91 Ls400 Idle Problem Is Back

[Quack11]

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.

[curiousB]

Just when you think you have it solved.... Murphy is alive and present.

Well maybe the clue is the low fuel tank. Maybe some water or contaminant got sucked into fuel pump and has clogged fuel filter and/or injectors. A clogged fuel filter would be progressively worse at higher engine loads though so if the problem goes away on the highway it likely isn't the fuel filter.

I had a car several years ago that had a leaky injector that caused an emissions failure. I guess the injector wouldn't shut off so at low speeds was making the car run too rich (at least one cylinder) at idle.

They make fluids to pour in your tank to eliminate the effects of water in the tank. I think its just alcohol but you might want to throw a dose of that in the tank.

Could you have some air/vacuum leak in the throttle body or connecting hoses/couplings that is causing air to bypass the MAF sensor? If a vacuum leak bypasses the MAF the ECU doesn’t sense the true air arriving at the cylinders and will make improper air fuel settings. I’ve heard you can test this with a propane torch. Just turn the torch on very low setting and don’t light the flame. Move the torch around suspected leak areas. If there is a leak the engine should speed up a bit when the propane is consumed in the leak. Obviously you must do this test outdoors and be careful. Propane is heavier than air and will fall to the ground and collect if you do it in a garage. That can lead to a dangerous boom… or worse.

[VBdenny]

It could be some bad gasoline. Was it a name brand (top tier) station? Some of the cheap places load the fuel up on alcohol which runs terrible in cars.

[Quack11]

It could be some bad gasoline. Was it a name brand (top tier) station? Some of the cheap places load the fuel up on alcohol which runs terrible in cars.

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"?

[curiousB]

I am looking at what the ECM is telling all the sensors to do!!!!!!!! Any suggestions, ideas or even "what if"?

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.

[Quack11]

I am looking at what the ECM is telling all the sensors to do!!!!!!!! Any suggestions, ideas or even "what if"?

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.

[curiousB]

OK then I think the IACV investigation is a red herring then. It only impact engine while idling and if you can't get engine to run at more open throttle it isn't IACV to blame.

Since you don't have ECU codes it doesn't sound like a sensor problem as the ECU should be able to diagnose that so rule out MAF, temp sensor, throttle position sensor, ... for now.

So what's left;

1) Fuel system. Well your 2.5 gallon test seems to prove that fuel filters and fuel pump aren't plugged or you wouldn't have good flow. What you don't know is if the fuel gets to the proper pressure (pressure regulator faulty or pump worn and can't get to pressure level. If you have a fuel pressure gauge or could borrow one from an auto parts store you could double check pressure of fuel to rule out this leg.

2) Injectors. Could be faulty but given an eight cylinder engine one faulty injector wouldn't cause the dramatic performance you are seeing. Seems highly unlikely more that one would fail at a time.

3) Ignition. These cars are know to have problems with dual banks of distributors where one side can fail causing a loss of 4 cylinders. Could be faulty coil, frayed and shorting wiring to chassis/block. Why not disconnect one coil and start. If runs rough but similar (IE no worse) to before then reconnect and disconnect other side. if won't start at all then maybe first bank is dead (or vice versa) for either of coil, wiring, rotor, ....

4) ECU. You've confirmed ECU is functioning although can't tell if 100%. I guess an output could be dead but seems less likely.

5) Wiring. Harnesses with broken wires or corroded connectors. Could be a root cause for (3) above.

[Quack11]

OK then I think the IACV investigation is a red herring then. It only impact engine while idling and if you can't get engine to run at more open throttle it isn't IACV to blame.

Since you don't have ECU codes it doesn't sound like a sensor problem as the ECU should be able to diagnose that so rule out MAF, temp sensor, throttle position sensor, ... for now.

So what's left;

1) Fuel system. Well your 2.5 gallon test seems to prove that fuel filters and fuel pump aren't plugged or you wouldn't have good flow. What you don't know is if the fuel gets to the proper pressure (pressure regulator faulty or pump worn and can't get to pressure level. If you have a fuel pressure gauge or could borrow one from an auto parts store you could double check pressure of fuel to rule out this leg.

2) Injectors. Could be faulty but given an eight cylinder engine one faulty injector wouldn't cause the dramatic performance you are seeing. Seems highly unlikely more that one would fail at a time.

3) Ignition. These cars are know to have problems with dual banks of distributors where one side can fail causing a loss of 4 cylinders. Could be faulty coil, frayed and shorting wiring to chassis/block. Why not disconnect one coil and start. If runs rough but similar (IE no worse) to before then reconnect and disconnect other side. if won't start at all then maybe first bank is dead (or vice versa) for either of coil, wiring, rotor, ....

4) ECU. You've confirmed ECU is functioning although can't tell if 100%. I guess an output could be dead but seems less likely.

5) Wiring. Harnesses with broken wires or corroded connectors. Could be a root cause for (3) above.

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.

[91LS400pilot]

OK then I think the IACV investigation is a red herring then. It only impact engine while idling and if you can't get engine to run at more open throttle it isn't IACV to blame.

Since you don't have ECU codes it doesn't sound like a sensor problem as the ECU should be able to diagnose that so rule out MAF, temp sensor, throttle position sensor, ... for now.

So what's left;

1) Fuel system. Well your 2.5 gallon test seems to prove that fuel filters and fuel pump aren't plugged or you wouldn't have good flow. What you don't know is if the fuel gets to the proper pressure (pressure regulator faulty or pump worn and can't get to pressure level. If you have a fuel pressure gauge or could borrow one from an auto parts store you could double check pressure of fuel to rule out this leg.

2) Injectors. Could be faulty but given an eight cylinder engine one faulty injector wouldn't cause the dramatic performance you are seeing. Seems highly unlikely more that one would fail at a time.

3) Ignition. These cars are know to have problems with dual banks of distributors where one side can fail causing a loss of 4 cylinders. Could be faulty coil, frayed and shorting wiring to chassis/block. Why not disconnect one coil and start. If runs rough but similar (IE no worse) to before then reconnect and disconnect other side. if won't start at all then maybe first bank is dead (or vice versa) for either of coil, wiring, rotor, ....

4) ECU. You've confirmed ECU is functioning although can't tell if 100%. I guess an output could be dead but seems less likely.

5) Wiring. Harnesses with broken wires or corroded connectors. Could be a root cause for (3) above.

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.

I am glad you finally found the problem. Why not attach the PDF in your next posting?

[curiousB]

Great stuff. As for the parts save them for down the road or I'm sure ebay would be a way to get a good portion of the purchase price back. Whatever haircut you take in the unneeded parts is surely less than the labor you saved and you can't put a value on the personal satisfaction of getting it right yourself.

[Quack11]

Great stuff. As for the parts save them for down the road or I'm sure ebay would be a way to get a good portion of the purchase price back. Whatever haircut you take in the unneeded parts is surely less than the labor you saved and you can't put a value on the personal satisfaction of getting it right yourself.

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

[Quack11]

Great stuff. As for the parts save them for down the road or I'm sure ebay would be a way to get a good portion of the purchase price back. Whatever haircut you take in the unneeded parts is surely less than the labor you saved and you can't put a value on the personal satisfaction of getting it right yourself.

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?

[curiousB]

ouch, the second return of Murphy. Well it seems its an intermittent given the come and go nature of the problem.

Back to basics. Do you have enough fuel volume from pump to fill 2.5 gallon container still. If that still works with latest pump then you are 1/2 checked. Filter must be ok and volume good. Pressure is another matter and you can only test that with a fuel pressure guage. Fuel delivery has to meet volume and pressure specs. You need to borrow a guage or buy one to see if problem is fuel pressure related. If I were to guess given that you've tried two fuel pumps and still have problems my bet the problem is elsewhere. Highly suspicious two pumps have identical problem with engine. Really need pressure guage to see if proper pressure exists when engine misbehaving. Last thought here, do you have a bad fuel pump relay? Again pressure guage would clarify this.

On to ignition. If the idea that one distributor bank is going dead leaving you with only a 4 cylinder this can be tested. Get the motor into its poor running state. Then disconnect center wire of one rotor (or back at coil) and see if it gets worse or stays the same. Then reconnect center wire and do the same to the other rotor. The point here is if one of the distributors is dead due to bad coil or short somewhere then disconnecting wire to bad rotor shouldn't make any difference whereas disconnecting wire to a good rotor would cause serious engine stumbling (or maybe shut down as if you take the four good cylinders away then you are left with no working ignition).

[Quack11]

ouch, the second return of Murphy. Well it seems its an intermittent given the come and go nature of the problem.

Back to basics. Do you have enough fuel volume from pump to fill 2.5 gallon container still. If that still works with latest pump then you are 1/2 checked. Filter must be ok and volume good. Pressure is another matter and you can only test that with a fuel pressure guage. Fuel delivery has to meet volume and pressure specs. You need to borrow a guage or buy one to see if problem is fuel pressure related. If I were to guess given that you've tried two fuel pumps and still have problems my bet the problem is elsewhere. Highly suspicious two pumps have identical problem with engine. Really need pressure guage to see if proper pressure exists when engine misbehaving. Last thought here, do you have a bad fuel pump relay? Again pressure guage would clarify this.

On to ignition. If the idea that one distributor bank is going dead leaving you with only a 4 cylinder this can be tested. Get the motor into its poor running state. Then disconnect center wire of one rotor (or back at coil) and see if it gets worse or stays the same. Then reconnect center wire and do the same to the other rotor. The point here is if one of the distributors is dead due to bad coil or short somewhere then disconnecting wire to bad rotor shouldn't make any difference whereas disconnecting wire to a good rotor would cause serious engine stumbling (or maybe shut down as if you take the four good cylinders away then you are left with no working ignition).

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.

[Quack11]

ouch, the second return of Murphy. Well it seems its an intermittent given the come and go nature of the problem.

Back to basics. Do you have enough fuel volume from pump to fill 2.5 gallon container still. If that still works with latest pump then you are 1/2 checked. Filter must be ok and volume good. Pressure is another matter and you can only test that with a fuel pressure guage. Fuel delivery has to meet volume and pressure specs. You need to borrow a guage or buy one to see if problem is fuel pressure related. If I were to guess given that you've tried two fuel pumps and still have problems my bet the problem is elsewhere. Highly suspicious two pumps have identical problem with engine. Really need pressure guage to see if proper pressure exists when engine misbehaving. Last thought here, do you have a bad fuel pump relay? Again pressure guage would clarify this.

On to ignition. If the idea that one distributor bank is going dead leaving you with only a 4 cylinder this can be tested. Get the motor into its poor running state. Then disconnect center wire of one rotor (or back at coil) and see if it gets worse or stays the same. Then reconnect center wire and do the same to the other rotor. The point here is if one of the distributors is dead due to bad coil or short somewhere then disconnecting wire to bad rotor shouldn't make any difference whereas disconnecting wire to a good rotor would cause serious engine stumbling (or maybe shut down as if you take the four good cylinders away then you are left with no working ignition).

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.

[curiousB]

I guess it could be you have two intermittent fuel pumps but it seems a tad unlikely. Clever idea using fuel source from other car.

I would take a moment to check the fuel pump relay (PN 90987-03003 http://www.toyodiy.com/parts/p_U_1996_LEXU....html?hl=28380) and the fuel pump dropping resistor (PN 23080-50030 http://www.toyodiy.com/parts/p_U_1996_LEXU....html?hl=28380A ). These parts, if flaky, could be giving you the fuel pump problems and they were common for both pumps you tried. Maybe a bad or corroded connection or the relay contacts are burnt up some from all the open and closing of them. The relay is cheap compared to a new pump so might be worth a try. The resistor should be easy enough to test with a DVM.

Be careful around open fuel with electrical connections. A small arc could ignite fumes/vapors.

[python]

bad ecu?

[Quack11]

bad ecu?

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.

[curiousB]

ONVEIN.pdf

INSPEC.pdf

fuel_electrical_diagram.pdf

It looks like the pump resistor is to allow a two speed fuel pump under ECU control. So if you provide 12VDC from battery to pump bypassing the relay and resistor then you would be isolating the fuel pump which is what you did I think. I don't think it would be safe to leave it this way but it useful to test. Car should run and if it doesn't then it might suggest pump is a problem. Harbor Freight sells a fuel pump pressure guage kit for $17.

I attached a couple PDFs relating to 2004 year but maybe similar in this part of the design.

[Quack11]

ONVEIN.pdf

INSPEC.pdf

fuel_electrical_diagram.pdf

It looks like the pump resistor is to allow a two speed fuel pump under ECU control. So if you provide 12VDC from battery to pump bypassing the relay and resistor then you would be isolating the fuel pump which is what you did I think. I don't think it would be safe to leave it this way but it useful to test. Car should run and if it doesn't then it might suggest pump is a problem. Harbor Freight sells a fuel pump pressure guage kit for $17.

I attached a couple PDFs relating to 2004 year but maybe similar in this part of the design.

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.

[yoichisoma]

I am following this thread closely. My 1990 LS has no idle, It dies right after start and must keep foot on the gas to keep the engine idling. For now, I turned the screw on the throttle mechanism to keep the plate from closing and allowing it to idle. Now I'm looking for a real fix.

Yo

[curiousB]

I am following this thread closely. My 1990 LS has no idle, It dies right after start and must keep foot on the gas to keep the engine idling. For now, I turned the screw on the throttle mechanism to keep the plate from closing and allowing it to idle. Now I'm looking for a real fix.

Yo

I would start by cleaning throttle body and the IACV. Good chance this is all you will have to do. Search this site for detailed instructions. Its a frequent topic.

[Quack11]

I am following this thread closely. My 1990 LS has no idle, It dies right after start and must keep foot on the gas to keep the engine idling. For now, I turned the screw on the throttle mechanism to keep the plate from closing and allowing it to idle. Now I'm looking for a real fix.

Yo

I would start by cleaning throttle body and the IACV. Good chance this is all you will have to do. Search this site for detailed instructions. Its a frequent topic.

I think the problem is now fixed and hopefully "Murphy" is forever gone. Thanks CuriousB and all other for your help during this long and trying times. The problem was a combination of fuel pump and pressure regulator. Changed both after many troubleshooting steps and I am able to drive with confidence again. Pump was delivering at start up but would not continue under engine demand/load, could hear it stalling and engine would respond as such. Already completed a 170 miles trip and was able to put more than half tank of gas in the car. Glad she is back on her feet and I now have enough used parts to start buliding another LS400, i.e ECU, MAF, TPS, etc. Thanks and lets continue to assist each other.