Power Loss, Loud Exhaust Noise When Kickdown Engaged

[dpellet]

Hi All,

Stil trying to figure this one out. ANy comments will be greatly appreciated. Car is a 91 ls400. This is a sporadic problem that has gotten less sporadic and more normal as time goes on. Car operates fine until I engage the kickdown switch, in the past it simply got up and went very quitely and powerfully. Now it kicks down and makes a god awful amount of exhaust noise as it appears and steadily but slower than normal picks up speed. The power loss occurs only when I engage the kickdown and sometimes it does not do this at all and acts just like her old self.

This one has had me scratching and reading every blog and forum I can find to try and correct the situation. Otherwise the car runs flawlessly, I have done a complete tune up, cleaed throttle body and plenum and intake runners etc... Checked the egr tube out and no cracks or leaks there.

Has anyone had this happen to them and if so what steps did yo take to correct it. No codes are thrown when this occurs and when the initial kickdown occurs and winds out to the next gear the noise stops and acceleration returns to normal...

Help!!

[steve2006]

It causes nearly every other weird fault so maybe this one too, check the drivers side trunk hinge wiring ( there is a loom going around it, open the loom carefully and check each wire for damage in particular the white wire)

[stcist2]

It causes nearly every other weird fault so maybe this one too, check the drivers side trunk hinge wiring ( there is a loom going around it, open the loom carefully and check each wire for damage in particular the white wire)

It sounds like you might have a bad engine/tranny mount allowing the engine to tilt when under hard acceleration, when the engine moves its possible that its opening the exhaust system, hence the noise.. try "power braking" when your in drive at a stop and if you hear the exaust get louder or feel the engine rock( you might even be able to see the engine move if you have the hood open ) youv'e found the problem

[steve2006]

It causes nearly every other weird fault so maybe this one too, check the drivers side trunk hinge wiring ( there is a loom going around it, open the loom carefully and check each wire for damage in particular the white wire)

It sounds like you might have a bad engine/tranny mount allowing the engine to tilt when under hard acceleration, when the engine moves its possible that its opening the exhaust system, hence the noise.. try "power braking" when your in drive at a stop and if you hear the exaust get louder or feel the engine rock( you might even be able to see the engine move if you have the hood open ) youv'e found the problem

Good point I never thought of that, if the engine moves it may have fractured the exhaust and as it rocks the fracture opens up more producing more noise, less back pressure and therefore less power.

Have you tried reving the car up high in neutral or park to see if you get the same noise from the exhaust?

[dpellet]

It causes nearly every other weird fault so maybe this one too, check the drivers side trunk hinge wiring ( there is a loom going around it, open the loom carefully and check each wire for damage in particular the white wire)

It sounds like you might have a bad engine/tranny mount allowing the engine to tilt when under hard acceleration, when the engine moves its possible that its opening the exhaust system, hence the noise.. try "power braking" when your in drive at a stop and if you hear the exaust get louder or feel the engine rock( you might even be able to see the engine move if you have the hood open ) youv'e found the problem

Good point I never thought of that, if the engine moves it may have fractured the exhaust and as it rocks the fracture opens up more producing more noise, less back pressure and therefore less power.

Have you tried reving the car up high in neutral or park to see if you get the same noise from the exhaust?

Thanks for the replies, Fixed the trunk wires a while ago and I did double check them the other day and all was well. Changed the tranny mount a few months back perhaps its time to get the motor mounts done as well.. I never thought about this senario althought I did have the mechanics change the gaskets in front of the cats a few months back as well. It's odd that it is so sporadic, the mount idea is worth checking out.

Thanks again, I'll update once there in.

Cheers. Any other thoughts in the interim would be great.

;)

[17LexusOwner]

Once your done checking those ideas I have a question for ya.

When you say loss of power, do you mean that your car isn't shifting down like normal when you stomp on the gas? Or is it more like it shifts down but you have less power for accelerating than you used to?

[dpellet]

Once your done checking those ideas I have a question for ya.

When you say loss of power, do you mean that your car isn't shifting down like normal when you stomp on the gas? Or is it more like it shifts down but you have less power for accelerating than you used to?

The latter, and because the noise is so loud, leaking exhaust like and directed towards the passenger foot area right where the cat is (likely need to replace the darn gaskets again :{). me thinks the motor mounts are indeed the likely culprit. As long as I stay off the kickdown the car is just like new no noises and plenty of torque. Power is lost when that exhaust noise appears and this only happens when the kickdown is engaged, shifts down fine and back up fine but on initial shift down the noise is terrible and goes away as soon as it shifts up with power returned.

My Regards!

[17LexusOwner]

replace em then! :)

[dpellet]

got the new motor mounts, changed them out, bit of a bugger to get to the drivers side mount but patience and persistence pay off. Excess noise and powerloss now gone!!!!

No vibration between 2000-3000 rpm either. Both mounts were shot, tops were seperated completely.

Many thanks to those who posted there ideas. stcist2, you rock, I've been trying to figure this out for months.

This car is now completely restored, full tune up, starter, pw steering all changed out, brakes/rotors all around, timing belt, water pump, idlers, new frnt & rear bumpers, alternator, wiring (trunk) all fixed, cluster bulbs all changed, stereo bulbs all changed (boy was this fun), alignment & new bushings for sway bars, all fluids changed, strut bar added, throttlebody, plenum & intake removed and de-carboned, motor & tranny mounts new, stainless brake lines. Everything on this car works as new.

Gee, you think there's anything else I should look after? EGR delete is my next task, still in good shape but me thinks the lower part of the egr tube is gonna give at some point in the near future.

It's taken almost 2 years part time to restore this fine automobile and I was able to drive it the whole time the thing would never leave me high and dry.

Gotta say, if your in the market for one of these either be willing and able to do the common issue work yourself or verify the common issue work has been completed otherwise you will not have a great experience with this car. Otherwise as they say this car is one of the best ever built, luxurious ride, sturdy, reliable & good guts for a small 8.

Thanks again & Cheers!