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Ls400 Idle, Tps And Damper


cleverdick
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Hi guys,

I'm hoping you can help me with a minor problem affecting my 1990 LS400.

The idle speed when warm is about 600 r.p.m. This in itself is not an issue, however at that speed there's a very slight 'boom' from the engine, possibly the exhaust. Increasing the revs to 650 - 700 on the gas pedal causes this noise to vanish completely. I'd therefore like to raise the idle speed ever so slightly to take care of this. I've checked the position of the TPS as per the official instructions (with some miniature clip leads), but at either 0.4mm or 0.85mm the continuity is either completely on or off at both gap settings. In other words, if I set the position of the TPS for zero Ohms at 0.4mm, when I insert the gauge of 0.85mm it's still zero. I have to turn the TPS round a bit for it go open circuit. It's then still O/C when I check it again at 0.4 mm! I've reset it as close as I can to its original position so that the car runs smoothly enough, although now, when slowing to a stop, you can feel the trans shift down whereas I don't think you could before. It's pretty subtle though. I'm wondering if, being a RHD model, there is some slight difference in the gap settings when adjusting the TPS position. Being a UK version, the throttle stop screw is on the top as the cables enter from the opposite direction (unlike in the tutorial on lexls.com). However, looking at the throttle stop screw, it doesn’t appear to have a direct effect on the throttle position anyway.

Closer inspection suggests that winding in the throttle damper dashpot screw might do the trick. However, it was found that even in the ‘foot off gas’ position, the damper was nowhere near bottomed out, even with the screw wound right in. I’m therefore thinking of adapting the damper bracket in some way to give a definite ‘backstop’ to this screw. (Will let you know if this works.) In fact, removing the damper completely seemed to have no effect on the driveability of the car whatsoever.

To sum up: (1) Does the position of the TPS affect smoothness of gear changes at slow speed? Have I buggered it up? (2) Has anybody noticed any detrimental effects from removing the throttle damper?

Final question: Is it normal for the car to idle at a higher speed (say 1300 rpm) for a couple of minutes when started warm?

Comments appreciated!

Thanks,

Rich :unsure:

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Hi guys,

I'm hoping you can help me with a minor problem affecting my 1990 LS400.

The idle speed when warm is about 600 r.p.m.  This in itself is not an issue, however at that speed there's a very slight 'boom' from the engine, possibly the exhaust.  Increasing the revs to 650 - 700 on the gas pedal causes this noise to vanish completely.  I'd therefore like to raise the idle speed ever so slightly to take care of this.  I've checked the position of the TPS as per the official instructions (with some miniature clip leads), but at either 0.4mm or 0.85mm the continuity is either completely on or off at both gap settings.  In other words, if I set the position of the TPS for zero Ohms at 0.4mm, when I insert the gauge of 0.85mm it's still zero.  I have to turn the TPS round a bit for it go open circuit.  It's then still O/C when I check it again at 0.4 mm!  I've reset it as close as I can to its original position so that the car runs smoothly enough, although now, when slowing to a stop, you can feel the trans shift down whereas I don't think you could before.  It's pretty subtle though.  I'm wondering if, being a RHD model, there is some slight difference in the gap settings when adjusting the TPS position.  Being a UK version, the throttle stop screw is on the top as the cables enter from the opposite direction (unlike in the tutorial on lexls.com).  However, looking at the throttle stop screw, it doesn’t appear to have a direct effect on the throttle position anyway.

Closer inspection suggests that winding in the throttle damper dashpot screw might do the trick.  However, it was found that even in the ‘foot off gas’ position, the damper was nowhere near bottomed out, even with the screw wound right in.  I’m therefore thinking of adapting the damper bracket in some way to give a definite ‘backstop’ to this screw.  (Will let you know if this works.)  In fact, removing the damper completely seemed to have no effect on the driveability of the car whatsoever.

To sum up:  (1) Does the position of the TPS affect smoothness of gear changes at slow speed?  Have I buggered it up?  (2) Has anybody noticed any detrimental effects from removing the throttle damper?

Final question:  Is it normal for the car to idle at a higher speed (say 1300 rpm) for a couple of minutes when started warm?

Comments appreciated!

Thanks,

Rich  :unsure:

Hi

According to my manual, the reading with no clearance between the lever and the

stop screw is 0.2-1.2Kohms

The dashpot[damper] has a set speed adjustment setting ,it has a cap and internal filter.

Yes engine starts approx 12-1500 revs and drops down rapidly

Hope this is not more confusing than useful! :unsure:

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Thanks ROYT: Looks like my TPS might have had it. It certainly doesn't give a reading of anywhere between 200 and 1200 Ohms - definitely either short or open circuit across the lower two pins, going from one condition to the other by the tiniest rotational movement. So what symptoms would one expect from a worn TPS? Idle speed is steady at 600.

Cheers... :duh:

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Thanks ROYT: Looks like my TPS might have had it.  It certainly doesn't give a reading of anywhere between 200 and 1200 Ohms - definitely either short or open circuit across the lower two pins, going from one condition to the other by the tiniest rotational movement.  So what symptoms would one expect from a worn TPS?  Idle speed is steady at 600.

Cheers... :duh:

Hi!

According to the w/shop manual chart of problem symptons for the

TPS, you would have one or more of the following problems:hi engine idle speed,

hesitation/poor acceleration, back fire,surging,engine stall after accelerator pedal

depressed,poor fuel economy.The diagnostic code would be 41.

As you don't seem to have any of these problems...do you think you should

throw the Avo or whatever tester you are using away??!!!!!

If you have traction control there is a sub-TPS as well.......... fault code 47

cheers ;)

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Hi,

It's true, I don't have any of the problems mentioned, except perhaps that you have to squeeze the accelerator slightly before anything happens. But I wouldn't call its performance hesitant in any way! MPG is around 25 on the motorway, traction control is not fitted. I don't have a code reader, hence the need to resort to my faithful old DMM. But all I ever wanted to do was raise the idle speed slightly, which I was LED to believe you could do by rotating the TPS. It would not be surprising if the the TPS had a dead spot - let's face it, potentiometers do wear, even those with conductive plastic tracks (which I'm sure these have). I'll get a diagnostic check done next time I visit my independent Lexus garage.

R.

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Unless the MKI is different the idle speed is controlled by the ECU and is not adjustable. If if is not correct then there is a problem with the idle up system etc.

Unless the TPS is not set correctly you shouldn't try and adjust it to compensate for a problem elsewhere.

It is quite possible for the TPS to have a dead spot given it's age. Could well be just dirt that a spray of electrical contact cleaner would solve if you can open it up.

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Unless the MKI is different the idle speed is controlled by the ECU and is not adjustable. If if is not correct then there is a problem with the idle up system etc.

Unless the TPS is not set correctly you shouldn't try and adjust it to compensate for a problem elsewhere.

It is quite possible for the TPS to have a dead spot given it's age. Could well be just dirt that a spray of electrical contact cleaner would solve if you can open it up.

Agreed above, just to complicate matters a bit more, there is an 'ISC' idle speed control valve,looks like it's at the front of the engine,[have'nt looked, just looking at a picture in the manual] looks like a little motor, has two water pipes connected to it.

Apparently after engine warm up it should make a clicking sound

There are checks given to test if it moves in both directions.

Think maybe we are going in too deep.

When the aircon is switched on does the engine 'idle up' ie increase revs a bit?

:unsure:

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  • 2 weeks later...

As far as I know the ISC valve is functioning correctly, with the idle speed falling from about 1300rpm after a cold start to just over 600 when warmed up. Switching on the [working] a/c doesn't really affect the idle speed.

However, this whole thing's taken a back seat recently because I've just had to shell out £500 for a new front upper wishbone (to stop a knocking sound) The bushes can't be got separately, so I've been told. So I'm not inclined to spend any more money on something which (at the moment) is relatively trivial. But thanks for all your advice anyhow.

:driving:

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As far as I know the ISC valve is functioning correctly, with the idle speed falling from about 1300rpm after a cold start to just over 600 when warmed up.  Switching on the [working] a/c doesn't really affect the idle speed.

However, this whole thing's taken a back seat recently because I've just had to shell out £500 for a new front upper wishbone (to stop a knocking sound)  The bushes can't be got separately, so I've been told.  So I'm not inclined to spend any more money on something which (at the moment) is relatively trivial.  But thanks for all your advice anyhow.

:driving:

Check with other LOC members, I thought you could get the bushes you want.

Maybe from the USA, not sure , worth a seperate post

cheers :unsure:

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