SN95 Accelerating On Its Own?

Does the car have a high idle or hanging idle? Is the throttle pedal or cable sticking?

Is this an auto or 5spd?

I would start by trying to reset the idle and getting it down to where it should be from the factory. Maybe @jrichker can give you the reset procedure and provide a few more things to look at in regards to your high idle. The high idle is likely responsible for what you are describing

Why don't you try gassing the throttle cable from the engine bay at the throttle body and see if the cable is sticking. This is a dangerous situation, if so. I am surprised by the ignorance of the people driving the toyotas and lexuses that ended up dieing because of sticking throttles, but I just bring that up so that you think about how you would handle the situation if it happened to you. It should be easy: manual = push in clutch, put tranny in neutral, steer/brake to a safe stopped place, and THEN kill the engine (the rev limiter should protect the motor). Auto, same procedure after shifting the gear select into neutral.

The likely culprit is a TPS that has the creeps. TPS sensors have a bad habit of starting out with the voltage at normal range .5- 1.0 volt and creeping upward. When this happens, the idle speeds up until it maxes out. Turning the ignition switch off and back on when it is creeping resets the computer and the process will slowly start all over again.

The tip off is to put the transmission in neutral when it care is accelerating by itself and see where the idle RPM is. If it is higher than normal, then the TPS has problems. Replacing it is the only cure.

Setting the TPS: you'll need a good Digital Voltmeter (DVM) to do the job. Set the TPS voltage at .5- 1.1 range. Because of the variables involved with the tolerances of both computer and DVM, I would shoot for somewhere between .6 and 1.0 volts. Unless you have a Fluke or other high grade DVM, the second digit past the decimal point on cheap DVM’s is probably fantasy. Since the computer zeros out the TPS voltage every time it powers up, playing with the settings isn't an effective aid to performance or drivability. The main purpose of checking the TPS is to make sure it isn't way out of range and causing problems.

The Brown/White wire is the VREF 5 volts from the computer. You use the Gray/white wire (TPS signal) and the Black/White wire (TPS ground) to set the TPS. Use a pair of safety pins to probe the TPS connector from the rear of the connector. You may find it a little difficult to make a good connection, but keep trying. Put the safety pins in the Dark Green/Lt green wire and Black/White wire. Make sure the ignition switch is in the Run position but the engine isn't running.

Here’s a TPS tip I got from NoGo50

When you installed the sensor make sure you place it on the peg right and then tighten it down properly. Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can move it just a little in very small increments. I wouldn’t try to adjust it using marks.

(copied from MustangMax, Glendale AZ)

A.) Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and get it nice and warm.

B.) When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other to ground.

C.) Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the battery lead for 10 minutes.

D.) The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.

The TPS is a variable resistor, must like the volume control knob on a cheap radio. We have all heard them crackle and pop when the volume is adjusted. The TPS sensor has the same problem: wear on the resistor element makes places that create electrical noise. This electrical noise confuses the computer, because it expects to see a smooth increase or decrease as the throttle is opened or closed.

TPS testing: most of the time a failed TPS will set code 23 or 63, but not always. Use either an analog meter or a DVM with an analog bar graph and connect the leads as instructed above. Turn the ignition switch to the Run position, but do not start the engine. Note the voltage with the throttle closed. Slowly open the throttle and watch the voltage increase smoothly, slowly close the throttle and watch the voltage decrease smoothly. If the voltage jumps around and isn’t smooth, the TPS has some worn places in the resistor element. When the throttle is closed, make sure that the voltage is the same as what it was when you started. If it varies more than 10%, the TPS is suspect of being worn in the idle range of its travel.

TPS will not go below 1 volt

Note: Make all resistance checks with the ignition switch in the OFF position. Failure to do so will result in incorrect results and may possibly damage the meter.

Engine mounted sensor circuit: Check the resistance between the black/white wire on the TPS and battery ground. It should be less than 1 ohm. Higher resistance than 1 ohm indicates a problem with the 10 pin connector, computer or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire.



See the graphic for the location of the 10 pin connectors:
Diagram courtesy of Tmoss & Stang&2birds



See the graphic for the 10 pin connector circuit layout.


Unplug the white 10 pin connector to do some resistance testing. It is good time to clean the connector pins and examine the connector for corrosion, broken wire or other damage. See http://oldfuelinjection.com/?p=85 for help in this department.

If the resistance on the TPS Black/White wire and pin 1 of the white engine fuel injector harness 10 pin connector, is more than 1.0 ohm, you have bad connection or broken wiring. Repeat the test using the pin 1 of the white body side 10 pin connector and battery ground. You should have less that 1 ohm. More than that is a damaged signal ground inside the computer or bad connections or wiring.[/b]

Just note that the above from jrichker is for the '93 and earlier engines. Yours is a '94 so the TPS colors are the same except for the ground which is gry/red. TPS cannot be adjusted on the '94 and the spec range is from .65v - 1.25v. Also confirm that the reference voltage is 5v. If it's not remove and clean the battery to engine and engine to chassis grounds. This may help with the wiring.

dubbsix said:

Hi

Auto.

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Hey