Unusually High Cold Idle

CptYossarian · July 22, 2015

Good evening,

I'm dabbling in Lexus ownership mainly to get tick "Own a V8" off my things-to-do-before-I'm-30 bucket list, and at £900 I wasn't going to argue.

It was a risky purchase as there's no stamped history between 2005 and 2015, although the MOT history confirms that it only did 37,000 miles in that time, with only 1,500 miles or so between some MOTs.

There's no cambelt history either - the only recorded change being done in the year 2000 at 65,000 miles or so, so I'm being proper gambly here, and as I don't want to immediately fork out £hundreds on a £900 car, I'm happy to run the gauntlet of having to break the car for parts if it lunches itself.

Anyway, my problem stems from a very high cold idle. It's 1,800rpm for the first couple of minutes, then it gradually sinks until the car is warm (second or third line on the temp gauge?), and then it does the normal Lexus purr of 500-600rpm.

I bought it off a dealer who'd been using it for a month or so. They'd naively fitted a SUPER-DUPER eBay CHIP, which I've removed (it was doing somewhere between naff and all to the engine), and the gan'sta tints that were fitted to the fronts (illegal) and the rears (not illegal but stupid).

I first thought it was the eBay chip, so I was disappointed when removing that didn't fix it. However, further research suggest resetting the ECU, which I've done by removing the fuse under the bonnet for a few hours.

After putting the fuse back in it ticked over at 1,100rpm when cold - much more normal I thought. I gave it a good 10 mile drive and a bit of a booting to try to make the ECU realise it was going to get more usage now, and for a couple of days it was fine, always starting at 1,100rpm.

But it now idles at 1,800rpm again, and as I know have come to realise that it might be an "adaptive" ECU, it might still be thinking that it needs to warm up quickly which is why it's reverting back to this high cold idle speed.

A slightly more logical explanation is that the throttle body is utterly coked up because it was being driven short distances for nearly 10 years...this is causing it to idle high because it can barely breath when cold.

Not being mechanically minded, am I in the right ball-park? Would a friendly garage be able to cope with replacing a throttle body on a car like the Lexus? It doesn't look horribly electrical and complicated under the bonnet. It's massive but simple in a way.

Anyone else got ideas? I know this kind of problem is a pain to diagnose!

Other than that it drives like a dream. Just over 140k now, silky smooth engine, quad-tipped stainless steel exhausts for a throaty grumble at low speed, gearbox is perfect and kick-down is effective if not a little uncivilised for a big ol' car like the LS400.

Thanks for any help you can provide! :)

ColinBarber · July 22, 2015

I'd start with a throttle body clean, including the Idle Air Control Valve. The temperature sensors (coolant and air) should also be checked as these can fail and make the idle run high because the ECU thinks it is very cold.

Malc · July 23, 2015

............. seems as if you have a good buy there, the simple fixes above might well be all you need then if it still does 1800 for a very short while then drops to 1100 then idles at 500 or so then there's nowt to worry about really.

The cambelt will probably be ok until you can get around to changing it, be a pity to leave it too long even though it will likely last till 165k at least .......... fingers xd, and don't blame me if it just goes pop ........ I don't think they ever do

Malc

CptYossarian · July 23, 2015

Colin - is this something a local mechanic would be able to do cheaply? I have a very good local mechanic who avoids all complicated cars with electronic gubbins under the bonnet - I am hoping a 20 year old Lexus, while huge, is more basic and easier to work on than your average 10 year old saloon of a similar size.

Malcolm - I think it's a good buy, for £900 or so it's almost "disposable" as long as I can get 6 months or more good motoring out of it. It's worth £900 even without a cambelt change to someone who's an enthusiast, but I'm not thinking about selling it at all. I love it!

I know that Lexus cambelts are buried deep and hidden from dirt and muck very well...I'm not so worried about the mileage it's done since the last change in the year 2000 as it's just barely reached the next interval, and a lot of that mileage was done obviously at a low speed and over short distances...it's just the age of the belt itself. 15 years!!! *gulp*

Malc · July 23, 2015

hahahahaha ............... the original belts in the early Mks were made of, effectively, kevlar ....... and this is just about indestructible, they make bomb proof bins etc from it !

I have tried to get from Lexus the make-up of more recent cambelt materials but haven't been able to elucidate a sensible reply.

If your last cambelt was OEM then you're probably ok for a while yet.

Regarding servicing and all things Lexus, my local indy guy has been looking after my cars very effectively and economically for approaching 8 years ( Mk1, Mk2 and now my Mk3 cars ) and he does what he does very well indeed.

What did let me down quite catastrophically was the front cooler pipes for the pas fluid, these rot out about now but mine also rotted out the steel complex pipework up into the pas pump area too ....... Lexus bits would have been prohibitevily expensive, repair effected by copper tubing ( no pressure ) and flexi hoses and clips .......... . but get it done, it's a cheap fix and will save a huge amount of grief. This is an achillies heel of the Mk3

Malc