ttyR2

So the wifes car decided to die on her in the middle of an intersection a couple weeks ago. She was able to get it restarted and get to work. Once there, I had a look and if I let off the gas completely, it'll drop below 1k RPM, and go lower and start to buck and then die. If I catch it and hit the gas it'll rev up and runs fine. Out on the highway it runs perfect as well.

Fast forward a week, it got worse and frequently wouldn't respond to the pedal at all, just dropping lower, start bucking, and then die no matter what you did.

Had the mechanic have a look and he though the fuel pump relay was bad. Ordered one through Sewell (very nice getting the Club Lex discount, btw) but the problem came back. He used the fuel system schematic to see how he could simply bypass the fuel pump relay so the pump always ran with the key on. The car runs perfect after he did that and never had a problem in a couple days of testing.

This car has 320k miles on it and I'm not really interested in chucking big $$$ at it. So...I'm wondering if I can just leave the jumper plugged in and forgo the pump relay completely. In the wiring schematic, the fuel pump relay appears to have two contact positions, one goes to a resistor then to the pump, and the other position goes straight to the pump, both under ECU control.

All the tech stuff (downloaded from Toyota's tech manual site) doesn't explain exactly what function the resistor has in the system. There's a seperate pressure regulator so I'm not sure why a resistor would be used inline with the pump (probably not to cut pressure). The normal position of the relay is to give direct power to the pump bypassing the resistor.

Any ideas? Here's the partial schematic:

ttyR2

A buddy of mine interjected that maybe the resistor functions similar to the ballast resistor in some American cars. When you hit the key, full voltage goes to the ignition, and when the engine is running it drops back to a lower voltage. Could the pump get full voltage to start and then kick back to a lower voltage for pump longevity?

python

iTrader: (4)

lmao....u r just asking for trouble if u bypass a relay, they are there for a reason

ttyR2

Sure. I fully understand and accept any possible outcome. But...I'm still interested in knowing when the resistor would come into play with the fuel pump (and not just supposition).

Vander

Any update on this?

EUBT

I'm just not getting this! You replace a pump that's not defective, but don't want to replace a relay that is?

JTom

I have the same issue on my LS 400 1992 and it's the fuel pump relay resistor. There's a thread on it here on this site https://www.clublexus.com/forums/mai...em-solved.html and that solved this guy's issue, I have the same problem as you and him, he put the part number, but I don't know where the fuel pump resistor is at to replace it, can anyone give me an idea?

Bill Dowd

I rencently had a problem like yours THE CAR WAS BUCKING AND QUITING I found it to be the MAF was shot I cleaned it and it made no difference I got one at a a used lexus parts shop in Santa Ana Ca phone 714-569-999 it's called 999 used lexus parts I paid $140.00 for a used one and the dealer wanted $1,000 for the same thing. and it is now working perfectly

nthach

But you can in the name of diagnostics - a quick check for a bad fuel pump on these motors is to turn the ignition to run/on but don't start the engine. Bridge the FP and B+ terminals in the diagnosis connector and watch the fuel pressure damper on the driver's side(passenger side on RHD cars) - the screw in the middle should be flush or stick out somewhat with the diaphragm cover - if it doesn't, you got issues.

JTom

Update on this, I replaced it with a new fuel pump relay resistor and same issues. If you have any updates, let me know, replacing parts is getting expensive so for now I'm leaving the fuel pump relay bypassed and it running at 12V and my car runs without issue but I'm in the same exact boat you are. Will keep tracking this thread for updates from others and my self.

JTom

Thanks for this I'll try it on mine.

Chin

iTrader: (34)

I have a similar problem. 1993 LS400. My car will crank/turn over all day, but it may take 20 or so tries before it starts. If I place a jumper where the EFI Relay goes, the car will start up and run fine every time.

I wondering if it could be the ECM that is bad? Could that be the original posters problem as well?

I don't want to keep grasping at straws because I don't want the starter to die on me in the middle of all this testing. I've been reading up on how fun those things are to replace.

freegard

The instrument cluster and the ecu and the whole world got bad polarized electrolytic capacitors
from 1990 through 1995. Yes, your ECU is no longer working correctly due to bad capacitors.

MattLS400

I believe we have similar issues,

https://www.clublexus.com/forums/ls4...s-to-half.html

After driving my 1996 Lexus LS400 for the past couple weeks, I have observed the following problems;
FIRST START on a morning (engine is cold) --- The vehicle works beastly perfect in traffic and on highway. No smoke, no rough idling. Normal Idle at 500 RPM

After 20 minutes (when the engine is warm) ----The vehicle begins to rough idle. No smoke.
------------------------------------------------------------When in traffic, it rough idles and the RPM drops below 500RPM and consequently cuts off. UNLESS while in traffic, I put the vehicle in Neutral and Accelerate\ REV till the Tachometer needle reaches at least the 2000RPM mark then I release the acceleration, only then the rough idle stops and idles normally again at 500RPM….and I work it like this “coasting” in traffic. However, if I drive in traffic accelerating under 2000RPM, the rough idle and shutting off would persist.
-------------------------------------------------------------When driving on highway (80 mph and more) which is above 2000 RPM on the Tachometer, the vehicle does not rough idle, it runs smooth. However, If I am driving slowly on the highway (approximately below the 2000RPM) the rough idle and jerking would persist.

My thoughts--- The mere fact that I am experiencing this problem when;
1) The Vehicle/Engine/Engine components are getting heated;

Leads me to believe that I can narrow down to either (A) Ignition coils (B) Fuel Pump


2) When heated, only runs smooth when I accelerate approximately above 2000RPM;
Leads me to further narrow down to (B) Fuel Pump or Fuel Pump Resistor

Your suggestions please and a part number for this fuel pump resistor would be appreciated

LScowboyLS

My suggestion is to fix the known factory defect issue with 1996 models, that stands about a 95% chance of being your issue, fixing the leaking ECU capacitors


here is the biggest clue of all - NONE of those components tend to fail on a 1996 LS400, but ECU's are dying like there's an epidemic! - the oldest secret of professional mechanics is something called pattern failure, in other words, the same parts tend to go bad on your 96 LS400, as go out on everyone else's 96 LS400!

There is even a company that has made hundreds of millions of dollars with this concept, the company that every serious mechanic uses, whether dealership or independent - their name is Identifix.

Even on the slim chance that your ECU capacitors are not the problem, you will not have wasted any effort, as they soon will be!! - this issue is not a matter of "if", but rather "when"!