1JZPWRD

iTrader: (59)

I have noticed the last three days or so, that when I let off of the gas at ANY amount or speed, it bucks or shakes the whole car for about 1 or 2 seconds. It does it everyime I let off of the gas or I want to just keep very light pressure on the gas pedal. Its worse around 40 mph, but noticeable at all speeds. The last kinda problem I had simliar to this, was that 2 of my rear axle bolts were loose going into the rear end. I retightned them back up and it quit for about 3 months. Well I thought the problem was the same, but its not, ALL of my rear end axle bolts are tight.

I have done some research on here leading to throttle body, MAF, 02 sensors, ignition coil, etc. I dont think its any of these. The car runs smooth and doesnt miss or anything. I am thinking its something else under the car somewhere!

I was leaning towards the driveshaft has play in it, maybe the rubber grommet is going out, or the rear end is going out AGAIN. I am on the 2nd one now. The car has 177K, 1992 SC400.

Any help or area to check, please chime in, its growing very old.

ch1ck3n

This is probably a fuel system related. Have you try to search for fuel mod? https://www.clublexus.com/forums/per...tures-faq.html
Maybe that will solve your problem.

Also check for hoses for leaks around the engine bay. That's all I can think of for now.

1JZPWRD

iTrader: (59)

I will look more into to today or tomorrow. Can someone explain to me why POSSIBLY doing the above mod would help my symptoms. The problems that I have read is start then die, hard starts, strong gas smell on start ups, etc. NOT any signs of car bucking when letting off throttle at any speed. I did notice a some sort of filter under the throttle body is missing. I dont know what it is, but I have seen where it has a black cap over it to keep the filter in place. I had to search and search to find a 400 on here to see what its supposed to look like. I wonder if it controls some of the air fuel ratio mixture to cause the bucking. Again, its directly below the throttle body behind the TRAC control sensor, beside the TPS.

Any more thoughts?

1JZPWRD

iTrader: (59)

I just returned from the test drive from driving my 400. I cleaned the MAF, adjusted the trac and the TPS, and blocked off the opening part of where I was missing a black cap of some sort. Its just under the TRAC and TPS with a black cover over some small filter. It likes the EGR filter.

Anyway, first start up, Trac light stayed on. Revved a few times to 4K and it sputtered a bit. I was thinking cause it was cold, been sitting overnight. Turned the car off, and restarted it immediately and the same results. Let it sit for about 30 minutes, and went for a test drive. It bucked and kicked, like a Texas rodeo, but then I floored it a few times up to about 80-90 mph. It's back to driving like a Lexus again. Its smooth, idles good, no trac light, and no more bucking when you left off of throttle at any speeds. I did read a thread under "hesitation" on here, to try and floor and let the computer learn your driving habits again. Well it worked, I hope it lasts a while. Under this thread listed above, people did the fuel mod, EGR block off plate, coils, plugs, wires, and caps, and still had the problem. There was a few that still believed it was in the MAF. After my cleaning and adjusting, its silky smooth again. You might want to clean the MAF and give it a try if your car has similar symptoms.

Ali SC3

iTrader: (10)

you could have a vacuum leak. thats why your car will drive horribly until you beat on it and force the ecu to adjust for the extra air.

The ecu knows how much air is going in via the idle air valve / throttle body from the maf sensor reading. If its getting air from anywhere else the ecu will not be able to calculate the fuel accurately and will result in jerking until it learns the trim it needs to arrive at the real amount of air. This is most noticeable at idle because the least amount of air is flowing past the maf so any unmetered air plays the largest percentage it can under these conditions. throw in the extra vacuum created by engine decel at 40mph which sucks in even more air and your ecu has no idea what to do at first.

If It starts giving you the same symptoms in a couple of days you will know. Also when the ecu does drastic compensations like that to correct for a leak it may cure up the initial symptom, yet create other symptoms like running richer or leaner in other areas. The stock tune is safe so its probably nothing to worry about.

I would suggest taking a picture of the thing you capped off and maybe the whole throttle body area. also check all the vac lines. the Idle air control valve should not have a filter on it, it should be attatched to the intake pipe, for your sake I hope the thing you are talking about is not the Idle valve.
If you are talking about the little round black thing that has a little piston sticking out the bottom of it and a vacuum line on the top (where a cap and filter also is) that is called a throttle opener, and its purpose is to crack the throttle plate under high vacuum conditions like decel to burn off any extra fuel, results in smoother decel and better emissions. IF yours is totally shot, that could also give you a similar symptom. sometimes they just need to be set to oem spec along with the throttle plate, some mechanics needlessly touch the throttle stop screws to "fix" stuff.

1JZPWRD

iTrader: (59)

The black cap that I am talking about is just under/behind the TRAC and TPS. If you remove the cap, there is a round filter, looks just like the EGR filter on top. Actually the cap for the EGR looks the same way as this cap, but just a bit smaller. I placed the filter back in there when I was adjusting everything.

It does make sense about revving it up and letting the ECU compensate. I will know more in a few days and post back of the symptoms, regardless of what I have found. I will also check for a vaccuum leak as well. The only way I know how to check that is to spray carb cleaner or WD-40, while its running, and listen for any change in the idle. Is there another simpler way to check for a leak. I have changed all of the rubber lines on top that I could reach over a year ago. Some of them seem loose to me, not like snug like the factory ones did. Is there a correct size that I need to look for? I will buy some tomorrow on the way home and replace all of them again. Thanks for the advice, Ali SC3.

Laslty, I did notice today when pulling the air assembly out of the car, that the flex tube, just behind the MAF, has a small crack, but its behind the clamp. I dont think its leaking there, but will check again tomorrow.

Ali SC3

iTrader: (10)

I am pretty sure you are talking about the throttle opener which has a little spongey thing inside and should also have a cap. you should have the cap on it too as it will release too fast without the cap and sponge in place. this could be part or the problem. Im a 2jz guy so im not so familiar with the v8 throttle body but it sounds like the throttle opener.

1JZPWRD

iTrader: (59)

I will try and post what it is exactly. I remember reading about somewhere on here. Thanks for your help. I will try and find a vaccuum leak this afternoon.

shern

iTrader: (6)

I had similar issues in the past on an LS400. The culprit ended up being completely worn diff bushings that would allow the diff to rock back and forth when load was let off of it. If the feeling is mechanical, I'd start with them and then move to the tranny and motor mounts.