To those of you with this issue, one that seems to only effect your vehicle during the summer months, I may have your solution. Mine is a 1994 LS400 196k miles.

Quick rundown of MY problem. During cooler months, say temps 85F or below, vehicle runs like a champ. Temperatures in excess, engine shudders or stalls seemingly at random, whether at high rpm on the highway, or just sitting still, idling. Many have described is a fuel starvation or vapor lock., while others likened it to a kick in the gut. Either way, vehicle stalls for no apparent reason, gives no codes, and precious few clues to help isolate the issue.

Solutions I have seen touted ranged from ingition modules to timing, to replacing the fuel pump, along with a number of sensors.
Solutions I have attempted, and failed to address the problem:
New plugs
ECT sensor.. both
new timing belt, along with new roters
replaced fuel pump and filter
EFI Relay, among others

A quick aside, with all the research I had done, I opted for the fuel pump based on the information I had gleaned from others with this seemingly same issue, primarily because it was noted that at a 1/4 tank of fuel or below they noted having the problem. And after changing the pump, I didn't experience the problem again for some time, however the temps outside had also lowered below what I call the threshold for the event. So not entirely certain my problem was cured. That was a year ago.

This year, very hot summer in Missouri, and here comes the problem yet again. This time with many a hot day to experiment, I went back and started testing other ideas. But this time I had a good clue that the fuel pump was not a likely culprit. It just so happened that the stalling hit me when I had a 1/4 tank of fuel, so I filled it up, thinking if the pump was the issue (due to overheating) it would go away. DIdn't.

So the problem in my eyes now was, what could potentially cause the ignition system in its entirety to fail, and I eventually arrived at the Crankshaft Position sensor as the most likely candidate. Coming home from school yesterday, the outside temp was 92F, and the car hiccupped and bucked all the way home, but fortunately didn't stall and leave me on the side of the highway. Got the autoparts store where I had prearranged to have the sensor waiting, picked it up, took it home, put it in. Problem has not resurfaced as of yet. Today, temps again were in excess of 90F, and coming home from school the outside temp was again at 92F according the vehicles outside temp sensor, so I felt I had a valid test. Zero indication of the engine stalling I'd had the day before.

Given the likelyhood of the problem to occur with a certain driving style, namely WOT upon entering the highway, it would nearly always exhibit symptoms with the appropriate outside temperatures. I gave it hell all the way to school and back and couldn't detect any hint of a problem.

Is it cured, not entirely sure, but I did note that the resistances were not the same between the old unit and the new. I coudln't tell for sure what they were exactly, because the cheap pos multimeter I have isn't very accurate. I'll update this over the next few weeks as I keep an eye on the weather so I can push the limit when its hot outside.

One other side note, if your LS400 doesn't have that underbody shield on it, changing the Crankshaft sensor shouldn't take more than a few minutes, one 10mm bolt, and a connector is all that you need to handle. I drove mine up on ramps, had plenty of room, and had the change done in under two minutes.

 

Did you check or replace your main engine temperature sensor? It's not the same as gauge sensor.

 

You missed the part where I said

"ECT sensor.. both"

One is for the gauge, one sends a signal to the ECU. So yes, to answer your question, I did.

 

Seeing how it has to do with ambient temp, it could be a bad Air Intake Temperature sensor (AIT) inside the Air Flow Meter. It doesn't always throw a code when it goes bad. It's also a common problem with these cars.
Hopefully the CPS was your problem though, I know what it's like to try and hunt down a problem for more then a year. GL.

 

Congrats !!
I hope it is solved permanently!!
Mine was the fuel pump but my symptoms would always go away immediately after getting gas.
Atleast now you have a bunch of new parts on the car lol.

 

Yesterday, the outside air temp guage read 94 on the way home from school. No evidence of a repeat yet. I'm going to put the old one back in, if temps remain high, and see if the problem returns. That way I can definatively say one way or the other that this sensor was the culprit.

Dean, if it hasn't been solved, I'll certainly look into the AIT as my next step. There aren't many options remaining, certainly not inexpensive solutions, to be sure. My opinion of how the problem is affecting the car leads me to believe it is something that effects the entire ignition system, in this case not allowing it to spark.

 

Update: Its been over 30 days, with no repeat of the symptom, temperatures in excess of 95F.

 

I'm glad I found this and I'm having the same problem. Mine hasn't done it in a minute but my lexus through a code 12 a week ago and it dealt with the CPS...so I'm having it check by my mechanic on today along with some other things dealing with that code....I will update on the repairs and findings as well...congrats on your fix....

 

I'm thinking this is my 93's issue.

The symptom is the car completely dies and will not restart when it is very hot out (also mid to high 90s). I've had it die twice and had it towed to my mechanic twice. Both times it started for them when they got to it and reported no codes.

I'm going to try to replace this sensor. I just dislike the uncertainty of knowing this is the problem. Thanks for the information though, this sounds like my car's symptoms except my engine at low speed.