mjkwee

I have a1992 LS400 with over 186,000 miles. I have an intermittent problem with the car sputtering after I have driven over 50 mile on a warm or hot day. There is no diagnostic code. I took the car twice to my mechanic and they cannot do anything without a code and the car runs fine after I let the car sit overnite.. I upgraded my ECU and swapped the capacitors. Recently, the problem surfaced again. I've replaced the Fuel Pump relay, replaced the ECT and also cleaned the MAF sensor. The Fuel Pump relay is a recent change after the third occurrence.

Today, I am thinking of replacing the FP resistor, P/N 23285-50010. However, this part is now discontinued. I have been looking for another replacement part.

My question is, is P/N 23080-50030 a 95-00 LS400 Fuel Pump resistor a direct replacement? The resistance is .73ohm. The mid point to the resistance range of .7 - .76 ohm.

Some members have fixed similar problem by replacing this FP resistor. I believe this may be the most probable faulty part because the problem occurs when it is a warm and hot day. After driving the car for a long drive the temperature in the engine compartment must be hot. Increased temperature Increases the resistance. Based on the schematics, the Fuel Pump runs without the resistor or 12V or with resistor and the voltage is within 8.4 - 9 volts. If the FP resistor is close to the upper tolerance, then an increase in the resistance on a hot day can feed a lower voltage below the acceptable tolerance.

I am hoping the 23080-50030 is a replacement part for my '92 LS400 Fuel Pump resistor. Thank you for any comments.

Yamae

Are you sure that the resistor is really causing the problem?
It seems to me that you are acting without a good confirmation to step next. I would precisely check the resistance change.

Usually resistors tend to increase the resistance due to the characteristic of metal materials as temperature goes up, but that should not cause any noticeable problem.

mjkwee

Yamae-San,

Thank you for your reply.

Two different mechanics were not able to help me because they cannot read or get any diag code. I am physically challenged with one hand to run my own test on my LS400. So I try to debug my problem one step at a time towards the Fuel Pump with the simple things
I can do. I suffered a stroke in Tokyo in 2006. They took me to Shiowa Hospital.

Questions,

1. Therefore, as temperature increases overtime the resistance will increase from .73 ohms to 1 ohm and therefore the voltage to the FP will approach 12 V or greater? What happens if voltage is below 8 volts? I think the resistance maybe within the resistance range on a cold day.

2. I have a 1992 LS400 with 186,000 miles, is it possible the FP resistor can feed more than 12 V to the Fuel Pump if R>1? I am not sure if the '92 has a FP ECM that switches the FP relay directly to the Fuel Pump. Limiting the voltage to 12V max.

3. Can the FP or how does it affect the pressure in the tank and fuel system, or will the ECU shutdown the engine to prevent a catastrophic failure?

4. Can the FP increase the pressure in the fuel system and flood the engine causing a stall condition?

5. What is the operating voltage specs of the Fuel Pump, what is the effective operating range?

I do not have the schematics for the Fuel Pump relay switches for the 1992. Given several schematics floating on the web, the schematics cannot be very different. If my assumption is close or correct, the only components between the relay and the pump is either the FP resistor or FP ECM. I believe the 1st generation LS400 may not have the FP ECM therefore some temperature may be affecting the FP and the fuel delivery pressure.

Here is a summary of what I have tried:
1. Replaced the ECU from eBay with the electrolytic capacitors changed. Later I discovered the ECU has the TRACTION version. My car does not have this feature. However, the car ran fine with this ECU.

2. As part of maintenance, I replaced the ECT sensor and cleaned the MAF sensor.
After 3 months I got the same symptom. My car stalled during a long drive on a warm day. The engine stalled.

3. I took the car to my friends mechanic. He could not read any Diag code so nothing was done because the next day the problem disappeared. This has always been an intermittent problem on a warm day.

4. I swapped back the original ECU with the caps changed. I purchased enough caps for two ECU.

5. I replaced the FP relay too.

The weather has been cold and the problem, if it still exist, has not shown. I have been assuming my problem still has to do with my fuel system or the Fuel
Pump. My car has 186,000 miles. So wisdom may say it most likely be the Fuel Pump. Am I reading too much in my problem or forcing a probable reason?

Thank you in advance.

dicer

So far not had that sort of problem with Lexus, but I have with other makes. It most always was the ignition module. A resistor usually works or it doesn't or can be destroyed by heat and change value. It takes lots of heat to ruin them.

Yamae

Didn't you check the fuel pressure yet, mjkwee?
You ask me new questions next to next without informing me the result of my initial suggestion. I think you are walking a long distance although there is a short cut.

mjkwee

Yamae-san,

I have not checked the fuel pressure because I do not have a diagram nor where and how to achieve this? Would anyone have a diagram so I will know what to look for?

Will testing the fuel pressure require I test it under certain temperature condition or can I test it while running the engine and still get a good reading given my problem occurs under certain condition?

Thank you.

mjkwee

Thank you for your comments. In your experience with the ignition module, was it intermittent when it gets hot or after running the car for a while on a warm day? Are you referring to the ignition module besides the FP box and FP relay and above the FP resistor? There are two components side by side and which one are you referring?

I will try to figure how to check the Fuel pressure and you have given me another area to investigate.

Yamae

The measurement method is in the service manual. Check the pressure when normal and abnormal of the engine condition. A real trouble shooting will be starting after these two measurements. You have been wasting time before standing at the starting point. I must say your mechanics are not well skilled to deal with your car.

If the fuel pressure measurement can't be done quickly, an alternative method is to check the voltage difference at the fuel pump when normal and abnormal. This can be done at the connector of the resister. It would be best to hook up the voltage meter all the time while you are running extending the cable to the meter.

dicer

I've seen intermittent problems when hot, and also complete failures. I've also seen a TPS DTC caused by a bad ig module.

randal

mjkwee, since there is no schader valve on the fuel rail, so in order to tap into the system, you will have to loosen the existing banjo bolt and connect a union pipe to the LH fuel delivery pipe. Then run a small rubber hose to a fuel gauge. Not the most convenient but thats the way Lexus chose.

The fuel pressure should be between 38-44 psi.

mjkwee

Thanks for the tip and fuel pressure range info.

mjkwee

Yamae-san,

I am beginning to think so. My mechanics are Toyota specialist and they said fuel pump do not typically go bad easily. That is why I decided to try a second mechanic when the last problem occurried. Without a diag code they do not want to second guess the problem or replace parts because they are expensive for an aging car.

I have to find another way to get these test and fix done. I live in the Bay Area in Sunnyvale. If there are any knowledgeable and reasonably priced Lexus mechanic who can help me locally? I will be very grateful for any recommendations. Thank you again.

mjkwee

Thanks. I will consider and explore this path.

gngai

Mjkwee, are you still in Sunnyvale? Did you ever find a skilled Lexus mechanic? If so, may I have the name/shop contact info as I'm in San Jose. How and did you resolve your issues? Thank you.

Cman99

has anyone solved this issue? My car seems to stutter around a 50minute drive on a °80 day. I do have a aftermarket fuel pump from oriellys and new filter