1992 Mercury XR2 (Mazda 323 drivetrain).
Engine starts and idles perfectly. Stumbles and sputters when accelerator is pressed quickly and won’t rev up. When accelerator is pressed very slowly engine revs up properly. Was running perfectly yesterday. What went wrong?
Check the resistance of the throttle position sensor while operating the throttle by hand.
If the resistance jumps all over the place while doing so, the TPS needs replacing.
Tester
If you find uneven change in resistance, as Tester explained, you can first try electronics parts cleaner spray in the TPS. It worked for me and I avoided having to replace the TPS.
You might also clean out your idle air control valve (IACV). Even if it’s not a problem - it’s pretty easy and it never hurts to clean stuff!
The IAC valve is out of play once the accelerator is depressed.
Tester
Yes. It’s supposed to be. But if it’s sticky, then it can … well, be sticky.
Whatever the hell you typed?
Tester
Say what you want. I’ve had a sticky/sluggish IACV cause acceleration stumbles. I just gave a cheap and easy maintenance item that wouldn’t hurt
Lighten up King mechanic.
Is the car drivable when accelerating slowly? Or is this while in park/neutral?
The fuel pump might be weak, measure the fuel pressure while accelerating.
Possible MAF, unplug it and see if it helps any with acceleration, the MAF sensor can make older cars act all kinds of strange sometimes…
Well, it’s a 1992 car. Does it have a carburetor? If so, the accelerator pump might need tweaking. That was the cause of hesitation back in the carb days.
Throttle body
Thanks! Sounds reasonable. I’ll see if I can do that. It’s been over 100 degress here for the past few days and I don’t have a shop. Think I’ll drive my other car until it cools down a bit.
Yes another possibility. Pressing down slowly allows the oxygen sensor to correct for the lean condition.
It acts exactly like a accelerator pump problem. But it has port fuel injection, one injector located in each intake manifold leg where the manifold bolts onto the cylinder head.
Wouldn’t you know it? I started the engine today and it runs perfectly, at least in my yard. I’m afraid to take it out on the road. Maybe I’ll try it early tomorrow morning when I’ll have plenty of time to tow it home if necessary.
We had a storm the night before the problem occured. The car leaks a little water into the passenger compartment. Maybe the computer got wet. I don’t even know where the computer is. But this car has been out in the rain for years with no problems.
Thanks to all of you for your thoughts and recommendations. I’ll report back if there’s any news.
A simple vacuum leak could cause that symptom as well. Do you notice any split or disconnected vacuum hoses in the engine compartment? If your car uses vacuum actuators for the hvac, another common source for a vacuum leak. This may or may not turn the check engine light on. Is it on? A problematic EGR system is something else to consider.
The vacuum hoses show signs of age but none that I’ve seen are broken. I don’t hear any hissing sounds like a vacuum leak.
I’ve wondered about the check engine light. I don’t recall ever having seen one. If there is one it’s not on now, or maybe the bulb is burned out. I also wonder if there’s a diagnostic connector and if a reader can be plugged in to check for trouble codes. Unfortunately I don’t have a shop manual. I need to investigate this further because I seem to recall that the car has a standard Ford engine control computer. If that’s true it should have a check engine light that can be used to blink trouble code numbers. I’ll have to check into that. Thanks for reminding me.
This provides instructions how to retrieve the OBDI codes from your ford, and their definitions.
Tester
That brings back a lot of memories…