jerking and hesitation at low acceleration

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I have a 2000 OBW, 2.5L-5MT, approx. 180k km. Recently it's been exhibiting some wierd jerking and hesitation behaviour. It seems to occur mainly under slow acceleration or cruise control mode. If I'm slowing down, a slow steady release of the throttle would at some point produce a hesitating jerk. When accelerating, if I'm accelerating slowly, it would also jerk. If I'm giving it some heavy gas, then there would be no problem.

It also happens while in cruise mode. It seems that slight changes in throttle position create disproportionately large acceleration or deceleration. What could be the cause of this, throttle position sensor, fuel, spark, etc.?

    Yousuf Khan

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Curious, did some numbnuts recently change your timing belt? Tensioner?

I had these symptoms and over teh course of 9 months having gone through changing plug wires, cam sensors, crank sensors, o2 sensors, having the head rebuilt and the problem persisted.

Turned out to correlate back to a timing belt change with a new subaru tensioner, and the belt jumping one tooth intermittently.

Getting any check engine lights/codes? In my case the subtle annoying crap yer talking about happened for a long long time before the car would finally throw a code, usually misfire on a cylinder or 4, or something pionting to crank or cam sensors. All leading one on a wild goose chase.

-- Todd H. 2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA

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On Jun 13, 1:25 am, snipped-for-privacy@toddh.net (Todd H.) wrote:

No, I haven't had any numbnuts changing my timing belt yet. :-)

However, now that you mention timing belt, I remember I was told by some techs at Mr. Lube, where I get my oil changed that they noticed that the timing belt was getting frayed. I normally don't trust them with anything other than oil changes, so I semi-ignored them.

No, there's no check engine warning yet.

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YKhan wrote:

They couldn't find anything wrong with the drive belts, and the timing belts were never exposed.

Actually, I decided to take the car into the dealer finally, and the day I took it in, the check engine light finally came on.

The dealer did some diagnostics and they are certain that the problem originates with throttle position sensor on the throttle body. This one is going to expensive, they are talking about $1300 for the part!

    Yousuf Khan

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On Thu, 12 Jun 2008 23:33:01 -0400, Yousuf Khan wrote:

sounds like a problem with the ignition system. I would start by looking at spark plug wires, then plugs themselves, then the ignition coils. If those are all good, then check the timing belt.

If all that is good, then start looking at the gas system like the fuel pump/filter though its a bit soon for that.

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All of the sparkplug wires were changed a little over a year ago, when I noticed a hesitancy early in the morning when first starting the car up. Turned out that there were holes in the wire which were causing short circuits when early morning moisture coated them. However, the timing belt might be something to look into, as I was told during a recent oil change that the belts were getting frayed.

Yousuf Khan

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On Sat, 14 Jun 2008 11:43:21 -0700, YKhan wrote:

How well does it shift, or is this a manual?

I would look at fuel before timing to be honest. especially since it is not a continuous problem. Does the car run any hotter?

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Also, what plugs were put in a year ago. If they were Bosch Platinums, pull them out and throw them as far as you can. Put in the recommended NGK or Nippondenso plugs. I have had that kind of trouble with just about every set of Bosch platinums I've ever run across.

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Hmmm, the Bosch plugs my father installed in his 71 Nissan Patrol, when his car was relatively new, had the same problems. German tradition keeps up, ;)

clare at snyder dot ontario dot canada wrote:

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clare at snyder dot ontario dot canada wrote:

No, I've stayed away from all of the gimmicky platinum spark plug stuff, I have only replaced them with standard-issue NGK plugs so far. Besides, the problem didn't occur after a spark plug change.

    Yousuf Khan

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dnoyeB wrote:

It is a manual, and it shifts horribly. That is one of the reasons I noticed this problem. I'd often get a hesitation when starting off slowly in 1st gear, but it would be fine if I floored it.

The car isn't running any hotter, the temperature needle has remained at its normal position throughout.

The dealer has told me the problem lies in a throttle position sensor inside the throttle body. Seems to be a rare problem, as they had to wait over a week to get the part in.

    Yousuf Khan

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YKhan wrote:

I would treat that information with a pinch of salt The timing belt is enclosed and there is no way of seeing "that the belts were getting frayed" without removing the covers. There is no need to remove those covers when changing the oil.

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On Sun, 15 Jun 2008 14:54:59 +1200, Bugalugs wrote:

I assumed he meant the belts he could see were worn. But true enough my visible belts were work and I replaced them all. Then I opened up my timing belt cover and that belt was fine..

CL

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Bugalugs wrote:

Yeah, I found that out in a short enough time. When I took it back to the place that said the belts looked frayed, they told me that the timing belts are actually inside the engine and not exposed, so they were probably only talking about the drive belts which run the alternator and AC, etc. They also took another look at those belts and said that this time they couldn't see any problem with those belts! Well, that closed that avenue of investigation.

    Yousuf Khan

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I had similar problems with 94 Legacy and turned out to be a small pinhole burned into one of the spark plug insulator boots. Good luck.

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On Sat, 14 Jun 2008 11:10:46 -0700, Dan wrote:

This is why I hang on to my old timing light. You can put the light on each wire as close to the plug as possible, and look at the flashes and see if there are any misses. Lets you know the cables are bad right away.

Of course it wouldn't catch a boot problem.

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Try disconnecting your front O2 sensor and taking it for a test drive. Power will be down but if it's now smooth, replace the sensor. It's a common failure item on these cars.

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Dave wrote:

The dealer seems to have traced the problem down to a throttle position sensor on the throttle body. I hope they're right because this is going to be expensive, they're talking about $1300 for a new throttle body!

I've been waiting over one week for them to ship the part into the dealership. Interestingly, during that time, I've been driving the car and the problem has been gradually going away all by itself. So I called the dealer today, and asked if it's still necessary to get it replaced? And he said he's absolutely certain that the part is going to fail again. This dealer has been relatively reliable for me, so I'm inclined to believe them, but crap, $1300 for a sensor problem?!?

    Yousuf Khan

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On Wed, 25 Jun 2008 10:23:57 -0400, Yousuf Khan wrote:

DAMN!! You need a whole new throttle body just because of a bad throttle position sensor? That sounds crazy. I never had to replace one though. Just sees high for a "sensor".

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dnoyeB wrote:

I would definitely investigate the neccesity of a TPS repair that expensive. mayeb get a second opinion. If it IS required, try to get your dealership to at least match an online dealer's price for the part - check with jamie thru www.subarugenuineparts.com

Carl

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