Throttle Position Sensor?

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I'm buying a 91 Plymouth Laser this Wednesday. It has the 4G63 and the 5 Speed. Ok heres the problem, the guy says it needs a Throttle Position Sensor and thats what I think too, but I thought I would ask everyone anyways. So heres what its doing, at idle it revs up and down evenly from 900 rpm to 1500 rpm. So are we right or on Crack? Thanks in advance.

posted by  x1/9-rally

I would think it more likely that the car has a vacuum leak somewhere or it needs a new IAC unit (Idle Air Control). When the car's at idle, the TPS signal isn't changing.

posted by  Bino

Maybe it is changing? There are so many things that make an engine hunt, but you could be right about the IAC, or it could be a bad sensor or signal, etc

posted by  Wally

I guess that could be the case (I considered it), it seems pretty unlikely but still within the realm of possibility.

posted by  Bino

A couple of months ago my car "died" and I had it looked at by a mechanic. They put on a new cap and rotor and fixed it. I also had the same idle problem as you, and they said that it was the IACV (idle air control valve). I never had it fixed because they were going to charge about $300 for the part. The problem actually went away for about three weeks, then came back for another week, then went away again. I still haven't changed it, it doesn't really do any damage to the car, its just really annoying. But I'm pretty sure its the same problem as yours.

(Mine's an '88 Intregra)

posted by  Mathew

:2cents: if the IAC is bad your car will idle high( in some cases) , sounds to me like a vacuum leak check all of your Vacuum lines/hoses you can also check the gasket seal around your intake manifold by spraying some carb cleaner directly at it going around the entire parimeter if possible ( the idle will drop briefly) guess what, you found the leak :thumbs: .

posted by  wrxsti2000

This is your third post and the misinformation campaign has already begun. A defective IAC may cause the idle to hunt, it may cause the idle to be high, it may cause the idle to be low, and in some instances and situations may not affect idle quality at all. Furthermore, a vacuum leak may also cause a high or low idle depending on where and what size the leak is, and using the carb cleaner approach might raise or lower said idle.

You are apparently well meaning, but you "help" really isn't. If you know as much as you claim to...
... then you would know that different situations require different approaches. You need to think out all possibilities before offering advice.

posted by  vwhobo

i had the same problem with a 92 TSi that i used to own. never really figured it out though. guess it could be the IAC.

posted by  glagon1979

Do you guys know where the IAC is?

posted by  x1/9-rally

On the throttle body.

posted by  Bino

Thanks :thumbs:

posted by  x1/9-rally

Ok so I bought the car and had some time to play with it. I replaced the IAC and it still did the same thing. So I messed with it some more and pulled off the wire to the soleniod thats on the other side TPS on the thottle body (I belive it raises the idle when the motor is cold) and it stopped reving up and down and started idling at 2000 rpm and when I grounded that wire it would rev up and down. So I replaced the soleniod and it did the same thing. And when the fans kick on the rpms drop to 900 and it idles normal. Any ideas?

posted by  x1/9-rally

hm, i remember my car used to do something like this. i dont know right now if it will keep doing it since im removing my ISC (Idle Speed Control, thats how its called in my car). yet, when it was cold, it would raise up to around.... maybe 1,500 rpms for a minute. then it would drop down to 800-1,200 and play around for a while. after it warmed up though, it calmed down at an almost even 980-1,000rpms.


im not sure if it applies to all cars when they age a bit, but mine used to do that due to being too cold when starting sometimes, and it just wanted to bitch a bit.


might even be a lack of voltage (or too much of, though i doubt that) through the cables for the sensors (have u cleaned them btw?).


another thing to consider, and i say this since it happened to my friend when he bought his 93 eclipse with the 1.8 sohc engine. the previous owner had tempered with the ECU quite a bit, and it caused the car to act somewhat cracked up. it has recieved a whole array of new sensors, and still tends to idle a bit rough (though it idles WAY better than when he first bought it). if nuthin else works, u could get the ECU checked, just out of curiousity, ya know?

posted by  Inygknok

If the replacement of the Idle air control valve does not work replace the map sensor.

posted by  napaman2486

Before replacing anything else (though MAP is a likely suspect) check all your vac lines. Most are made out of the crappiest crap plastic that get brittle and break super easy. Always check the obvious and cheapest first. And spraying carb cleaner *as said above* is a good idea just to rule out a failed manifold gasket.

posted by  TokyoJoe

TPS could be out of adjustment. On my car it's two screws (which are a huge PITA to get to). We loosened them and slowly turned the TPS until the check engine light started flashing (which indicated the ECU was getting a good signal I guess). Then it ran fan after that, used to do the whole 800-1700rpm thing over and over and drove me crazy.

posted by  TanyasMkIISupra

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