Princejr86

my ls 400 recently stared having issues accelerating and sometimes dies when accelerating. I have been exploring several different avenues and have the following possibilities:

• Bad Spark Plugs
• Bad Fuel pump
• fuel system cleaning needed
• or i just plain purchased some bad gas

I need some suggestions from people who may have experience these symptoms. thanks guys

mitsuguy

iTrader: (2)

Just need to start checking things...

Start with spark plugs - pull em, inspect em, while you are at it, might as well replace them... you can get platinum NGK's for like $3 each, so, $24... Your spark plug gap may be too big, causing the spark to not jump the gap under heavy acceleration.

Since you are going to replace those anyways (and make sure they are gapped properly), might as well take it for a run and see if there is any change...

Then, problem still exists - might not be a bad idea to check the fuel pressure... At idle, you should see 35 psi on a fuel pressure gauge... Disconnect the fuel pressure regulator vacuum line and it should rise to 43ish psi...

The last two are unlikely, at least not to cause the issue you are having... There are other possibilities, but you are on the right track and you should at least start there...

Last but not least - check engine light on?

j2b4o

I pulled my MAF to clean it (was all plastic so I don think I actually did anything) but when I re installed it the car did what you describe randomly. I would give it moderate throttle and it would completely loose power the rpm would drop below idle and than it would come back instantly.
I pulled the ECM fuse to reset it and that cured the issue. I'm only mentioning this because I would also check the MAF if I were you.

GS4_Fiend

iTrader: (22)

94 Lexus LS400, you are NOT suppose to clean the MAF. You probably already ruined the MAF by cleaning it. Try disconnecting the MAF sensor and see if it runs better, if yes, it's time for a new MAF sensor. New MAF on this particular car is about $1055. GL

j2b4o

If you read my post it says the problem disappeared after an ecu reset so definitely not ruined. Pretty impossible to ruin plastic anyway there is no exposed element to destroy.

g4ebguy90

I know this was about a week ago. But think about what you just said. If the MAF sensor was just a peice of plastic and not a complex instrument that is used to measure air, I am pretty sure Toyota would not have put it on your car. There is something interally that meters the air, and anything you done could have possibly messed it up. On these I heard they are alot more easy to ruin then a standard hotwire type MAF.

GS4_Fiend

iTrader: (22)

That's correct, I was refering to the hotwire inside that is very delicate. Only on certain vehicles you can clean the MAF but with the OBD 1 type MAF sensor on the LS400 you cannot clean it.

freegard

Well, mine is doing this: If I drive down the freeway and accelerate gradually the car responds perfectly, but if
I attempt to accelerate hard the engine cuts out; I back off the the throttle and she purs along. This happened right after I filled the tank with cheap gas; but I do not believe that cheap gas literally quits burning under hard
acceleration like that. I have had cat problems with other cars and it is not that. Another thing: If it is in park or
neutral I can give it gas and it has no hesitation through the RPM range where if under a load it will cut out.

So, will someone direct me to the ecm fuse alluded to in the former post that fixed his problem?

freegard

I think I am getting a better understanding of all this: The ECU controls via a coil relay a two speed fuel pump.
If the ECU capacitors are bad the relay does not induce the coil to the contacts eliminating the resistor in thje circuit. If you try to accelerate quickly the fuel pressure remains at low pressure and not enough fuel gets to injectors. Sure, you can, or did perform a reset on your ECU but I believe that this issue will occur again. Resets usually indicate that there is something wrong.The reason the resistor is getting so hot is that it is never given a break, the fuel pump is always on low pressure. Resistors drop voltage. Drop voltage to the fuel pum and the pump will slow down. That is what is going on with the ECU, the resistor, and the relay. I could be wrong but I do not think so.

chadr

I have a 93 ls400

Did you determine the trouble? I have the same symptoms now. I can floor the car in neutral and it will not accelerate. The rpms drop to about 1000-2000 rpm and the engine catches back on and speeds up a bit but then quickly looses power till rpms drop again. I just sold my fuel pressure gauge...but it would have been handy now to verify I really do have a fuel pressure issue. I would still need that unique banjo bolt and fitting to do the check shown in the shop manual.

freegard

Yes I did. My ECU had several leaking capacitors. They were causing that and other issues with the car since I bought the car two years ago. I had bad mileage (15MPG); the engine would hunt for idle and jump from 500RPM to 1000RPM then back to 500 RPM; the last straw was when the engine would not allow me to accelerate hard; could not get codes from ECU, could not diagnose vehicle. That is different from the issue you describe. Nevertheless, a bad ECU presents a ton of variegated symptoms; so the one that is most telling is: It won't give you the correct, or sometimes any codes.
REGARDLESS! It is almost impossible to fix any engine problem if the ECU caps are failing. Replacing all of them is very tedious, it is not for someone who has no electronics experience; no soldering experience. But if you do have experience then it is the best thing you can do and the first thing before you beat your head against the wall and drain your wallet replacing items that may not need replacing. You wont spend more than 60.00 replacing your bad ECU caps, and that figure factors in a soldering iron, flux, isopropyl alcohol (to clean circuit board where old caps leaked) solder, magnifying glass, and all the new capacitors (at most there are eight in your ECU).
A remanufactured ECU merely has replaced capacitors, that's it. Everyone knows that capacitors are the culprit with these ECUs over 99% of the time.

So, the short answer is YES, all of my problems went away after I replaced my bad capacitors about ten days ago. Every last one of them--gone!!!

For more details go here:
https://www.clublexus.com/forums/ls4...capacitor.html

chadr

Thanks for the update. I was getting tps and sub tps codes, but after removing and testing the tps I could find no trouble. I even checked the output of the tps on a scope to see if there was any bad spot that was too hard to see on a ohm meter. I didn't bother verifying the wiring from the tps to the ecu yet. I ordered the 8 caps from digikey last Friday at a total cost of $6 shipped. After a couple hours last night I was able to remove all 8 caps. The smallest ones and maybe another one were clearly leaking but luckily didn't destroy any of the nearby traces.

The tps codes on the ecu may be unrelated to my trouble but I agree it still makes sense to replace the ecu components known to fail. Are there other components that have been known to fail on the ecu or it is just the caps?

chadr

Got the caps in the mail today and installed them... Problem solved. Glad I found the fix mentioned so often here. I would have spent money testing fuel pressure and checking injector pulses before trying to replace all the capacitors in the ecu.

RickyGee

I know this thread is nearly a year old, but my 94 LS400 that I gave to my student son is now exhibiting the same symptoms. I was planning a trip to North Carolina to probably replace the ignition system, but the capacitor problem fits all the symptoms, including a false TPS code.

Did any of you guys write down the capacitor specs/values so that I could order them before leaving and tearing the ECU apart? Thanks heaps. -Rick

lexuspaul

Change the fuel filter sounds like it getting clogged with crud. It's a very awkward job to do if you dont have a ramp.