1990 Ls - New Purchase

BristoLS · October 3, 2005

Hi, new member having just bought my first Lexus. It's a 1990 LS400, 117K, full service history and in imaculate condition. I am very pleased with my purchase, but have noticed what I hope is going to be a fairly minor problem.

When started from cold, the engine idles at almost 2000prm. Obviously it shouldn't be that high! Once the engine is up to temperature is idles perfectly and the engine is absolutely smooth and quite.

Any ideas? Idle valve? Is it something that I can tinker with myself or shall I just take it straight to my mechanic? Thanks!!

chips229 · October 3, 2005

Hi and Welcome to the club :)

Have a look through the LS 400 section of this site should be some answers there for you,never had that idle problem with my car but im sure ive read posts with them sort of problems.

Cheers chips.

cleverdick · October 3, 2005

Hi!

I also have a 1990 LS - and you're already one step ahead of me because my car is not in immaculate condition! ;)

You may need to clean the throttle body, as shown on www.lexls.com. I'm not saying that it will definitely solve your problem, but it's likely that it won't ever have been done - at least off the car and properly. The job isn't as bad as it looks, and you don't have to drain the coolant as the TB is the highest point in the cooling circuit. The only trouble I had was in removing a hose - I cut through it in the end and replaced it with new during reassembly. You can make a TB gasket from gasket paper - about £1 a sheet from your local motor factors. There are three very small vacuum holes in the body which could easily get blocked I suspect, and you'll be surprised at just how carbonised it all is on the inside. Allow about half a day, and if possible do when the shops are open in case you need to go get some hose or something.

Good luck...

Leadfoot · October 5, 2005

Welcome to the club, please don't get intimidated by the amount I have replied with, it will most likely be easy to cure. I do tend to get a bit carried away.......

The throttle body getting choked up with carbon deposits is a common occurence on the high mileage cars, but normally it affects the hot idle quality, not the cold.

I will check what mine revs at in the morning but is the 2000rpm in neutral or is it idling that high in gear?

I had a look in my lexus manuals and the troubleshooting matrix listed the causes for an "incorrect first idle" as, in priority of likelyhood

You might want to print this...........if its out of your league at least it gives you plenty of ammo for dealing with a mechanic.

1) Accelerator pedal link - don't know about this as there was no section referred to so just check the throttle linkages for ease of movement.

2) Dash pot - the dash pot is a device that is connected underneath the throttle position sensor and the book says to

INSPECT DASH POT(DP) SET SPEED

(a) Warm up the engine.

( b ) Remove the cap and filter (and a little plastic separator for the filter) from the DP.

( c ) Race the engine at 2,500 rpm for a few seconds.

(d) Plug the VTV hole (its the only hole in the dashpot body and a finger will do so an assistant will be needed for the rpm readings and throttle control) .

(e) Release the throttle valve.

(f) Check the DP set speed should be 1,500 rpm.

If not at specified speed, adjust with the DP adjusting screw ( at the back of the dashpot body) The manual also says DP speed only should only need adjustment when a new DP is installed.

(g) Reinstall the DP filter and cap.

3) Power Steering Idle up valve, if this malfunctions it can leak in extra air causing a faster idle. I don't have those pages though, sorry.

4) Water temperature sensor circuit, big job, manual says

(a) DRAIN COOLANT

( b ) REMOVE INTAKE AIR CONNECTOR PIPE

( c ) REMOVE UPPER HIGH–TENSION CORD COVER, TWO

ENGINE WIRE COVERS, TWO NO.3 TIMING BELT

COVERS AND LOWER HIGH–TENSION CORD COVER

(d) DISCONNECT RADIATOR INLET AND OUTLET HOSES

(e) REMOVE RH IGNITION COIL

(f) REMOVE WATER INLET HOUSING

(i) Remove the connector from the RH distributor.

(ii) Remove the water inlet hose from the water inlet

housing.

(iii) Remove the two bolts and remove the water inlet

housing from the front water by–pass joint.

(iv) Disconnect the water hose.

(g) REMOVE WATER TEMP. SENSOR

(i) Remove the connector from the water temp. sensor.

(ii) Remove the water temp. sensor and gasket. 5) Intake air temp sensor

At 20 c should read between 2 and 5K ohms

5) Air intake temp circuit - don't know about this one.

6) ISC Valve - (idle speed control???) this is the valve that you were asking about but its actually a little DC stepping motor controlling a valve with 128 positions. It could be this but the detail is too much to write out and its more likely to be one of the above.

7) Engine + ECT ECU - doubtful given it runs fine.

You should get a lot of enjoyment out Lexus ownership, just be patient when things go wrong as it can take some time to sort if your mechanic isn't good at reading instructions.

Cheers,

Leadfoot.

BristoLS · October 6, 2005

Thanks very much for the detailed replies, printed and ready to call in to mechanic. He's an excellent guy and I have no doubt that he'll sort it for me. I will have a crack at the Throttle housing job myself first though. I have to say that it doesn't feel like an expensive problem as the Revs drop down as soon as the engine is up to temperature and the runs like a dream! A good sign I hope! Thanks again, very helpful.