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Annoying shaking during deceleration
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toyota-v8
New User
May 8, 2024, 7:45 AM
Post #1 of 6
(744 views)
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Annoying shaking during deceleration
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Hi fellow enthousiasts!
The car is a 2008 Lexus LS600h with a 5.0 V8 (2UR-FSE), 100k miles.
This car drives great! Only not in one condition, and that is during deceleration, in this case the car starts vibrating, most noticable at highway speeds. Brakes are not operated, only foot off the gas.
No codes are present.
Fuel trims at idle, cruising and during mild/hard acceleration are near 0. Fuel trims at deceleration are -10% LTFT and -10% STFT (both cyl. banks).
Strange thing is that while STFT and LTFT are -10%, AF sensors report a lean mixture (3.4 - 3.5V, or am I misinterpretating this?). During all other conditions, AF reports 3.2-3.3V.
Car reports "Multi Cylinder Misfires" (10 every second) during decel. Individual cilinders stay at 0.
Vibration is only present when fuel pump duty is greater then 0 during decel. (car is injecting fuel). Car has dual injection system (port and direct), during vibration direct injection is used (I am not able to test it with port injection, (not sure if scan tool is capable to do this, and if it is allowed by the ecu)).
When disconnecting/connecting battery, the vibration is gone for a few drive cycles. LTFT builds up in negative and after some time the vibration reappears.
I am not sure why the ECU is injecting fuel during deceleration, maybe the hybrid nature of the car is causing it.
What has been done?
- MAF sensors replaced by genuine Toyota.
I was using OEM MAF sensors (Hella) before, that seemed to trick the ECU to run more rich. This took away the vibration, the car was not injecting fuel in this case during decel. for some reason. However, this lead to the car reporting a "lean condition" during idle (Fuel trims +25%). When replacing the OEM's with brand new Toyota ones, the "lean condition" was resolved, but the vibration reappeared and the car started injecting fuel again during decel. - just as with the old (Toyota) MAFs.
- Sparks and coils replaced;
- Cleaned throttle body;
- Checked for vacuum leaks (not found).
- Checked VVTi(e) system and timing chain (no issues found);
- Replaced eCVT transmission oil and transfer box oil.
Dealers and specialists have not been able to find the problem.
If more info is needed, I would be happy to connect the computer and collect all the data needed.
(This post was edited by toyota-v8 on May 8, 2024, 8:03 AM)
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Hammer Time
Ultimate Carjunky
/ Moderator
May 8, 2024, 8:56 AM
Post #2 of 6
(731 views)
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Re: Annoying shaking during deceleration
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This doesn't make much sense at all. I don't know why you are blaming the engine at all if this only happens on deceleration. The engine is pretty much out of the picture at that point.
My best guess on this is you are dealing with a bad inner C/V joint that is binding on decel. They usually show up more during acceleration but a force in either direction can bring it out.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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toyota-v8
New User
May 8, 2024, 9:33 AM
Post #3 of 6
(728 views)
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Re: Annoying shaking during deceleration
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Hi Hammer Time,
Thanks for your reply! We've already looked at C/V joints. I am convinced the issue is caused by a bad air/fuel mixture, or bad behavior of the ECU (why should it inject fuel anyway). Because the issue is only present when the ECU is injecting fuel during deceleration. I know this is weird, but this is realy the case. Fuel pump duty is 7% and scan tool shows trims and injectors being active during decel, foot off the gas. I think this may be nature of The hybrid system. When fuel is not injected, engine is off (hybrid) or trans is put in manual gear (higher rpms, no fuel injected) , the issue is NOT present during decel. This make a trans/drive train issue less suspectible.
I am thinking maybe a bad throttle body TPS could be the issue. Car may think fuel is needed due to throttle position is above fully close? I don't think it is caused by the gas pedal position, because issue is present at (adaptive) cruise control as well.
I will check TPS value during decel and vibrating, compared to decel and non vibrating. It doesn't make sense sometimes fuel pump is active and sometimes fuel pump is off during decel. I would suspect it to be always off during decel. However other LS600's show the same issue during decel. Some heavy vibration (as mine), some less. In heavy causes, multi cyl misfires count up more rapidly.
(This post was edited by toyota-v8 on May 8, 2024, 9:35 AM)
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Hammer Time
Ultimate Carjunky
/ Moderator
May 8, 2024, 10:35 AM
Post #4 of 6
(717 views)
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Re: Annoying shaking during deceleration
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This is a case where a little bit of knowledge can be dangerous.
Your TPS and throttle body theory is just ridiculous. This is a drive by wire car which means it has dual, opposite TPS's. If at any time they are not in synch with each other it will set a code and the computer limits it to 1/2 throttle and if it loses one completely, it shuts down the throttle all together.
If you are determined to pursue that theory I'll just close this question and move on.
The one thing you haven't even looked at is the Inverter assembly. I'm not well versed on that part of the system but I have owned a Lexus Hybrid and I know they recharge on decel and the issue could be there.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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toyota-v8
New User
May 8, 2024, 11:56 AM
Post #5 of 6
(709 views)
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Re: Annoying shaking during deceleration
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Ok, forget about the TPS in that case. Invertor, I thought about it too. However, it recharges as well when decelerating while fuel pumps are off or when engine is completely off. In those scenarios there is no vibration. A problem with the invertor assembly would not match these negative fuel trims I think. Or is -10% st and lt in these conditions normal? And what about the misfires? It makes sense they are not real misfires but just fluctuations in rpm caused by the tranmission/invertor whatever. I dont know, its a Sh!tty problem where nobody seems to have an answer.
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Hammer Time
Ultimate Carjunky
/ Moderator
May 8, 2024, 12:02 PM
Post #6 of 6
(707 views)
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Re: Annoying shaking during deceleration
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You just aren't getting the answer you want.
I advised you to forget about fuel trims. They have nothing to do with your problem. Since you are determined to keep pushing that scenario, then there is no point in continuing this any further here.
I'm closing this question now.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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