Ls400 Idle Speed

[Lexcob]

Hello

I'm new to this forum but I was looking for a little advice and guidance on the normal idle speed for the LS400 engine. My V8 is no longer in a Lexus but transplanted into something else. The engine idles around 1200 -1400 rpm even when the engine has warmed through, is this typical? It is steady at this rpm so it is driveable, it just seems a bit on the high side to me. But since I have no experience of the LS400 power plants I am not sure.

Or is this one of the symptons of the Idle control valve on the front of the engine perhaps needing a clean? If so can anyone point me at a 'how to ' guide. Some the guides and videos on the web seem somewhat inconcise to me. Am I right in saying that you can't get anything like a Haynes manual for the LS400 as they never printed them?

Thanks in advance for any help.

David

[Malc]

................ mine starts at about 1400 revs from cold, quickly reduces to 1000 and idles after a short while at about 500 ish ... the rest I don't know

Malc

[Lexcob]

OK thanks Malc, lets see how that matches up with others but it does sound more what I would expect.

[steve2006]

Fully warmed up the book idle speed is 650 RPM +/- 50.

[Lexcob]

Ok thank you for that.

[ColinBarber]

Either the control valve needs a clean or it just needs to settle down. Older Toyotas take a long time to reduce their idle speed to the correct speed after an engine ECU reset. I find the best approach is to drive the vehicle and at every junction stop completely, put it in Neutral and wait at least a few seconds before moving off again. Every time you come to a rest and the transmission is out of drive it will adjust the idle speed, lowering it if it is too high.

[Lexcob]

Thanks for that, that is interesting what constitutes a reset of the ECU? For instance if the Battery to totally flat for a long period of time or has been briefly disconnected can this cause a reset of the ECU? This did occur recently and since then only around 10 miles has been covered? Could the ECU still be 'learning' again?

Just as an additional point the engine is no longer in a Lexus and it is not mated to its original auto box, but on a conversion to a Supra 5 speed manual box, but I guess the ECU will still make the adjustments to the idle speed?

Thanks for your responses.

[Mybikes a cx500]

There's an engine ECU temp sensor, could be failed, they do age. If the ECU doesn't see the engine warm up it will keep the idle high, and run over-rich

[ambermarine]

The point Ian made regarding the coolant temp sensor is relevant .It as a direct bearing on the idle speed and operates through the ECU . The sensor is located to the left of the water pump (looking from the front of the engine) and is usually coloured green.

Haynes have never attempted to cover the Lexus LS400 the workshop manual is as thick as a telephone directory.The Lexus publication is available on eBay and comes in two volumes .you just need the engine and transmission section .

The sensors are about £12 aftermarket. All the other points have been answered

[Lexcob]

Thanks guys I was wondering where the ECU got its signal from to adjust the idle valve in the first place so that answers that. I'll start there to make sure that is sending a signal first. Then I'll move on to the other points.

Thanks I'll report back.

[malcolmdc]

Interesting to note the design idle speed of 650 +/- 50, cos my LS400 has 250rpm (indicated) in gear, about 450 in N, smooth as a nut, but seems to be slow by general standards! Only done 30k so perhaps all the cylinders are closer together in compression!

[alien1974]

mine random in gear, sometimes 400 sometimes 500 and sometimes around 650 just depends what mood its in

[runsgrateasanut]

Mine idled on the high side until I decoked the Throttle body. That may not have been the cause and it did idle rather high when I put back the throttle body with the Sensor possibly in a different position owing to my marking it poorly..

There's a good vu tube video on checking and adjusting Throttle sensors, albeit not on a Lexus. Confess that after the engine seemed to idle rather high I changed the position ever so slightly and now idles when warm at around 600rpm. Start from cold about 1100. If I have the Climate control selected this does increase the idle revs somewhat, so it gets turned off when likely to be stationary or when freewheeling downhill into Town to keep to 30mph.

[steve2006]

Interesting to note the design idle speed of 650 +/- 50, cos my LS400 has 250rpm (indicated) in gear, about 450 in N, smooth as a nut, but seems to be slow by general standards! Only done 30k so perhaps all the cylinders are closer together in compression!

A 1991 LS400 with only 30,000 miles on it ?

Would be nice to see some pictures and a little more history of your timewarp LS.

Favourite is the TB clean, there is a little hole in the butterfly and if this gets clogged it can cause low idle despite the IACV rerouting the engine air supply at idle.

[malcolmdc]

Thanks for that little tip - will check TB if it ever stops raining outside, but not an urgent problem! Mine is the Mk 1 from Autotrader back in January if you recall my introductory post - Came from a dealer in north London who thought it had spent time as an embassy car, hardly used/laid up but fully doc service/mot/tax from new, all in posh Park Lane garages in London!. Had only 22k on it then, but it was bought to be used, and has now just passed 30k, total ave mpg 22.63, all motoring use (can manage 30 on a rare long run). Having new rear discs and pads today, but otherwise one ball joint only needed for mot, and have got some 16 inch nicer wheels to go on in the spring, when piccies will be posted! I'm looking forward to a lifetime's motoring with the old girl - sure she'll outlast me. As far as Classic car shows are concerned, at 24 years its a classic car, and has twice been entered as such (at much reduced admission charges) sitting alone as an exhibit - where do all the Lexus ls400/Soarer owners hide themselves away? Would be nice if a Surrey/Sussex meet was arranged, or even meet at a car show (Bromley, Cranleigh, or wherever)