Very rough start, idles fine, stalls as soon as I touch the pedal

1995 Mazda Protege
1.5 L
153K miles.

My car was running fine, then one day it started sputtering while driving, mostly when I accelerated. It stalled and I could not get it started. This happened on a day when it was raining hard, but not sure that had anything to do with it. A few days later when I went to pick the car up it started right up, it had stopped raining and been dry for 2 days prior. But about a mile into the drive back home it stalled and I could not get it started.

Here is what I have tested so far.

  • put in new fuel filter.
  • tested the fuel pressure and pressure regulator
  • tested the injectors with a noid light.
  • tested that the spark plugs were producing a spark.

After I pulled the plugs and cleaned and dried them (they were wet with gas) the car started, but I think not because of anything really related to the plugs. What I’ve noticed is that when cranking the engine if I give it gas it will not start but if I keep off the throttle it will reluctantly start. But once it starts the idle is smooth and as it should be. Then if I give it any gas it immediately stalls. It seems to me given these symptoms that it is getting enough air to idle but not enough air to match the increased gas.
The only CEL code that I’m getting (which I had before this issue occurred) is the “closed throttle position switch” (I forget the number).
If it was an MAF, or IAC, or EGR issue would I not get a code?

Anyway, can anyone give me advice on what this could be and how to test it. Again my guess is its an air intake issue but not sure where.

Probe the Throttle Position Switch, and check the change in voltage as you manually open and close it. The TPS is an important part of the fuel mapping system, and a bad one can throw off the fuel mix as you try to accelerate. The ODB-II system is pointing to it as a suspect.

“It seems to me given these symptoms that it is getting enough air to idle but not enough air to match the increased gas”

You’ll want to swap that around in your head. The “gas” pedal is really an “air” pedal. So when you press it down this isn’t putting more gas in. It’s putting more air in. The computer doles out the fuel. So the computer may not be feeding enough fuel to keep up with the extra air your are allowing to be pumped in when you open the throttle.

As @BustedKnuckles said, one thing that can mess up the whole thing is a TPS problem so test it. FYI: [this is generic info] the typical TPS is 3 wire. The PCM sends it a 5V signal over one wire. Another is the signal return that generally varies from about .5V (closed) to 4.5V (open). The third wire is ground. You want to check for the 5V ref, and then find out if the signal return is smooth and within appropriate range. If you find any issues, you assume a wiring problem first and don’t spend the $$ on a new TPS until you know that’s what it is.

You apparently have a throttle position sensor error code so that’s the natural place to start. Note that it might not be the sensor itself. There could be a problem with the wiring (e.g. a frayed wire shorting to ground).

Cig made a great point about reversing your thinking. When you press the pedal, the throttle directly lets more air into the engine, but its attached Throttle Position Sensor (TPS) also sends a signal to the car’s computer that you’ve pressed the pedal. The computer immediately sends a signal to the injectors to push more fuel in. It sounds to me like you’re opening the air passage but the computer isn’t getting the signal to push more fuel in.

The reason it’s smooth once it starts and then stalls when you push the gas is because once it’s running the car’s other sensors, the mass airflow sensor and/or manifold absolute pressure sensor, and the crank or cam speed sensor, and the upstream oxygen sensor can provide sufficient data for the computer to adjust the injector output (the “pulsewidth”) to the engine’s needs, but when you push the gas pedal these sensors alone will not compensate to the loss of the TPS signal, the engine will go lean, and it’ll die.

Others have already given you the answer. This is only an attempt to help you better understand the apparent contradictions in the symptoms. They aren’t really contradictions at all once the way the parts work is understood.

My shot in the dark/silver bullet guess? Replace the distributor cap and rotor and plug wires.

If the code is for closed throttle position switch, then this probably has a 6 pin TPS instead of a 3 pin TPS. It has the three wires as described above, but also a single pole double throw switch (IPS) that closes for idle position and WOT. This was common during the era this car was made.

The idle position can be broken due to a throttle not fully closing because of gum or someone adjusting the throttle stop screw like it is an idle adjustment or it can be a bad contact or broken wire.

There is also a condition with this system where the TPS losses contact only when the IPS is closed, you get a P0220 code. I had this on my 97 Nissan PU. You can play with the throttle position screw and if lucky, find a spot where you make the IPS and still have contact with the TPS. If I remember correctly it was about an eighth or quarter turn in from the factory setting. It was four years ago and still working just fine.

In this system, if the IPS isn’t making like it should, the computer never knows that it is supposed to take control of the idle speed, so it doesn’t send any signals to the IAC motor.

Thanks all for the feedback. I just tested the sensor with a multimeter. The power wire gave a reading of 5v and then I plugged the sensor in and tested the return and it read about .5 with the throttle closed and about 4.41 with WOT. So, I guess the wiring is ok. I called Oreilly’s auto parts about a sensor and it was 139 friggin dollars. I’ll be damned if I’ll pay that, found a new one online for $25, but will have to wait a few days for it to get here. Is there anything I can test in the mean time?

Cig and Mountainbike thanks for clearing up my misunderstanding about the throttle, it is clear now.

Regarding the TPS, I did research it a bit when the CEL came on and it does not seem like the TPS alone would cause the car to stall like this, and particularly would not cause the hard start. I did an experiment today with my friends car and we unplugged his TPS and the car started and ran, although the idle was a little off, and when he pushed the throttle the engine responded fine. Also the TPS was being reported bad by the system about a month before the problems with starting happened so why was in not stalling then?

So, as a humble newb I respect the feedback from you veterans but is it really likely this is a TPS issue? And what other possible causes could this be?

Thanks very much.

Keith, my TPS is a 3 pin. The error code is P0510 “Closed Throttle Position Switch”.

P0510 has nothing to do with the TPS. It is solely for an IPS. Somewhere on your throttle body, you have a separate 2 or 3 pin switch as I described. It can be located at the end of the bell crank instead of the throttle plate shaft. Look for a push button switch that the bell crank hits when in the idle position. But you are right that it should not cause the problems you are having, the usual problem is dying at lights or dying when you blip the throttle when stopped.

What does IPS stand for?

Idle Position Sensor or Idle Position Switch.