Fuel cut? afm trouble? transmission control isue? i'm in need of help

[Gnarles_Manson]

Hi, this is my first tech post, hope i'm doing everything according to hoyle.
I've got an 86.5 n/a automatic, 104,000 miles on the odo, and when its up to operating temp, and i'm cruising under light throttle, i get a quick "buck" or hesitation, then it's fine for awhile sometimes, sometimes not. If i vary my speed between 50 and 60 mph , never staying at one constant speed, it doesnt do it.
Sounds like a TPS? I purchased a new one and have adjusted it per tsrm specs and also tried my own adjustment wherein i set the idle switch to be very sensitive to throttle movement (trying to rule out the possibility of the idle switch being actuated under light throttle ,possibly cutting fuel).
When the described symptom occurs, i dont notice any fluctuation of the tachometer.
I am so frustrated with this car, and very hesitant to throw any more money at it without some solid diagnosis of the problem. Transmission fluid is clean, trans shifts very well, torque converter events seem correct according to throttle position, kickdown is beautiful. The engine performs great.
if its of any help to those who read this, here is a list of things i have done to the car:
new TPS
new coolant temp sensor
new O2 sensor
new distributor, cap, rotor, plugs, wires
new valve cover gaskets
new air filter
new fuel filter
added an intake elbow to replace mine that cracked
added an MSD streetfire ignition, blaster 2 coil, and 8910 tach adapter (problem existed before adding the aftermarket ignition)
No trouble codes reported for engine or transmission.
The only way i can seem to get the jerking activity to stop is to unplug the TPS, leaving me with poor performance and a barely functional transmission.

-I am forever indebted to anyone who can help me out here

 

[Gnarles_Manson]

Edit- performed resistance test on tps terminals VTA and E2, found 4k-5k instead of .2-1.2 per tsrm. Tried to fine tune that value, and found that it would spike to 15k with very slight movement, idle switch still closed. Reinstalled worn OE sensor and readjusted. I've put on 30 miles so far without the jerking symptom. Looks like maybe that original engine management brand tps wasnt such a good deal. Any veteran supra battlers out there want to confirm or deny my suspicions before i fork out big bucks for a toyota tps?

 

[Nick M]

There are two sweeping potentiometers on the car. The TPS is one, and it is an important one, but a correction factor. The other is the airflow meter. It is possible to have a worn spot on the resistor, but I would think you need more miles to have wear like that.

You can check both with a scope. Do you know what a sweep test is? You are looking for a flat spot. This is assuming the car is in working order as you testify is.

 

[Gnarles_Manson]

Thanks for the reply , Nick. I dont have access to a scope, only a digital multimeter, so its a real pain trying to test a potentiometer of any sort.
The replacement TPS (the one in question) felt much lighter and of cheaper construction than the OE unit. In retrospect ,the replacement tps seemed to only cure the driveability issue for a couple days or so. Theoretical auto repair, gotta love it. I read a post somewhere on this site (sorry no link) stating that an irregular input to the ecu from the afm could cause fuel cut, wondering if the same principle applied to the tps.
-Edit
I think the jump from 4-5k to 15k and right back down would be evidence of a flat spot or manufacturing defect in the tps, may buy an analog meter this weekend to help with this, but my results with the digital were very repeatable.

 

[Nick M]

If the idle contacts close, you will send the ECU mixed signals, and it won't know what to do. And that does not set a code 51. Of course the idle contacts can't close with your foot on the loud pedal, but you might have a wiring problem at the connector.

 

[Gnarles_Manson]

thanks nick, that confirms what i thought understood about the function of the idle contacts. I tried setting the questionable tps so that the idle contacts would open if you even gave the throttle a dirty look, and same thing occured (the jerk) . Tried setting it so the idle contacts were always open, too, same symptom, just higher idle. The irregularity of the signal sweep on E2 and VTA terminals makes me now suspect that the sensor is just a pile of crap. Putting the original (old) sensor back in and just doing a quick adjustment of the idle switch made the symptom dissapear so far. I didnt try adjusting the old sensor before i relpaced it, i just assumed it would be a good thing to try because the shift points and shift quality were all weird (for lack of better terminology) and the jerking symptom came and went mysteriously. The jerking symptom got worse after installing and properly adjusting (hell even improperly adjusting) the new tps.

 

[Nick M]

I would make sure it is plugged in good, and nobody has ever messed with wiring before you. On a side note, this air flow meter turns on the fuel pump. You won't stall if the switch loses continuity, it would have to hang open and run out of fuel. But maybe it is failing, and you are just getting the begining stages. I will say that the fuel pump switch isn't common as far as I know.

 

[Gnarles_Manson]

that's something else i've pondered.the connector is questionable, its cracked up a little from heat, and the metal bail is missing from it, but you have to give it a tug to get it to come off of the sensor. any idea if toyota sells a new pigtail for the tps?
My shift quality is still goofy with the old TPS so I guess it's going to need changed anyhow?

-edit (damned bunch of editing i'm doing here)
I'm pretty sure the afm is functioning properly, i should probably bring it in the house and test it to confirm. must get an analogue meter!
Thanks so much for all of your input, Nick, sometimes it just takes a little spark to get someone kickstarted who has exhausted their scope of a problem.

 

[Gnarles_Manson]

Its been almost a week with the old worn out TPS installed and adjusted, and the bucking/jerking has completely vanished. I will report back once i get the new toyota TPS. Hope this post helps someone with a similar issue. Lesson learned; even newly installed parts should be questioned.

 

[Gnarles_Manson]

Installed new Denso (toyota) TPS today, problem solved. Beware of aftermarket TPS's.