Quack11

I can relate to your misery or a CuriousB calls it, "Murphy". Continue to isolate the many possibilities and I am sure your problem will soon be identified and fix. Good luck.

Did a check with and without resistor and concluded that the resistor allows the pump to run at 8-9V and at 12V. Bypassing the resistor will only allow the pump to operate at 12V which seem to be ideal at higher speed due to more fuel demand. Resistor ohms at specs, so is the pressure and all electrical connection going to the fuel pump is in correct working condition. More to follow after installation of a new pump.

Swapped ECU with known good one and it did the same, think I have the problem down unless it's a pump resistor problem which I did not troubleshoot.

A professional would simply do a fuel system pressure check and scan the pump for current draw and rpm....... 100% agree. Professionals also have all the required tools and a price chart ready to make hard times harder.

I've added this post to another thread but i'm updating the one I start since it's being tracked by a few people. I've done extensive and indepth troubleshooting, fault isolation and test/check of this problem up to today and all problem is pointing to the fuel pump. Ordered one two hours ago and will post the results after installation. Currently I have two of everything that will cause that problem and nothing seem to work so I got a used fuel pump and the car drove fine for a day until after I 1/2 fill the tank with premium gas, then the issue came back. No CEL and nothing stored in the ECM, i've tricked the system to trip the CEL and it came on and was picked up by the scanner tool. Every sensor, wire, hose, etc that I have access to was thoroughly inspected. Mounting surface where EGR valve connects to intake was badly clogged but did not affect functionality, not sure why. Isolate both engine banks and individual cylinder and the operational difference was felt, not to mention the electrical shock from the plug wire. What I did here was verify that ignition/spark was not a factor or contributor to this problem and I totally ruled it out. Troubleshoot fuel flow system to include, pressure, relay, regulator and nothing was changed. swapped ECM and results are the same so I went a bit further because this problem will not conquer me one bit. I removed the rear seat and open the fuel tank and start the engine. With a constant 12V going to the pump it was surging and the engine would react to the high/low fuel delivery pressure. This tells me that the voltage is correct and consistent but the fuel pump is either cavitating or worn internally. So I did another stunt and nailed what I think is the real culprit and now I awaits my new fuel pump and hopes a new bug (Murphy) will not awake. I totally disabled the fuel pump and disconnect the fuel supply line going to the engine. Hooked up a fuel line from my SUV to the LS400 so I can use it as the source of fuel supply and turned ignition key to the "ON" position. Next I attempt to start my car and bang, the problem was solved instantly. I was unable to take it for a drive with a fuel umbilical string attached but she works 5.0. Next I submerged the suspected bad fuel pump in gasoline, connect it directly to power and the fuel supply line and start the engine, it went back to the normal headache. I think this is the problem and I will provide an update after installation and a few operational miles.

Very common issue and i'm currently having the same problem with my 91 LS400. I too have refused to take my car to any mechanic and have invested in an OBD-I&II scanner for future use due to my hobby of fixing the cars that I can handle. I've done extensive and indepth troubleshooting, fault isolation and test/check of this problem up to today and all problem is pointing to the fuel pump. Ordered one two hours ago and will post the results after installation. Currently I have two of everything that will cause that problem and nothing seem to work so I got a used fuel pump and the car drove fine for a day until after I 1/2 fill the tank with premium gas, then the issue came back. No CEL and nothing stored in the ECM, i've tricked the system to trip the CEL and it came on and was picked up by the scanner tool. Every sensor, wire, hose, etc that I have access to was thoroughly inspected. Mounting surface where EGR valve connects to intake was badly clogged but did not affect functionality, not sure why. Osolate both engine banks and individual cylinder and the operational difference was felt, not to mention the electrical shock from the plug wire. What I did here was verify that ignition/spark was not a factor or contributor to this problem and I totally ruled it out. T/S fuel flow system to include, pressure, relay, regulator and nothing was changed. swapped ECM and results are the same so I went a bit further because this problem will not conquer me one bit. I removed the rear seat and open the fuel tank and start the engine. With a constant 12V going to the pump it was surging and the engine would react to the high/low fuel delivery pressure. This tells me that the voltage is correct and consistent but the fuel pump is either cavitating or worn internally. So I did another stunt and nailed what I think is the real culprit and now I awaits my new fuel pump and is hoping a new bug will not awake. I totally disabled the fuel pump and disconnect the fuel supply line going to the engine. I hooked up a fuel line from my SUV onto the LS400 so I can use it as the source of fuel supply and turned ignition key to the "ON" position. Next I attempt to start my car and bang, the problem was solved instantly. I was unable to take it for a drive with a fuel umbilical string attached but she works 5.0. Next I submerged the suspected bad fuel pump in gasoline, connect it directly to power and the fuel supply line and start the engine, it went back to the normal headache. I think this is the problem and I will provide an update after installation and a few operational miles. Sorry for the long write up but I hope it will help.

Very common issue and i'm currently having the same problem with my 91 LS400. I too have refused to take my car to any mechanic and have invested in an OBD-I&II scanner for future use due to my hobby of fixing the cars that I can handle. I've done extensive and indepth troubleshooting, fault isolation and test/check of this problem up to today and all problem is pointing to the fuel pump. Ordered one two hours ago and will post the results after installation. Currently I have two of everything that will cause that problem and nothing seem to work so I got a used fuel pump and the car drove fine for a day until after I 1/2 fill the tank with premium gas, then the issue came back. No CEL and nothing stored in the ECM, i've tricked the system to trip the CEL and it came on and was picked up by the scanner tool. Every sensor, wire, hose, etc that I have access to was thoroughly inspected. Mounting surface where EGR valve connects to intake was badly clogged but did not affect functionality, not sure why. Osolate both engine banks and individual cylinder and the operational difference was felt, not to mention the electrical shock from the plug wire. What I did here was verify that ignition/spark was not a factor or contributor to this problem and I totally ruled it out. T/S fuel flow system to include, pressure, relay, regulator and nothing was changed. swapped ECM and results are the same so I went a bit further because this problem will not conquer me one bit. I removed the rear seat and open the fuel tank and start the engine. With a constant 12V going to the pump it was surging and the engine would react to the high/low fuel delivery pressure. This tells me that the voltage is correct and consistent but the fuel pump is either cavitating or worn internally. So I did another stunt and nailed what I think is the real culprit and now I awaits my new fuel pump and is hoping a new bug will not awake. I totally disabled the fuel pump and disconnect the fuel supply line going to the engine. I hooked up a fuel line from my SUV onto the LS400 so I can use it as the source of fuel supply and turned ignition key to the "ON" position. Next I attempt to start my car and bang, the problem was solved instantly. I was unable to take it for a drive with a fuel umbilical string attached but she works 5.0. Next I submerged the suspected bad fuel pump in gasoline, connect it directly to power and the fuel supply line and start the engine, it went back to the normal headache. I think this is the problem and I will provide an update after installation and a few operational miles. Sorry for the long write up but I hope it will help.

Murphy, Murphy, Murphy......I did the ignition test yesterday and almost set the car on fire from the high spark output from both coil, did individual plug wire/cylinder and the problem is not in that area. Fuel pump relay could possible be the culprit and I agree with you about two pumps doing the same thing and the problem being intermittent. This fuel pump seem to be stronger but I will verify it tomorrow after checking with the pressure gauge. The tank is clean with no foreign objects inside it and filter was changed in June. I'm doing some reading on the fuel system and will tackle this problem head on again tomorrow. More to follow.

I sure did save on all the labor cost and the self satisfaction is worth far more. Thanks again. Well, the critter was not gone for too long. Went for a ride this evening and I did not even make it out the garage. Check my CATS to see if there are restricted or glowing in the dark but they are fine. The car will run a bit longer (almost a minute now) and it will stay running sometimes if foot is on the gas pedal but will instantly shut down. I'm back to believe that the used fuel pump is not delivering as it should, injectors are not opening (which should trip the check engine light I think) fuel pressure regulators are shot (both at the same time) or it's just time to go back to Nissan. My 91 Honda Civic with 200+K and riding lawn mower are the most reliable vehicles on planet earth right now. Changes PCV today and that did not make a difference, what else can be done before handing this problem to someone else?

I sure did save on all the labor cost and the self satisfaction is worth far more. Thanks again.

Well I got up with a game plan today to go deep into this problem and hopefully fix it. I changed the MAF Sensor, (again) TPS, COIL, IACV, ECM, fuel pressure regulator, secondary TPS, hoses, wirings, etc, etc, etc. I did all the possible fault isolation I could think of and nothing did the trick. After tricking the fuel system I changed the fuel pump and the car is back in business. So now the car is working fine again but I have all these LS400 parts in my garage, all of which works fine but none of which can be returned. I may open a used parts store until I can get rid of them. Thanks for the assistance all, CuriousB you devotion to helping others here is note worthy and I appreciate you time. I have plenty of troubleshooting steps and maintenance for the 91 LS400 in pdf file format if anyone need some information.

Well I am sure just an oversight but the ECU responds to sensors as inputs (MAF, temp, throttle position, ..) and controls outputs such as ignition, fuel pump and injectors. The coincidence of the fuel being empty and the problem arriving seems suspicious still. I can't help but think of contamination in fuel system such as tank filter or post fuel pump filter. Again a partially plugged filter would yield worse problems at higher rpms though. I think you need to differentiate between an idle problem (where IACV could be culprit or the IACV driver circuit within the ECU) and a more general fuel problem. That said can you get the car onto the road and running at a higher speed such that IACV valve is irrelevant (ie throttle plate open enough that it swamps any air bypass via IACV). If it stills stumbles at road speed and something north of 1300 RPMs you can put the IACV related theories aside for the time being. If you have no apparent running problems at higher RPMs then you can focus on the IACV and related items (ECU driver circuit outputs, cable, connectors and harnesses to IACV, and possible vacuum leaks). I sense the frustration but this is solvable. Here is exactly what the car is doing, upon start the car will not run past 5 seconds now, whereas before it was right about 9-10 seconds. It can immediately be restarted and does the same thing repeatedly. I tried to keep it running by pressing on the accelerator but this does not help. A repeated depressing of the gas pedal will keep it running but only for less than a minute. I've been over the entire troubleshooting steps for idle/running problems, tests both with and without codes and nothing seem to work. It is very frustrating but as you've said, it can be solve.

Could be possible but this is the same place i've bought gas for many years. Today I removed the EGR valve and did a vaccum test, it works as advertised. Cleaned the ports and check all the hoses and surrounding areas, all seem to be fine. Removed the fuel pressure line and regulator, turned the ignition key and the pump is working just fine, almost fill my 2.5 gallon botttle in no time. Disconnected the MAF sensor and attempt to start the car, nothing happen and the check engine light came on. This was a simple test to ensure the computer is working and codes are being stored when a problem is identified. Removed the IACV again and checked for proper function, inspect the throttle body and all hoses, electrical connectors, etc, ect, all seem to be in good order. Noticed that I did not hear the fuel pressure released when I removed the filler cap, this seem strange as I am used to hearing it on all vehciles. So with no codes and all the possible troubleshooting/inspection complete, I am looking at what the ECM is telling all the sensors to do!!!!!!!! Any suggestions, ideas or even "what if"?

Thought I had this idle problem under control but I was wrong. After almost three weeks of solid driving and 183 miles, yesterday the car stalled within 15 seconds of starting. It restarts fine on the first turn of the key but will not idle past 10 seconds with foot on or off the gas pedal. I had this same problem in June and I did all the troubleshooting steps out there, include changing the IACV. I narrowed down the problem to the MAF Sensor and the issue went away after changing it. No maintenance was done since Aug 4th (change the MAF Sensor) and the problem started after fueling the car on Sunday and driving only 2.5 miles. There are no codes in the ECU memory and none is found/displayed on engine scanner which leaves me with no clues. Any ideas? Does anyone know if the fuel pump can be heard when the key is turned on but not started and then goes away after a few seconds? I can only hear the pump when the engine is running and it sounds fine. Is it possible for the fuel quantity to interfere with pump operation? I purposely waited until the low fuel level light was on before refueling, could it be that i've picked up residue and may have a clogging problem with inturns create a fuel starvation problem? Any ideas before I start lookinh for a new ride? Thanks.

Let us know if this helps in anyway. traction_contrl_system.pdf

See if this information will provide any help. g___tests_with_codes.pdf

Agree with Python, do all the necessary checks before spending money on replacements parts only to get the same result. Your car should have been converted to the R-134A by now and if it was not properly vaccumed it will not function regardless of what you do. A system leak check will determine any leaks using a special dye and from there you should be able to find the problem. Becareful if doing it yourself and observed the warnings for R-134A as it will knock you out cold.

This is a very commom issue my LS400's including mine. The seat back motor feeds a shaft that raises both side together, however the right side of all the shafts are stripped and therefore does not allow the right side to raise to its full upright position. A high speed, tough drill bit will do the trick. If you are going to try the drill bit method, lower the seat back to where both sides are even. The drill portion can be done with the seat in or out of the car. Drill a small hole through the horizontal shaft and support hole together, put a small nail or cotter key in it and your problem is solve. Let us know the end result.

The ignition switch is the only thing that will allow the engine to continue running after turning off and removing the key. When the ignition switch is turned to the OFF position, it basically kills primary power to the coil and stalls the engine. Unless the coils are getting 12V power from somewhere else, you have a bad ignition switch. Also by disconnecting battery cable your vehicle should continue to run from power provided by your alternator and should not shut down. If you have a remote start system on your car this could also be the cause so do your research and let us know how this problem is solved.

New or used? How much was the part? Thanks, glenmore I got a used one, the MAF sensor and two hood struts cost me $14.00 (Yes, fourteen bucks comparing to the prices i've seen on the internet which exceeds $1,200.00) This seem to be a needed item in the LS400 community. I visited five junk yards, looked at over 13 cars and found it on the very last one I checked. The MAF sensor and the IACV was already removed from all the other LS400.

Forum, I got a MAF sensor and installed it on my car, check for codes and start it up. The car is back to normal and is driving just fine. Thanks for all the assistance and I am very excited to have it back on the road.

Forum, I got a MAF sensor and installed it on my car, check for codes and start it up. The car is back to normal and is driving just fine. Thanks for all the assistance and I am very excited to have it back on the road.

Forum, I got a MAF sensor and installed it on my car, check for codes and start it up. The car is back to normal and is driving just fine. Thanks for all the assistance and I am very excited to have it back on the road.

There goes another happy ending to a LS problem. Randy sure seem happy and it's because of everyone's effort. This forum is a great place.

Ok, so I am finally back on the East coast and jumped right into fixing this problem. I purchased an OBD I&II scanner and hooked it up to the diagnostics port and it gave me a code 31 (MAF sensor). Took the sensor off and did some more cleaning and even open it to see the setup, too complicated for me so I re-assembled and installed only to find the same problem. I've now noticed that the car in burning rich (light blue smoke and that rich smell of gasoline) Tried to drive it and got the same problem so I will be on the market for a new MAF sensor and see if the problem goes away. I changed the IACV also since I had an extra one sitting around. I'll post the final results when I get and install the MAF sensor. Thanks for all your help.