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Posted by Micah in NC [Email] (more from Micah in NC) on Mon, 12 Jan 2004 09:08:24 Share Post by Email
In Reply to: Figure out this one!, Kevin A, Mon, 12 Jan 2004 08:02:18
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Kevin,

Hi. I don't have any firsthand experience with the 2.1L motor, but I have read a bit about it, and might know of an item that could cause such a problem for you...

In short, it might be the TPS. If that sounds like Greek, read on and hopefully I can clear it up...

The 2.1L motor has the LH 2.4.2 fuel injection system and this incorporates a "Throttle Position Sensor" (tells fuel injection computer how much the throttle is opened) versus the older "Throttle Position SWITCH" of the 2.0L engines (CIS, LH 2.2 and LH 2.4 systems).

The difference between the two is that the older "switch" simply tells the fuel injection computer if throttle is: 1) closed, or 2) more than 72 degrees wide open, not anything about HOW MUCH open the throttle is.

Now, to make it confusing, both the sensor and the switch terms get interchanged all the time. And both are known by the "TPS" acronym.

Anyway, the TPS(ensor) on 2.1L motors is a potentiometer (from what I understand through a bit of reading, a "pot" is a variable resistor, giving variable resistance (ohms) in the electrical circuit). A known voltage is sent through the "pot" and makes its way to the fuel injection computer; this known voltage is altered by the resistance in the TPS, depending upon the angle the throttle valve is opened. So, a higher or lower the resistance is in the TPS, .

If the TPS is dirty or worn, it might have a "bad spot" where it would not provide the correct resistance at certain point(s) along the throttle opening range (like at the partly open throttle position you describe).

My Mom's old '88 Jeep Cherokee 4.0L 6-cyl. motor uses a similar setup. I haven't isolated this as the culprit, but it seems logical, since the TPS was replaced several years ago and the problem stopped after that (only to return of late). Her Jeep is acting the same way you describe your '91 900 problem.

That said, all of the above is purely supposition on my part, because I haven't worked on a 2.1L motor.

However, if you decide to test your TPS with a (digital) ohmmeter, I have a 1992 Chilton's Car Care Manual which has the specs on resistance for the TPS on a 2.1L engine. I think it gives specs for throttle closed (idle position) and WOT (wide open throttle). However, those figures might match your TPS, even if your TPS was faulty at that part-throttle opening where your trouble is occuring. You would check for this "bad spot" by slowly opening the throttle (ohmmeter connected, of course) and watching the meter for either a sudden zero or infinite resistance reading. If you see either of those, then the TPS either needs to be cleaned (douse it w/carb cleaner?) or replaced.

--Micah in NC
'84 900S, 249K mi.

posted by 65.80.14...
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