1991 Lexus Ls400 Hard To Start, Jumpy Rpms, Stalling, And Sputtering

knoxvegas · September 6, 2007

Where do I start except that buying my Lexus was a mistake. I have a 1991 LS400 ran good for a week and then out of no where died when I came to a stop. I pushed it to the side and tried to start it for 30 minutes. It finally started and I got it home. It was sputtering really bad. I let the car sit for a couple of days and it fired right up. I drove it for about a mile or so and it started sputtering and trying to die on me. I got it home and shut it off and tried to start it, but it took forever to get it cranked and then the rpms would jump up and down. Eventually the engine would die. This is what has been replaced below:

spark plugs

spark plug wires

distributor caps

ECU

timing belt

water pump

I called the independent Lexus mechanics that worked on my vehicle and they said what they did to make it quit sputtering, stalling, etc.. is cleaned off the cables going to my crankshaft position sensor and reconnected it. They told me that the sensor could be going bad. So what I did is ordered the Crankshaft position sensor and should be getting it tomorrow. I've done threw to much money into it to back out now, so has anyone else ran into this problem. Any information is appreciated.

sunshineknox · September 6, 2007

Dang is there no one that can help me? I feel about as lost as a Lexus mechanic. Out of all the certified mechanics out there the Lexus mechanics are just as lost as me.

IanCornwall · September 7, 2007

If the Main Stealers did what they said they did and cleaned the connectors to the sensor and it worked well even for a few miles, then I would think that is where the problem is

The problem with electrics on any Lexus is that unless the brains in the car gets the right info it gets all confused and tries to make up its own mind what is wrong or just shuts down

The next problem is that if they cleaned the wires and connectors once because they had either corroded or just got covered in the proverbial cr*p or dead budgies from the road, cleaning them once may not have been enough

In fact if they cleaned them only once there is a high probability that they may even have dislodged some more dirt that has fallen on the sensor from the surrounding area

First thing to do is to get to the sensor and really spring clean everything in the area, not just the connector or sensor itself

Now get some electrical contact cleaner and spray that into over and onto anything that looks like it is a connector or plug and inside the plug

Don't be tempted to shove screwdrivers or metal things in the plugs to clean the contacts unless you disconnect the Battery and wait a few minutes, as you may short a circuit to the ECU and have a real problem and huge expense

Next spray liberally WD40 over everything in the area, wires connectors sensor to stop any more water or dirt being a problem

Put it all back together and try it, if it works great, if it doesnt or the problem comes back then it will be on to the nice auto electrician and his laptop to sort it out

sunshineknox · September 8, 2007

I appreciate your help. I replaced the crankshaft sensor and it made it harder to start and still died when in gear. If i put it in neutral before I came to a stop it would idol fine. Once it warmed up it was easier to start. I'm lost.

IanCornwall · September 8, 2007

next quick test if you have bad starting from cold but good from warm

get under the bonnet and pull the EFI fuse

Turn the engine over for about 10 seconds with the fuse out and no throttle

This cleans all the cylinders of fuel as the EFI fuse out "turns off" the fuel injectors

Ignition off and nip around the front and put your EFI fuse back in

quickly try and start the engine

If it starts easy and runs ok then you have a fuel management problem, the brains are probably trying to cold start or overfuel from cold

Most probable cause is the Engine temperature sensor reading that you are in the antarctic and throwing fuel in by the bucketfull, once warm this cause usually dissapears

This is what happened to me, do not get confused with the radiator coolant temperature sensor, the engine temperature sensor is a different thing, and a quick cheap fix if that is the problem

sunshineknox · September 8, 2007

Well I disconnected both EFI fuses, There was a round fuse and a 20 amp fuse that were labeled EFI. It started, it would still die. The rpms jumping up and down.

Rob · September 9, 2007

may sound simple but have you checked all the intake and vac hoses? it can be easy to miss a split hose. If the ls400 has a MAF sensor has it been cleaned or swapped with a known working one?

No idea if the 400 has obdII but if it has then that would have been my place to start before replacing anything.

IanCornwall · September 9, 2007

yep next stage is the vacuum hoses or a bad wire on the MAF

The MAF could also just be full of the proverbial cr**, there was a section on here somewhere about cleaning the MAF, anyone help with a better memory ?

Mark W · September 9, 2007

http://www.lexusownersclub.co.uk/knowledge...ndex_v1&c=9

Gives instructions for the RX300 and GS430 ......they are both very similar so guess its not that different for the LS....

Halfords sell the isopropyl alcohol as "Electrical contact cleaner" - only about £3 (did mine 2 weeks ago)

ian11 · September 9, 2007

just a wild guess,have you checked for a dodgy fuel pump,or fuel filter,both of which could lead to the symptoms described. good luck !! ian

popsicals · May 1, 2008

Well I disconnected both EFI fuses, There was a round fuse and a 20 amp fuse that were labeled EFI. It started, it would still die. The rpms jumping up and down.

Had erratic rpm on our car,so I took it to the main stealership ,sorry dealership who new bugger all about this model.they could not sort it.£815 poorer .Right ,what I did was loosen the pozidrive screws on the throttle body sensor( small round black thing with a multi socket on it)on the intake body , and once you have undone the screws move the sensor by about 1 mm.and retighten the screws.start the engine for a millisecond then turn it off.

repeat this ten times. This re educates the cold start settings. you should then have a fast idle setting around 1800rpm.

If you have an even fast idle then as the engine warms it should automatically set the hot running speed.

hope it works for you as it cured ours.

P.s. Main dealerships appear to employ technicians and not proper mechanics.I have the bills to prove it.Good luck

Also try Cataclean in your fuel it is brilliant :

Yakuza · May 2, 2008

I know what i'm talking about here.

Have you considered replacing the Fuel ECU?

This really sounds like Fuel ECU failure from many other experiences with your motor.

Cost 200 quid or 2nd-hand maybe less than 100.

To diagnose before spending cash, you need to bridge the fuel ECU out at the diagnostic port.

Where are you at? where is the car and who's got it in for work?

Yakuza · May 2, 2008

I know what i'm talking about here.
Have you considered replacing the Fuel ECU?

This really sounds like Fuel ECU failure from many other experiences with your motor.

Cost 200 quid or 2nd-hand maybe less than 100. I can point in general direction if you ned help sourcing cheap.

To diagnose before spending cash, you need to bridge the fuel ECU out at the diagnostic port.

Where are you at? where is the car and who's got it in for work?

ROYT · May 2, 2008

.

...and finally after all that, check 'Idle Air Control Valve.'..looks like a small motor, at the front,middle,

of the engine near the top......

Alexander the o.k. · June 6, 2008

Hello all- new guy here, so be gentle.

Here's what fixed it for me:

Once I did it, it was fixed for good (so far).

What I did was to strip and rewire the wire harness at the left side hinge in the trunk, or boot as our UK friends say.

This appears to be a known design flaw which will eventually fray those wires, cause shorts and all manner of weird happenings, jerking, wild rpms, engine cutting off etc.

So, that's it!

Just wanted to share a cheap fix that worked for me.

Your mileage may vary.

Anyway thanks again to all and hello!

Manc · June 18, 2008

Hello all- new guy here, so be gentle.
Here's what fixed it for me:

Once I did it, it was fixed for good (so far).

What I did was to strip and rewire the wire harness at the left side hinge in the trunk, or boot as our UK friends say.

This appears to be a known design flaw which will eventually fray those wires, cause shorts and all manner of weird happenings, jerking, wild rpms, engine cutting off etc.

So, that's it!

Just wanted to share a cheap fix that worked for me.

Your mileage may vary.

Anyway thanks again to all and hello!

Hi Alex, without looking in my boot just now (its late and throwing it down), and maybe asking a silly question but u mean just the bundle of wires from the light cluster that run back into the car? must be if they near the hinge i suppose, they cant be fueltank related cos that on the other side anyway (over here anyway :).

Well im gonna take a look anyway cos i had a few gremlins in my car - a 93 Mk1 Ls400. It just jerks when i need more revs but lacks power when it does if you understand???? Its the only fault the engine has had and i dread to think of the expense.....but your info sounds a bit mad at 1st but i know electrics can do strange things so im gonna look and hope it is just a rewire.....

Thanks for the shout

Alexander the o.k. · June 22, 2008

Hello all- new guy here, so be gentle.
Here's what fixed it for me:

Once I did it, it was fixed for good (so far).

What I did was to strip and rewire the wire harness at the left side hinge in the trunk, or boot as our UK friends say.

This appears to be a known design flaw which will eventually fray those wires, cause shorts and all manner of weird happenings, jerking, wild rpms, engine cutting off etc.

So, that's it!

Just wanted to share a cheap fix that worked for me.

Your mileage may vary.

Anyway thanks again to all and hello!

Hi Alex, without looking in my boot just now (its late and throwing it down), and maybe asking a silly question but u mean just the bundle of wires from the light cluster that run back into the car? must be if they near the hinge i suppose, they cant be fueltank related cos that on the other side anyway (over here anyway :).

Well im gonna take a look anyway cos i had a few gremlins in my car - a 93 Mk1 Ls400. It just jerks when i need more revs but lacks power when it does if you understand???? Its the only fault the engine has had and i dread to think of the expense.....but your info sounds a bit mad at 1st but i know electrics can do strange things so im gonna look and hope it is just a rewire.....

Thanks for the shout

Hi Manc,

Sorry. Been spending most of my time over on the U.S. site.

Being stupid, when I first found LOC and immediately joined, I didn't realize it was international and that I had joined the UK club. I have since corrected the problem by joining the U.S. site. Just now popped back across the puddle to see what my UK mates were up to. So sorry for the delay.

When you open the boot, at the left boot hinge, there is a wire harness concealed there on the hinge itself.

With each open/close action it puts stress on those wires. Eventually they fatigue then start to short out. This causes all manner of strange behaviour with the beast.

Everthing from (in my case) the stereo going on the blink, to bucking, stalling when brakes applied, to rough idle to surging. Really terrible experience.

I thought my beautiful world class automobile was pure junk and the engine, toast.

I finally (being a bit slow) realized that it must be an electrical issue, but had no idea where to begin to look for the cause.

Took it into the dealership (I know that's a big no-no here ;) ) to have them run an electrical check.

After 1 full day, they diagnosed the short at the hinge.

(My guess is that they knew from my description over the telly what the prob was, as it is an inherent design flaw).

They then offered to replace the harness at $500.00US.

I paid them their diagnostic fee, and politely declined.

Later at home, it was a matter of removing the plastic hinge cover concealing the harness, and stripping, reconnecting and taping it all securely together. Do Not arrange the whole mess so that it is stressed again.

In other words, let it swing free, in the open, relieving any stress on the bundle.

Not pretty, but.... problem solved as Ross Perot used to say.

Hope others with rough running, weird behaviour issues will see this and try it.

It has been smooth as silk since this was done.

Best of all, it's free and it works!

Good luck

(by the by, please post back if you try this and it works. The more people who learn about this easy fix, the better)

"Beer. It's not just for Breakfast anymore "

popsicals · November 11, 2009

next quick test if you have bad starting from cold but good from warm
get under the bonnet and pull the EFI fuse

Turn the engine over for about 10 seconds with the fuse out and no throttle

This cleans all the cylinders of fuel as the EFI fuse out "turns off" the fuel injectors

Ignition off and nip around the front and put your EFI fuse back in

quickly try and start the engine

If it starts easy and runs ok then you have a fuel management problem, the brains are probably trying to cold start or overfuel from cold

Most probable cause is the Engine temperature sensor reading that you are in the antarctic and throwing fuel in by the bucketfull, once warm this cause usually dissapears

This is what happened to me, do not get confused with the radiator coolant temperature sensor, the engine temperature sensor is a different thing, and a quick cheap fix if that is the problem

popsicals · November 11, 2009

I know what i'm talking about here.
Have you considered replacing the Fuel ECU?

This really sounds like Fuel ECU failure from many other experiences with your motor.

Cost 200 quid or 2nd-hand maybe less than 100. I can point in general direction if you ned help sourcing cheap.

To diagnose before spending cash, you need to bridge the fuel ECU out at the diagnostic port.

Where are you at? where is the car and who's got it in for work?

Rickster22 · September 30, 2021

On 9/5/2007 at 6:46 PM, knoxvegas said:

Where do I start except that buying my Lexus was a mistake. I have a 1991 LS400 ran good for a week and then out of no where died when I came to a stop. I pushed it to the side and tried to start it for 30 minutes. It finally started and I got it home. It was sputtering really bad. I let the car sit for a couple of days and it fired right up. I drove it for about a mile or so and it started sputtering and trying to die on me. I got it home and shut it off and tried to start it, but it took forever to get it cranked and then the rpms would jump up and down. Eventually the engine would die. This is what has been replaced below:

spark plugs

spark plug wires

distributor caps

ECU

timing belt

water pump

I called the independent Lexus mechanics that worked on my vehicle and they said what they did to make it quit sputtering, stalling, etc.. is cleaned off the cables going to my crankshaft position sensor and reconnected it. They told me that the sensor could be going bad. So what I did is ordered the Crankshaft position sensor and should be getting it tomorrow. I've done threw to much money into it to back out now, so has anyone else ran into this problem. Any information is appreciated.