mbrickell Posted February 22, 2006 Share Posted February 22, 2006 Ok, I want to understand how the throttle works. ( 1990 Loyale FWD ) Is it correct that there is no actual link between my gas pedal and the engine? If not, am I correct that there is some kind of potentiometer like a throttle sensor on the throttle body or AFM somewhere? So, there would have to be a sensor at the gas pedal somewhere to take input from your foot, send a signal to the other potentiometer on the FI system somewhere, which would in turn send a signal to the injector how much fuel to use? I'm just trying to understand exactly how what you input with your right foot gets to the engine and makes the engine respond. The reason I ask is that my car runs real well, but there seem to be little flat spots at various points in light acceleration where the engine slightly hesitates before reaching a slightly different RPM range where things are 100% again. This is not a huge thing, like the car wanting to stall or bucking, not that blatant. This is like at a few small places along the RPM range when accelerating from a stop, or from 30-40 mph light acceleration, the car seems to struggle just a bid to have a smooth seamless acceleration curve. I am trying to understand the system to figure what might be old or worn out causing this minor stumble, if I can call it that. That is not the best way to put it. Rather, say every hundred or 200 rpm you hit a point where the car is putting out 80% effort, then back to 100% effort, like little subtle flat spots. You break through them, and you are fine. If you give heavy throttle this seems to be a lot less apparent than light throttle. Steady state cruise this is not apparent, nor at highway velocity. This reminds me of a similar situation on VW Vanagons, but I don't think the Subaru system is the same physically, but I will mention it here because it feels just like the common Vanagon syndrome. The Vanagon FI system has an air flow meter of some sort with like a grid, such as a screen door or window, in it. Across this grid there is a wiper arm with a contact against the grid at one tip. This is a lot like the "roof" when you ride bumper cars at the fair, if you have ever looked up there. There is an energized grid and a contact from the pole on the bumper car. Anyway, on the Vanagon, when you press the gas pedal, the wiper arm moves across the grid in a fixed arc, telling the computer somehow to increase throttle. As you accelerate and decelerate this arm wipes across the grid back and forth like a kind of potentiometer somehow. Since you are using the lower end of the grid more than higher ends ( you always have to start at a slow speed regardless of how fast you eventually go ), after tons of miles the lower end of the grid wears more and makes spotty contact with the wiper arm and you get crappy flat spots in acceleration as a result of poor contact at the worn areas. This happens at high miles and results to the driver as a wierd nondescript feeling of flat spots, or the engine not wanting to cooperate until you break through the dead zone on the grid. When you push the gas pedal abruptly through its range, frequently you momentarily get the grid functioning and all is well for a few moments. Anyway, sorry for the novel. The point is, the Soob feels a lot like the Vanagon syndrome, and the first thing that popped in my head was I wonder what the symptoms of a worn air flow meter or worn throttle position sensor or some other component I am not familiar with is in this setup ( worn injector? ) If someone is familiar with how this whole ball of wax works, and has time to explain it to me, awesome. If someone can give me common symptoms of faults or wear in the components, even better. Thank you! Trying to learn the Subaru idiosyncracies. Quote Link to comment Share on other sites More sharing options...
Numbchux Posted February 22, 2006 Share Posted February 22, 2006 I don't have the solution to the problem, but I can tell you that there is a very simple throttle cable. one end attached to the pedal, the other to the throttle body. no electronics Quote Link to comment Share on other sites More sharing options...
mbrickell Posted February 22, 2006 Author Share Posted February 22, 2006 Ok, cool, and thank you. I must not be too bright. I looked under the hood and did not see an apparent throttle cable. I'll look harder tomorrow. Thank you Matt Quote Link to comment Share on other sites More sharing options...
Adam N.D.J. Posted February 22, 2006 Share Posted February 22, 2006 I'm willing to point towards either a problem with the TPS (throttle position sensor) or the air flow meter. Is your check engine light on, or flashing?? That would help things out more. Quote Link to comment Share on other sites More sharing options...
mbrickell Posted February 22, 2006 Author Share Posted February 22, 2006 Thank you for reply. No CEL or abnormal lights of any kind. Vehicle starts fine, idles at 700 or so, normal idle with no fluctuation. When starting cold in the AM ( 20 F outside ) rpms will run 2200 or 2300 for a while. If you blip the throttle they will drop to 1800 or 1900, and shortly go to low teens or so ( by then I am usually driving and not paying attention to idle speed anymore ). The wierd flat spot issue seems almost like something is in a range where it is getting a degraded signal, the engine is momentarily running lean or rich, and there is just a 10-20% weakness or loss of power at a few rpm ranges. On the highway steady state speed totally non-issue. Cruising in town at 35mph non issue. Light acceleration from a stop sign or from 25-35 mph is when I notice it most. If you put your foot in it, it will pick up and go and I don't notice flat-ness. Quote Link to comment Share on other sites More sharing options...
85Sub4WD Posted February 22, 2006 Share Posted February 22, 2006 I have never expierenced this personally, but I have heard the the track for the throttle sensor (potientiometer-like circuit) gets worn with time, and can wear so that it has a "flat spot" on it - I would try setting up a JY throttle position sensor and see if you notice a difference - there is a specific proceedure in the FSM/other manuals on how to set it up properly (clearences & such) the throttle cable is on the passenger side, on the throttle body - it should be easy to spot with the spare tire out good luck Quote Link to comment Share on other sites More sharing options...
mbrickell Posted February 22, 2006 Author Share Posted February 22, 2006 I have never expierenced this personally, but I have heard the the track for the throttle sensor (potientiometer-like circuit) gets worn with time, and can wear so that it has a "flat spot" on it - I would try setting up a JY throttle position sensor and see if you notice a difference - there is a specific proceedure in the FSM/other manuals on how to set it up properly (clearences & such) the throttle cable is on the passenger side, on the throttle body - it should be easy to spot with the spare tire out good luck Good info, I thank you. Quote Link to comment Share on other sites More sharing options...
hooziewhatsit Posted February 22, 2006 Share Posted February 22, 2006 Here's the procedure to test the TPS http://www.ultimatesubaru.org/forum/showthread.php?t=24997&highlight=throttle+position +sensor (well, a pdf in that thread) If you want to replace it and you have AC.... I found it was easiest to take a file and make some clearance on the AC compressor. You only need about a quarter inch off the corner of it to be able to slide the TPS off, versus removing the fan, alternator, unbolting the compressor, so you have that 1/4" of clearance. a $5 TPS from the junkyard fixed my annoying low speed semi-mild bucking I was having. Quote Link to comment Share on other sites More sharing options...
Davalos Posted July 3, 2006 Share Posted July 3, 2006 OMG - Is the Throttle Position Sensor REALLY $600+ Dollars? ... Quote Link to comment Share on other sites More sharing options...
GeneralDisorder Posted July 3, 2006 Share Posted July 3, 2006 Maybe from the dealer - pocket item at the JY... $279.79 from www.rockauto.com GD Quote Link to comment Share on other sites More sharing options...
SuBrat84 Posted July 4, 2006 Share Posted July 4, 2006 Uhm.. when's the last time the car had a tune-up? It sounds like you may need new spark plugs / spark plug wires. Air filter? PCV Valve? Quote Link to comment Share on other sites More sharing options...
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