rbert

I have changed out caps/rotors//plugs with OEM Lexus parts and didn't make a difference so it's not that. The failure of a sensor does not always give codes, later OBD II (96 and on) seem to give more information on problems. OK ,so why does this car run so good with the mass airflow sen. disconnected?

rbert

I changed caps/rotors and plugs with factory OEM parts, sos. not the problem......

VSsc400

iTrader: (1)

rbert. I merged your threads since they were all related to the hesitation problem.

akforte

i pulled the following after searching on google. This is for another car - maybe GM, but who cares I think this may be the answer to our problems. If this is it you guys are buying me a beer!! Looks like it may be the speed sensor. See my comments after the excerpt.

Excerpt form site:

http://www.salemboysauto.com/faqs/d...?Action=Q&ID=28

" 2. As your are driving down the road, the computer is watching your throttle pressure, your engine speed, your vehicle speed in mph, what gear you are in and a/c operation. As you approach 38-42 mph, if you are on level ground, if you are feathering the throttle, if the engine is warmed up, if the car is in third gear, the computer will electronically lock up the torque converter inside the transmission which makes you have direct drive, so to speak.

On this day, the driver brings the car into a shop and complains the engine is bucking and missing . What is happening is the computer sees your throttle pressure stay at 32%, sees the engine rpms stable, but sees the reported speed at 36 then a second later the computer sees 43, then 38, then 41, then 37. The computer would be confused. How can the speed be fluctuating and the engine isn’t and the drivers throttle pressure isn’t changing either? As the speed is reported above 38 mph, the torque converter clutch (TCC) is locked and when the speed is seen below 38 mph, the computer unlocked and disengages the TCC. This rapid locking and unlocking of the TCC feels EXACTLY like an engine miss to the driver. But the computer THINKS the speed sensor is going wacky so it will set the SES light to tell the driver that it sees something wrong.

In this case, the speed sensor was replaced by 3 different shops, except the SES light kept coming on, the same speed sensor code was set and the engine bucked or missed violently when the speed approached 38 to 42 mph. Clearly, the speed sensor wasn’t the problem.

In this case, what finally fixed this car was when we pulled the speedo cable and lubed it. You see when the computer saw the speed vary, it was because the speedo cable was binding up, then releasing causing the speed sensor to report this "speed up, slow down" or 38-41-36-42-34-39 speed reading. Each time the reported speed exceeded 38 the TCC was locked, each time it dropped below 38, the TCC was unlocked causing the bucking or missing. You see, the computer can only guess on the repair, it really can not tell the tech if the speed sensor is bad or the wiring harness connection is loose or bad or if the speedometer cable is jerky and needs to be lubed. "

Now I don't think the speedo cable needs to be lubed in our case as it's probably electronic. Somebody help me out on this one, isn't there a seperate speed sensor that the trans uses for it's reference? I really think we're on to something

akforte

Try this(idle air control motor)

rbert

I think you may be on to to something. I drove with a co-worker yesterday and he seemed to think it could be trans related. I know torque converters are a problem on these cars. The IAC was one of the first things I checked. I tested it per the service manual (powering certain pins makes it drive open/closed) I could never get it to run per the book. I was thinking the tests in the book were wrong being it seems to operate (turn A/C on and and rpm increases ect.) It's also suppose to make a clicking noise when you shut the engine off, I think it drives the IAC open but I have never herd any clicking. I guess these do get dirty and bind up according to a Lexus service guy. I did clean mine, it wasn't dirty. I think IAC is about 600 bucks. According to him, the only problems he's seen on SC400s are IAC and TPS. I will look into this more. Thanks for the info.

UpInTheLex

iTrader: (5)

After getting my exhuast yesterday I noticed all of my hesitation is under the stock stall speed of 1900 rpms, after that 2K rpm and above no hesitation what so ever. I race my car a lot also. I think a fin or two is damaged in my TC, I have a Lextreme one 2800 stall speed that i need to install, so Once I get it in there I'll know for sure, but thats what i'm leaning towards.

akforte

I have the sameeeeeee problem ,let get to bottom of this problem. :egads

rbert

Anyone know of a Lexus tech rep I can talk to? Obviosly the two Lexus service centers I have been to have no clue. Pretty sad..........Well I'm going to stop there, I just wrote a book and deleted it. The moron factor is alive and well.

akforte

rbert did you find prob

rbert

I pulled the EGR valve off, opps checked it per Lexus MM and it wouldn't run. Took it apart and found the rotor cracked all the way through. Funny thing is with this failed, it's spring loaded closed and the hesitation problem seems like it's an excessive EGR condition. I can't believe someone hasn't figured this hesitation problem out.

rbert

Oh yea, disconnected the EGR connector, and it runs fine, so im going to make a block off plate and re-connect it and see how it does. Any other thoughts?

UpInTheLex

iTrader: (5)

Oh so the EGR valve is broken and you're going to disconnect it for good? Sounds good. I'm going to go out and take a look, is it hard to get to or remove? I could make a block off plate at my University I bet.

akforte

How to get to the EGR valve ,is this the prob. pic well help .

rbert

Might be part of the problem. Mine doesn't work at all. I still think theres another problem. Im going to block it off and see if anything changes.