ECM/ECU Coolant Temp Sensor

[Razor61]

I’ve just replaced the ECU Coolant Temp Sensor on my 96 LS400 as the idle on the car wasn’t right, it could be high around 800/900 rpm - low rpm - round about the correct rpm but wasn’t stable and the common 0 revs sometimes after a decent journey and stopping. It was worse at high revs when you take your foot off the gas and the car keeps driving forward. It started niggling me so I did some searching and came across some info pointing towards the ECU Coolant Temp Sensor that could be the cause. So the sensor was replaced yesterday, I drove to work and back today (50 mile round trip) and the idle seems to be a lot better. The rpm drops to 600 and stays solid at 600 with no movement up or down, yes it is early days yet and it might be just a fluke today but it doesn’t feel like it. The car also feels different but that may be all in my mind and it’s exactly the same, time will tell though I suppose. If it solves the engine cutting out sometimes when coming to a stop I’ll be well pleased.

[cruisermark]

Hi

I remember changing this on my Mark 4 LS400 - but couldn't see a difference

[steve2006]

There was a recall for the engine ECU on the 95-97 LS400 due to the engine stalling after a high speed run, for example coming off a motorway and slowing down at the end of the slip road.

I hope it has sorted your problem but if you had the Battery off this may have had the same effect as replacing the CTS.

The easiest way to check whether the CTS is working is to start the car from cold and watch the RPM, when the temperature  gauge gets to a quarter up you should see the RPM drop slightly and then as it approaches halfway the RPM should step down to around 650 RPM. This indicates the ECU CTS is reacting correctly to the temperature change and the ECU is receiving the signal and adjusting the idle speed.

[Razor61]

Thanks for all the information, I was aware of the ECU issue and the sensor was ‘working’ so wasn’t open circuit or anything. I replaced it mainly to see if it cured the high/low/erratic idle. I believe that they degrade over time or can do and replacing it can cure the issues I have or had, hopefully. I didn’t disconnect the Battery or pull the ecu fuse either, many people recommend that you do. I have another ECU (later issue) and the correct capacitors ready to replace if needed which I will do at some point when I find my soldering iron!

[Razor61]

Drove to work this morning, the idle is the same as yesterday and very solid so I took a very short video to illustrate when I arrived at work. I drive 25 miles to work, mostly motorway, and when I leave the motorway a couple of miles from work there are some lights at the top of the slip road. This is the place were I first noticed the engine cutting out when stopped at the lights, I paid particular attention this morning, it was fine and the revs went down to more or less 600 rpm and stayed there. I know that's no guarantee it's been solved at all though. But, previously I have noticed that when I stop at the lights the idle goes low and then most of the time it goes up a bit and engine doesn't stop but it stays lower than it should be. When the engine does stop you can see the revs just die and the engine stops, classic symptoms I imagine.

I have done a few other things over the last few weeks, I adjusted the throttle cable and removed the slack a week or so ago when I dismantled the air intake etc to replace the sensor (only to find I didn't have the correct socket to remove the sensor) so put it all back and adjusted the throttle cable. This did improve the cruise control 'smoothness' quite a lot in my opinion. I put some Toyota fuel system cleaner in the tank the last time I filled up which was 2 - 3 weeks ago and the tank is 1/3 full now. The air intake up to the throttle body has been removed a few times along with the 2 x vacuum hoses (one goes to the IACV/idle control valve and the other to the steering pump I think ) and all put back together after cleaning and jubilee clips tightened etc. Maybe that's had an effect but who knows.

IMG_0760.MOV

[ambermarine]

The problem is solved and you are correct in assuming this was the suspect  I had the exact scenario on my Mark1 in 2010 and used a OEM replacement bought in the states whilst on our annual Florida vacation(missing that now) and it cleared up the minute it was fitted.

[Razor61]

1 hour ago, ambermarine said:

The problem is solved and you are correct in assuming this was the suspect  I had the exact scenario on my Mark1 in 2010 and used a OEM replacement bought in the states whilst on our annual Florida vacation(missing that now) and it cleared up the minute it was fitted.

👍 that's good to hear and I'm glad you have done the same thing in the past. My gut instinct was that replacing the sensor has made a difference but after a couple of day's driving I'm pretty much convinced.

This post made me think the temp sensor could be the issue:

https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/376713-all-ls400-owners-with-bad-gas-mileage-read-this.html

[Malc]

15 hours ago, steve2006 said:

There was a recall for the engine ECU on the 95-97 LS400 due to the engine stalling after a high speed run,

My understanding is that this specific recall was in the USA but Lexus refused to do it in the UK

I do suffer this stopping syndrome too, but have managed it well all these past few years by just understanding when it's happening, pop the gearstick into  N  and then restarting the car ....  it's all rather seamless now and I can do it without even stopping and just gliding for those very few seconds she's in limbo ......  often entering a roundabout 

OR just using both feet, when braking with the left and slight right foot on the throttle approaching likely " stop " spots ...........  eg a roundabout

Now I'm reading it's likely to be the ECU Coolant Temp Sensor then I'll ask my indy to try to deal with it all for when she's next down there being looked after 

This is a very informative excellent Forum Post ..............  thank you contributors..........

Malc

[Razor61]

Here’s another link that was in the original post I read, this convinced me to change the sensor.

https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/377171-diy-coolant-temperature-sensor-change-pics.html

[Razor61]

It’s been 5 days, 250 miles going to work every day and the idle is still very stable with no erratic behaviour. Car hasn’t cut out when stopping after a decent run on the motorway but the cutting out issue was intermittent so can’t say if that’s been solved yet (my gut/instinct says it has though) What I can say is that I’ve been taking note of the idle speed when stopping after a decent run, or any time I stop for that matter, it’s not the same as before and is very stable and consistent. Definite change in behaviour, when you drive the same car everyday on the same route and roads you tend to notice any change in the way it drives and behaves. I certainly do but I still can’t find the cause of an annoying squeak from the passenger rear😬

[Razor61]

Just an update - another 2 weeks and 425 miles, going to work and back plus journeys round town. Idle is still stable with no cutting out after a decent run so looks like idle issues and the cutting out issue have been solved. Car is smoother and MPG is a bit better, before I was averaging 24 - 25 mpg but now it's 28 mpg based on the last tank full of fuel. But aircon isn't working at the moment so it hasn't been on at all. When it's fixed I'll do another check to see what difference using the aircon all the time actually makes to mpg, just out of interest. The original post I found was related to mpg, especially round town iirc, I changed the sensor because of the idle issues so not that bothered about mpg but it's a welcome bonus.