unstable tps-reading in idle

[joscorstjens]

The idle speed of my engine is not stable, sometimes it floats around 1000 rpm, sometimes around 900. So I measured the resistance of the tps with an ohm-meter. The fixed resistor part is stable but the variable part has a variation from 20 up to 100 ohms, depending on the way you release the accelerator pedal (gentle or suddenly). [You can see this on mega-logviewer as an idle tps-value floating between -2 and 0.]

How difficult is it to replace the tps (on an injected 2.0 pinto engine)? Is there some other way of fixing this?

[trakkies]

It's more likely to be the mechanics of the throttle mechanism not returning to the stop each time.

[joscorstjens]

So it sounds like a 'worn' spring? Where could I possibly find the partnumbers of the springs involved?

[kjones6039]

It's more likely to be the mechanics of the throttle mechanism not returning to the stop each time.

Here I go agreeing with Dave again! (which i seem to do a lot)

So it sounds like a 'worn' spring? Where could I possibly find the partnumbers of the springs involved?

I would not suspect springs. If the spring is adequate to return the throttle it should be fine. Most any auto parts store can provide throttle return springs that will work with your application.

I would be looking for a worn throttle shaft/bore (very common in my experience), or any other mechanical problem that is causing the throttle to not return to a consistent position.

Ken

[904svo]

Make sure your wiring to the TPS is correct. If the 5 volts is wired wrong it will cause crazy reading at idle.

[kjones6039]

IMHO, incorrect wiring will cause problems across the board. Not just at idle.

Ken

[joscorstjens]

wiring is correct as the problem only exists near idle and a worn shaft would also exhibit unstability across the board (I admit, this is more difficult to detect)

[kjones6039]

I am certainly not offering my comments as the only solution. From 50+ years of experience though, I do know that loose/worn throttle shafts are common. It should be very easy to detect and eliminate mechanical problems prior to going to the expense and effort of changing the TPS.

Ken

[trakkies]

wiring is correct as the problem only exists near idle and a worn shaft would also exhibit unstability across the board (I admit, this is more difficult to detect)

Not so - the percentage error would be much greater with the throttle near closed.

[904svo]

Make sure that the TPS pot has very little play between the shaft and the pot. Some TPS's do not have a return spring in them so if there is
play between the shaft at the pot your TPS reading will change at idle.