The definitive guide to fixing SC400 hesitation issues
03-17-11, 05:07 PM
#17
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wow thank you so much for this thread...i thought i lost all hope about the hesitation issue.
Just when i fixed the white smoke issue now my car hesitates. Damn these SC400
Just when i fixed the white smoke issue now my car hesitates. Damn these SC400
03-20-11, 12:26 AM
#19
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Nice info. Having a LS400 doing the same thing. Stumbles real bad around 2K rpms. Checked TPS, Coils, Changed coolant sensor for gauge and the other one(fan control maybe). Replaced the capacitors in teh ECU and still doing and throwing code 41. Will go the the bypassing the FP ECU next.
03-20-11, 04:34 AM
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I just did the trans mod on the speed sensor. i cut wire from ecu to trans clutch speed sensor and installed a switch. i have to retrain ecu and was wondering the best way to do it? and should i do it with carin normal mode or with the switch engaged cutting out the speed sensor input to the ecu?
03-20-11, 05:27 AM
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I just did the trans mod on the speed sensor. i cut wire from ecu to trans clutch speed sensor and installed a switch. i have to retrain ecu and was wondering the best way to do it? and should i do it with carin normal mode or with the switch engaged cutting out the speed sensor input to the ecu?
03-23-11, 04:40 PM
#22
OK, lets talk about what ultimately fixed my issue and is actually the easiest thing to check. The fuel pump ECU.
MIne was intermittent. Sometimes the ECU operated properly, sometimes it didn't. From what I can guess at low speed the pump was putting out less than the needed pressure, basically starving the engine. It idled nice. So maybe the ECU was not increasing the speed as needed. I don't know. Regardless my problem is gone with the pump operating in high pressure mode all the time.
So if you find this is your problem and you want a permanent fix you can;
- go on eBay and buy a used fuel pump ECU ($80-$100)
- bypass the fuel pump ECU
I chose to bypass at least for the time being. My understanding is that the multi-speed logic was only for pump life. YOur fuel system is designed to operate at high speed and the fuel pressure regulator just releases the unused fuel back to the tank.
MIne was intermittent. Sometimes the ECU operated properly, sometimes it didn't. From what I can guess at low speed the pump was putting out less than the needed pressure, basically starving the engine. It idled nice. So maybe the ECU was not increasing the speed as needed. I don't know. Regardless my problem is gone with the pump operating in high pressure mode all the time.
So if you find this is your problem and you want a permanent fix you can;
- go on eBay and buy a used fuel pump ECU ($80-$100)
- bypass the fuel pump ECU
I chose to bypass at least for the time being. My understanding is that the multi-speed logic was only for pump life. YOur fuel system is designed to operate at high speed and the fuel pressure regulator just releases the unused fuel back to the tank.
A little background info about bypassing a Toyota (Lexus) Fuel Pump ECU.
I don't know if this also applies to the LS400 or not, but when considering this option as a possibly permanent solution here's something to strongly consider...
I wanted to mention that the Twin-Turbo Supra community was all into this exact "mod" 10-15 years ago as one of the preferred free performance upgrades because they were able to increase the performance of their cars by doing this but over time they found that it drastically shortened the life of the stock Denso fuel pump on any Supra that had run the "12 Volt Fuel Pump Mod" for several years. Most Supras that retained their stock fuel pump ECU wiring instead of doing this "mod" are still running fine with their original factory fuel pump whereas most owners that chose to do that mod have needed a new pump at least once over the years, if not twice.
So given the choice of the minor performance gains it offered them versus killing their fuel pump very prematurely, most Supra owners who had done this decided to go back to running their car through that ECU instead of bypassing it, or chose not to do that mod if their car's Fuel Pump ECU was still wired properly.
So although this may seem to fix your hesitation, I suspect that for the long term it's a bad idea to leave it that way unless you like replacing fuel pumps.
I also suspect that you possibly have another issue and that this mod is simply a band-aid that's masking the symptoms of that other issue, but anything's possible...
I've had a bad stock Fuel Pressure Regulator or a bad vacuum line to the regulator on other Toyota vehicles cause either continuous or random low fuel pressure issues which wasn't apparent at idle but resulted in hesitation issues and the "12 Volt Fuel Pump mod" increased the pressure enough to compensate for the actual problem, giving the impression that the Fuel Pump ECU must be bad when it wasn't.
I've also seen a partially clogged fuel filter on a high mileage car or low mileage car that ran cheap gas all its' life end up getting the partial clogging overcome by the extra pressure of the "12 Volt fuel pump mod" and making things seem normal again and pointing to that ECU as a possible problem while the pump was jumpered to run at 12V all the time...
But then again, many of the electrical gremlins and numerous ECU related problems (which I'm finding that the LS400's seem to have a lot of) are either a rarity or are totally reliable without question on any other Toyota I've worked on over the years. So normally I would swear to you that the FP ECU simply does not fail on Toyota vehicles but I suppose it IS possible that the 1UZ's Fuel Pump ECU might actually be prone to failure after all. But I'm still inclined to doubt that until many LS400 owners report it as a failure...
I know the LS400 is not a Supra, but it IS a Toyota product.
So although I'm pretty new to the LS400 and 1UZ world I'm relying on my decades of experience with all 4 generations of Toyota Supras, 2nd gen Celica's, and 2nd/3rd gen Camry's and the 2 Lexus IS300's I've owned over the years when it comes to working on my "new to me" 94 LS400.
Other than an intermittent main ECU failure in my LS400 which is common in LS400's but super rare in Supras, so far applying my overall I4 and RWD I6 powered Toyota troubleshooting experience to this car is working out really well for troubleshooting the various old age issues my LS400 has once I finally figure out this whole V8 layout thing and figure out all the new places they moved stuff to on this car...
Last edited by zukikat; 03-23-11 at 04:44 PM.
03-23-11, 05:10 PM
#23
Nice info. Having a LS400 doing the same thing. Stumbles real bad around 2K rpms. Checked TPS, Coils, Changed coolant sensor for gauge and the other one(fan control maybe). Replaced the capacitors in teh ECU and still doing and throwing code 41. Will go the the bypassing the FP ECU next.
Did you try a different main ECU in this car yet?
Someone else has a thread about having a nagging TPS code and hesitation problem and replacing the TPS with a new one which didn't help and then sending their main ECU off to get serviced twice with no joy from that either until a desperate ebay purchase of a used main ECU actually cleared the code solved their problems iirc.
My car behaves noticeably different overall to me on the ECU that I put in it versus how it behaved on the ECU that was in the car when I bought it.
Even the tranny shifts differently now, smoother and much less agressive, waaaaay too slow on WOT acceleration though (slips the clutches now) which it wasn't doing before and I'm sure it's this other ECU doing that...
My car is still not quite right overall on this other used ECU but I've put almost 3k on the car in a month so far on this used ECU and it's totally drivable as-is (city AND highway commuting!) so I'm just rolling with it for the time being 'till I decide where to buy an ECU from next or this used ECU takes a dump too, whichever comes first...
It's still got a high idle (1,200 in P or N at all times) and it's running a little rich at all times and there's an occasional light momentary stumble on moderate accel requests or during sudden major decel on the interstate but it no longer goes from running fine and shifting smooth but crisp and agressively to bucking and stumbling or barely accellerating at all or flat out threatening to leave me stranded on the side of the road for no apparent reason and then back to "normal" again moments or minutes later without throwing any trouble codes since I swapped main ECU's in my car.
I definitely liked the other ECU's tranny shifting and throttle response behaviors a lot better but at least this ECU makes the car totally drivable again as-is and I never knew when the other one was going to leave me stranded...
03-23-11, 05:28 PM
#24
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Please pardon the length of this post!
A little background info about bypassing a Toyota (Lexus) Fuel Pump ECU.
I don't know if this also applies to the LS400 or not, but when considering this option as a possibly permanent solution here's something to strongly consider...
I wanted to mention that the Twin-Turbo Supra community was all into this exact "mod" 10-15 years ago as one of the preferred free performance upgrades because they were able to increase the performance of their cars by doing this but over time they found that it drastically shortened the life of the stock Denso fuel pump on any Supra that had run the "12 Volt Fuel Pump Mod" for several years. Most Supras that retained their stock fuel pump ECU wiring instead of doing this "mod" are still running fine with their original factory fuel pump whereas most owners that chose to do that mod have needed a new pump at least once over the years, if not twice.
So given the choice of the minor performance gains it offered them versus killing their fuel pump very prematurely, most Supra owners who had done this decided to go back to running their car through that ECU instead of bypassing it, or chose not to do that mod if their car's Fuel Pump ECU was still wired properly.
So although this may seem to fix your hesitation, I suspect that for the long term it's a bad idea to leave it that way unless you like replacing fuel pumps.
I also suspect that you possibly have another issue and that this mod is simply a band-aid that's masking the symptoms of that other issue, but anything's possible...
I've had a bad stock Fuel Pressure Regulator or a bad vacuum line to the regulator on other Toyota vehicles cause either continuous or random low fuel pressure issues which wasn't apparent at idle but resulted in hesitation issues and the "12 Volt Fuel Pump mod" increased the pressure enough to compensate for the actual problem, giving the impression that the Fuel Pump ECU must be bad when it wasn't.
I've also seen a partially clogged fuel filter on a high mileage car or low mileage car that ran cheap gas all its' life end up getting the partial clogging overcome by the extra pressure of the "12 Volt fuel pump mod" and making things seem normal again and pointing to that ECU as a possible problem while the pump was jumpered to run at 12V all the time...
But then again, many of the electrical gremlins and numerous ECU related problems (which I'm finding that the LS400's seem to have a lot of) are either a rarity or are totally reliable without question on any other Toyota I've worked on over the years. So normally I would swear to you that the FP ECU simply does not fail on Toyota vehicles but I suppose it IS possible that the 1UZ's Fuel Pump ECU might actually be prone to failure after all. But I'm still inclined to doubt that until many LS400 owners report it as a failure...
I know the LS400 is not a Supra, but it IS a Toyota product.
So although I'm pretty new to the LS400 and 1UZ world I'm relying on my decades of experience with all 4 generations of Toyota Supras, 2nd gen Celica's, and 2nd/3rd gen Camry's and the 2 Lexus IS300's I've owned over the years when it comes to working on my "new to me" 94 LS400.
Other than an intermittent main ECU failure in my LS400 which is common in LS400's but super rare in Supras, so far applying my overall I4 and RWD I6 powered Toyota troubleshooting experience to this car is working out really well for troubleshooting the various old age issues my LS400 has once I finally figure out this whole V8 layout thing and figure out all the new places they moved stuff to on this car...
A little background info about bypassing a Toyota (Lexus) Fuel Pump ECU.
I don't know if this also applies to the LS400 or not, but when considering this option as a possibly permanent solution here's something to strongly consider...
I wanted to mention that the Twin-Turbo Supra community was all into this exact "mod" 10-15 years ago as one of the preferred free performance upgrades because they were able to increase the performance of their cars by doing this but over time they found that it drastically shortened the life of the stock Denso fuel pump on any Supra that had run the "12 Volt Fuel Pump Mod" for several years. Most Supras that retained their stock fuel pump ECU wiring instead of doing this "mod" are still running fine with their original factory fuel pump whereas most owners that chose to do that mod have needed a new pump at least once over the years, if not twice.
So given the choice of the minor performance gains it offered them versus killing their fuel pump very prematurely, most Supra owners who had done this decided to go back to running their car through that ECU instead of bypassing it, or chose not to do that mod if their car's Fuel Pump ECU was still wired properly.
So although this may seem to fix your hesitation, I suspect that for the long term it's a bad idea to leave it that way unless you like replacing fuel pumps.
I also suspect that you possibly have another issue and that this mod is simply a band-aid that's masking the symptoms of that other issue, but anything's possible...
I've had a bad stock Fuel Pressure Regulator or a bad vacuum line to the regulator on other Toyota vehicles cause either continuous or random low fuel pressure issues which wasn't apparent at idle but resulted in hesitation issues and the "12 Volt Fuel Pump mod" increased the pressure enough to compensate for the actual problem, giving the impression that the Fuel Pump ECU must be bad when it wasn't.
I've also seen a partially clogged fuel filter on a high mileage car or low mileage car that ran cheap gas all its' life end up getting the partial clogging overcome by the extra pressure of the "12 Volt fuel pump mod" and making things seem normal again and pointing to that ECU as a possible problem while the pump was jumpered to run at 12V all the time...
But then again, many of the electrical gremlins and numerous ECU related problems (which I'm finding that the LS400's seem to have a lot of) are either a rarity or are totally reliable without question on any other Toyota I've worked on over the years. So normally I would swear to you that the FP ECU simply does not fail on Toyota vehicles but I suppose it IS possible that the 1UZ's Fuel Pump ECU might actually be prone to failure after all. But I'm still inclined to doubt that until many LS400 owners report it as a failure...
I know the LS400 is not a Supra, but it IS a Toyota product.
So although I'm pretty new to the LS400 and 1UZ world I'm relying on my decades of experience with all 4 generations of Toyota Supras, 2nd gen Celica's, and 2nd/3rd gen Camry's and the 2 Lexus IS300's I've owned over the years when it comes to working on my "new to me" 94 LS400.
Other than an intermittent main ECU failure in my LS400 which is common in LS400's but super rare in Supras, so far applying my overall I4 and RWD I6 powered Toyota troubleshooting experience to this car is working out really well for troubleshooting the various old age issues my LS400 has once I finally figure out this whole V8 layout thing and figure out all the new places they moved stuff to on this car...
03-23-11, 05:39 PM
#25
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Out of curiosity, it's been a few weeks since I read this whole thread and I forget...
Did you try a different main ECU in this car yet?
Someone else has a thread about having a nagging TPS code and hesitation problem and replacing the TPS with a new one which didn't help and then sending their main ECU off to get serviced twice with no joy from that either until a desperate ebay purchase of a used main ECU actually cleared the code solved their problems iirc.
My car behaves noticeably different overall to me on the ECU that I put in it versus how it behaved on the ECU that was in the car when I bought it.
Even the tranny shifts differently now, smoother and much less agressive, waaaaay too slow on WOT acceleration though (slips the clutches now) which it wasn't doing before and I'm sure it's this other ECU doing that...
My car is still not quite right overall on this other used ECU but I've put almost 3k on the car in a month so far on this used ECU and it's totally drivable as-is (city AND highway commuting!) so I'm just rolling with it for the time being 'till I decide where to buy an ECU from next or this used ECU takes a dump too, whichever comes first...
It's still got a high idle (1,200 in P or N at all times) and it's running a little rich at all times and there's an occasional light momentary stumble on moderate accel requests or during sudden major decel on the interstate but it no longer goes from running fine and shifting smooth but crisp and agressively to bucking and stumbling or barely accellerating at all or flat out threatening to leave me stranded on the side of the road for no apparent reason and then back to "normal" again moments or minutes later without throwing any trouble codes since I swapped main ECU's in my car.
I definitely liked the other ECU's tranny shifting and throttle response behaviors a lot better but at least this ECU makes the car totally drivable again as-is and I never knew when the other one was going to leave me stranded...
Did you try a different main ECU in this car yet?
Someone else has a thread about having a nagging TPS code and hesitation problem and replacing the TPS with a new one which didn't help and then sending their main ECU off to get serviced twice with no joy from that either until a desperate ebay purchase of a used main ECU actually cleared the code solved their problems iirc.
My car behaves noticeably different overall to me on the ECU that I put in it versus how it behaved on the ECU that was in the car when I bought it.
Even the tranny shifts differently now, smoother and much less agressive, waaaaay too slow on WOT acceleration though (slips the clutches now) which it wasn't doing before and I'm sure it's this other ECU doing that...
My car is still not quite right overall on this other used ECU but I've put almost 3k on the car in a month so far on this used ECU and it's totally drivable as-is (city AND highway commuting!) so I'm just rolling with it for the time being 'till I decide where to buy an ECU from next or this used ECU takes a dump too, whichever comes first...
It's still got a high idle (1,200 in P or N at all times) and it's running a little rich at all times and there's an occasional light momentary stumble on moderate accel requests or during sudden major decel on the interstate but it no longer goes from running fine and shifting smooth but crisp and agressively to bucking and stumbling or barely accellerating at all or flat out threatening to leave me stranded on the side of the road for no apparent reason and then back to "normal" again moments or minutes later without throwing any trouble codes since I swapped main ECU's in my car.
I definitely liked the other ECU's tranny shifting and throttle response behaviors a lot better but at least this ECU makes the car totally drivable again as-is and I never knew when the other one was going to leave me stranded...
03-24-11, 12:03 PM
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i wish you lived by me Brad.
This car has so many things that could cause my hesitation and vibration issues. Its hard to pinpoint what it is and replacing things one by one is putting a dent in my wallet.
My car gets treated so much better than me and the day i decide to forget everything about the car then i know it will die on me
This car has so many things that could cause my hesitation and vibration issues. Its hard to pinpoint what it is and replacing things one by one is putting a dent in my wallet.
My car gets treated so much better than me and the day i decide to forget everything about the car then i know it will die on me
03-27-11, 04:11 PM
#27
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i wish you lived by me Brad.
This car has so many things that could cause my hesitation and vibration issues. Its hard to pinpoint what it is and replacing things one by one is putting a dent in my wallet.
My car gets treated so much better than me and the day i decide to forget everything about the car then i know it will die on me
This car has so many things that could cause my hesitation and vibration issues. Its hard to pinpoint what it is and replacing things one by one is putting a dent in my wallet.
My car gets treated so much better than me and the day i decide to forget everything about the car then i know it will die on me
03-27-11, 04:22 PM
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I was really tired of having to turn on my SC with my foot on the gas pedal just to keep it running long enough to warm up. I decided that since I am not really a gear head, and after a few bucks spent on a complete overhaul of plugs, dist cap, plug wires, rotators, air filter etc, to take it to a pro. The ecu was wacky and needed reprogramming. You guys might not understand how wonderful it is (tears well up in my eyes) to have your car start up like its brand new! An ECU check up should be considered if all maintenance has been done without success. Just my 2 sense in the matter.
03-27-11, 04:50 PM
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I was really tired of having to turn on my SC with my foot on the gas pedal just to keep it running long enough to warm up. I decided that since I am not really a gear head, and after a few bucks spent on a complete overhaul of plugs, dist cap, plug wires, rotators, air filter etc, to take it to a pro. The ecu was wacky and needed reprogramming. You guys might not understand how wonderful it is (tears well up in my eyes) to have your car start up like its brand new! An ECU check up should be considered if all maintenance has been done without success. Just my 2 sense in the matter.
03-28-11, 10:23 AM
#30
Great Write-up. I have a question - would a bad fuel pump ECU cause low idle in Drive? Any comments are greatly appreciated.
Last edited by JOEYD2008; 03-28-11 at 10:25 AM. Reason: Eliminate QUote