Hi. I spent the entire day reading and reading before deciding to register and post.

The car is a 1994 LS400. All of a sudden went from running fine to extreme power loss, major hesitation, no acceleration, etc etc., however did not completely die.

I used a SnapOn Solus scanner and could not retrieve any codes. Furthermore, the CEL does not illuminate at KOEO. Also, manually jumping with a wire produces no result. THe CEL comes on while running and does occasionally flicker.

Underhood diagnostics: ignition coils have full and strong spark (ouch), and there is spark to all cylinders. All plugs are dirty but otherwise ok. Also, pulling the plug wires off individual cylinders produces no effect on running condition and the spark is weak, leading me to believe it is currently in limp mode. However, it should still display the CEL at KOEO and should put out codes.

I am ruling out a fuel or major mechanical problem because I have been doing this for long enough to know the difference.

From what I have read the rough running with the CEL failure/flickering and no codes, typically indicates a bad ECU. I removed the ECU and opened it up and it looks clean but of course that doesn't mean it has not failed.

I believe it needs an ECU. Just looking for some kind of confirmation from the community before I spend $200ish on a reman.

 

Welcome to CL.

Unfortunately the simple test is to plug in a known working ECU. We've had some past threads regarding these and so you take a chance buying from a wrecking yard. I don't know if anyone here has been able to trace a specific part in these.

You've probably seen this thread:
https://www.clublexus.com/forums/ls4...-repaired.html

GL

 

My '94 is doing the exact same crap, same diagnostic nightmare. I opened up my ecu and noticed 2 things ; the ecu is a total departure from the prior years, and secondly, there is no dialectric heat sink compound between the chikps and the heat sinks and none between the sinks and the chassis. As I looked on ebay for a used or reconn'd ecu I noted that almost everyone selling them claim the 93 and 94 are interchangeable, reality is they are not. That being said, go by the part numb on the ecu. Good luck, I think we will both need luck before this is sorted out.

 

They aren't interchangeable unless the part number is the same.

 

I made a post before about ecu not giving out any codes. My car starts up and kill when I give it gas. Is y'all doing that too?

 

Any chance it's a maf or o2 sensor causing limp mode and a burnt out cel bulb?

 

If you read the OP, the CEL comes on when the vehicle is running and flickers intermittently...

 

My 1990 LS is doing this too. I'm calling junkyards quick to see if I can find a replacement ECU for my car. Is there anything I need to look for when getting one from a jy?

 

Yea open it up before you buy it and make sure it's the right part.

 

I did read it. apologies for the oversight.

 

If you put the diag jumper in and it reports diagnostic codes then your ECU is probably ok.
If you put the jumper in and it totally ignores you even though you're 100% certain you've got good connections, and then later when the light has finally gone out it starts recognizing the jumper again then your ECU is in the process of dying... Been there, done that...

Within the first week that I bought my '94 LS400 it went from pretty much normal and drivable for the first few days to sometimes deciding to light up the CEL and do some stalling randomly and idling low and when it was in that state I could jog faster than it could accelerate in forward gears yet it would still fly in reverse the whole time the CEL was lit and it would also totally ignore the diagnostic jumper being inserted the whole time it was acting bad like that.
BUT whenever it started doing any or all of that stuff with the CEL lit and ignoring the diag jumper, if I parked it for about 20-45 minutes and walked away then came back and tried the diag jumper again it would eventually start acknowledging the diag jumper again and of course at that point the ECU always said everything was fine and it would go back to running normal again for a random duration before acting up again, sometimes minutes, sometimes days before it'd do that crap again.
It also would buck and stumble mildly to moderately sometimes or decide to act like I was literally turning the key on and off in about 1/2 second intervals while going down the road at any speed for a few seconds sometimes or a couple of minutes other times and it would also decide to randomly stall on decel at any speed sometimes while doing that on/of/on/off wierdness. The biggest annoyance was its' tendency to flake like that and stall without warning as I hit the brakes and started to turn the steering wheel to turn into a parking lot or driveway or just get off the road because of its' misbehaving, which gets very difficult to stop and turn the car when the engine has totally stalled mid-turn, NOT fun!

This car is a total base-model other than the sunroof/moonroof option. No TRAC system, no Air-Ride suspension control system, no seats/seatbelts/mirrors/steering-wheel presets or power telescopic steering (just power tilt/manual telescopic), no seat warmers, no CD changer, etc, etc, etc.

The car had a used 1994 model year -50142 ECU in it when I bought the car (had yellow junkyard paint-pen markings on the ECU housing and a missing rear cover plate!) and I replaced it with a -50141 junkyard ECU that I found in the trunk of an LS400 at a local junkyard and all of those symptoms instantly vanished the moment I swapped out the ECU.
I still have a bit of random light stumble on accel that feels like a wheel balance problem but definitely isn't, and a bit of shudder on downhill/decel at highway speeds occasionally but when I "floor it" the car goes just fine and almost always cruises amazingly smooth in town and on the freeway at 70mph so it's not quite perfect but it's been like that for 10 months and 14k miles since the ecu swap with no more undrivable car problems...

So if your CEL comes on and totally ignores the diag jumper the whole time it's on and the car will fly in reverse but is a slug in forward gears, but then magically the light goes off, the diag jumper starts working again but reports everything's fine and the car drives pretty much normal again 'till the next time the CEL comes on then there's a good chance that your ECU is dying.
There are also other symptoms of a dying ECU but they're different problems within the ECU with totally different behaviors like not revving above mid 2k rpm's and claiming the TPS is bad when it's not, among other "bad ecu" issues, some of which are fixable by the reman shops, some of which apparently are not.

Many of those reman places just change out all the electrolytic capacitors which fixes most problematic LS400 ecu's but not all of them. Some LS400 ECU's just seem to have deeper issues that those places can't seem to isolate and repair.

Some of the big name auto parts stores offer reman'd ECU's for these cars with a warranty for around double what the "we fix your bad ECU" places charge, or even 1.5 x- 2x what people sell plain old used ones for on ebay and it seems expensive at first but I'm really considering spending the extra money to get one from my nearest local auto parts store to see if the little minor issues my LS still has might finally be solved completely...

There appear to be 3 different ECU's for a US Model 1994 LS400 and they don't seem to care about TRAC or non-TRAC or Cali Emissions Edition or any of that. They seem to just run the car with whatever options it has and ignore the rest if it's lacking any, but I haven't personally tested them all so there may be exceptions. I can tell you that the -50142 and -50141 both run my base-model (NON-TRAC) 1994 LS400 interchangeably with no complaints about lacking features at all...

I know it's a lengthy reply with a bit of repetitiveness but hopefully it proves at least somewhat helpful for you.

 

Good to hear someone has resolved many of the symptoms my '94 has. Is it just me , or does it seem like the mech engineers did a great job on this car and the electrical engineers were asleep at the friggin wheel.

 

it's a 20+ year old car with one major electrical component that goes bad...

and it was a 3 year issue, 93/4+ cars are generally fine.

 

I remember when I found out about the ecu could be the problem I went to the junk a day before and I saw a 93 & 94 ls400 w/trac still in the cars I came home and read clublexus a lil bit and went back the next day and all of them was gone I was burned up. If you see one you better get it ASAP!

 

have this trouble too

No trouble codes/
new spark plug,new filters,fuel pump,but car have no torqe