewall, which could possibly let air into the plenum?
The only attachments on the throttle body are: ISCV, vaccuum for charcoal cannister, plug for TPS, plug for ISCV. There's no other way that air could get in.... Except for via the vaccuum hoses for brake booster and crank case ventilation....

One of my other thoughts, is that idle speed is partially dependant on a reading from the MAF sensor? If I disconnect the MAF sensor, the engine dies, even when idling. If I've got the sensor plugged in, but not in the airflow, the engine dies.....

[fivebob]

Okay I just tried unplugging it, to see what would happen. No change, idle stays exactly the same. Although I only unplugged it for a second or two.
There's no way that idle could be adjusted, apart from via this plug...

Not that I know of, you need to put the inputs onto an oscilloscope to see what happening.

The ISCV is different to the A/C idle up valve. the ISC is part of the throttle body assembly, the A/C idle up is on the firewall.

Okay, this is the part that I dont understand. I dont actually have anything on the firewall, which could possibly let air into the plenum?
The only attachments on the throttle body are: ISCV, vaccuum for charcoal cannister, plug for TPS, plug for ISCV. There's no other way that air could get in.... Except for via the vaccuum hoses for brake booster and crank case ventilation....

Hmmm, odd. IIRC it goes directly into the plenum, not the T/B, but it's been ages since I looked at it. Could be that on you engine the idle up is controlled by the ECU, but why that would be different from all the other 3S engine I have no idea.

Have you got a vacuum diagram fro your engine?

One of my other thoughts, is that idle speed is partially dependant on a reading from the MAF sensor? If I disconnect the MAF sensor, the engine dies, even when idling. If I've got the sensor plugged in, but not in the airflow, the engine dies.....

Not that I know of. It dies because it reads no airflow. No airflow = no fuel required, no fuel = no go.

[fivebob]

Is the Evap system the one from the Series 1 SW20 or the later model one with a VSV attached?

Just a thought but I'm not sure if it makes any difference on a NA engine though. Try pinching the line to the charcoal canister to see if that makes a difference.

[Mr Revhead]

from what i can see on the EPC.
the ISC is off the plenum/intake hose.

but yeah as above, what system are the you running?


looks to me sw20s all run the same system, just a coupla location differences

[RomanV]

Yep I have the VSV attached, doesnt make any difference either way though.

I've also tried blocking the evap hose, no difference.

I've got a spare throttle body here, I'll take some pics when I get home.

My thoughts about the MAF sensor. I'm running it in a non standard location/sized pipe. If it is getting a different signal to what it is expecting while at idle, then it could be adding more fuel/changing ISCV position, based on that.

I cant rule that out as a possibility, until I get the standard airbox to test/compare with. *cough*

Okay so basically, the only way that extra air could possibly get into my plenum is:

1. brake booster vac hose
2. crank case vent vac hose
3. evap vac hose
4. throttle body itself
5. idle speed control throttle bypass.

There are no other possible ways, apart from a leak.
I suppose I should try blocking them all off one at a time, and seeing if it makes a difference.

[fivebob]

Hmmm, odd. IIRC it goes directly into the plenum, not the T/B, but it's been ages since I looked at it. Could be that on you engine the idle up is controlled by the ECU, but why that would be different from all the other 3S engine I have no idea.

Have you got a vacuum diagram fro your engine?

Ok I just looked at the Caldina vacuum diagram and it has no seperate A/C idle up valve either, so it must use the ISCV (which is the duty cycle controlled version) to control idle when there is A/C demand.

With this in mind, I think you have a leak somewhere else in the system as regardless of A/C demand the ECU should try and keep the idle stable, not just increase it because of A/C.

[Mr Revhead]

working on it!!!



as for intake....
interesting thing i have learned.
the 3sge in the altezza. a few dealers have had altezzas in with funny surges/misses which they had a mass bastard of a time finding.
on a few occasions the casue was aftermarket airfilters.
or specifically the MAF sensor not liking what the filter did to the flow.
was only found when replacing MAF with a new one and filter with genuine
owner then removed genuine filter and symptoms restarted.
unfortunatly thats about the extent of info i have on it.
apprantly measuring the outputs of the MAF was how they cottoned onto it.

[Mr Revhead]

theres a device toyota call "valve, magnet (for idle up device)" listed for all sw20s.... showen with the aircon components
isnt that what your talking about?

should have a p/n on it of 90910-13003

[fivebob]

My thoughts about the MAF sensor. I'm running it in a non standard location/sized pipe. If it is getting a different signal to what it is expecting while at idle, then it could be adding more fuel/changing ISCV position, based on that.

I can't see how. If you the pipe is larger then the ECU thinks there is less air and runs it leaner than it should, result engine slows down or has a poor idle. If it's the other way around then the engine runs richer, which might increase the idle slightly, but not by 1000rpm. Either way the ECU should be able to use the ISCV to get the engine to idle at the target RPM. You mght get a surging RPM but not a constant high RPM.

Okay so basically, the only way that extra air could possibly get into my plenum is:

1. brake booster vac hose
2. crank case vent vac hose
3. evap vac hose
4. throttle body itself
5. idle speed control throttle bypass.

There are no other possible ways, apart from a leak.
I suppose I should try blocking them all off one at a time, and seeing if it makes a difference.

Could be a good start, at least you would be able to eliminate them as a possible cause.

[postfach]

have you had a look to see if the vacuum line going to the power steering is still attached? mine has come off a few times now and it does the same thing. just a thought.