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Knock sensor - overheating? Bad bad dealer...


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7 replies to this topic

#1 uniberp

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Posted 15 February 2005 - 02:07 PM

Ok, I'm pretty steamed now, in anticipation of bad news. Beware scam dealers.

I'm new to subaru but a pretty fair mechanic. Car ran fine, kept cool, seemed to have plenty power. Bought it, replaced axle for torn boot. No big deal.

Girlfriend complained about hesitation. I thought maybe O2 sensor. No dash light, can't be serious yet. Again, I only drove a few used ones, from dealers, os my basis for comparison and experience was limited.

She took it to her mechanic, he's a good guy. Knock sensor is disconnected, and get this: Dashboard CHECK ENGINE LIGHTBULB WAS REMOVED. I am actually unholy pi**ed. Mechanic said there was only the knock sensor code coming up on his scan, so I told he to go ahead and replace the sensor.

I bought it from a broker, so I'm being really friendly to him, to get my way, and he agreed to cover some expense in this, but that may not be good enough. He says he has a dealer service report, which I asked to see.

What more might they have been hiding? Would the additional ignition retard from an open knocksensor circuit mask an overheating problem?

Thanks all you gurus
Mpergiel99foresterelmhurstil

#2 blitz

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Posted 15 February 2005 - 02:39 PM

Curious as to how you arrived at the conclusion that the dissconnected sensor was in need of replacement? It might've been dissconnected by someone that thought they were "fixing" a problem that doesn't really exist. Seems a bit unusual at any rate. I'm curious as to how this all shakes out.

Also, retarded timing would tend exacerbate an overheat condition, or at least drive the exhaust temp way up.

#3 uniberp

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Posted 15 February 2005 - 03:19 PM

Curious as to how you arrived at the conclusion that the dissconnected sensor was in need of replacement? It might've been dissconnected by someone that thought they were "fixing" a problem that doesn't really exist. Seems a bit unusual at any rate. I'm curious as to how this all shakes out.

Also, retarded timing would tend exacerbate an overheat condition, or at least drive the exhaust temp way up.


Thanks for a quick response, really. I hate these new (to me) car doubts.
The mechanic reconnected it and ran a code scan that returned "bad knock sensor". I'm hoping it was an amateur hack and not really masking some serious problem.
Can elevated EGT at 4degress retard burn the valves over extended running? I got 500 on it since purchase.

#4 blitz

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Posted 15 February 2005 - 07:59 PM

Can elevated EGT at 4degress retard burn the valves over extended running? I got 500 on it since purchase.

It's possible, but I wouldn't sweat it. The exact amount of retard that the ECU mandates in the knock sensor fail-safe mode depends on intake air temp. In really cold weather (0* F) very little timing is pulled, but at elevated temps (100*F) I'd estimate that about 12 degrees gets yanked. The seats are hardened, so only the valve would suffer.

Chances are that everything's alright.

#5 Scottbaru

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Posted 15 February 2005 - 09:07 PM

Shouldn't "fail safe" be safe for the engine?

#6 blitz

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Posted 16 February 2005 - 06:46 AM

Shouldn't "fail safe" be safe for the engine?

Well, that was my term, but probably not well applied. I think it's actually the limp-home mode. Error/default/whatever mode.

4 degrees retard won't hurt anything, except throttle response and fuel economy.

#7 Gravityman

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Posted 16 February 2005 - 07:48 AM

Did the mech clear the engine codes after he reconnected the knock sensor wire? If not you will probably still see the sensor bad.

#8 uniberp

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Posted 17 February 2005 - 09:34 AM

Did the mech clear the engine codes after he reconnected the knock sensor wire? If not you will probably still see the sensor bad.


Thanks all, blitz, gravityman for the encouragement. I've never burnt a valve (that way). These engines are new to me.

The mechanic said there were no other codes, and he's very competent from our previous experience. I don't doubt he'll clear them (30 min, batt disconnect) and run a new scan. I'm waiting to hear the 'final' report. I still can't believe someone would pull a stunt like that, removing the 'check engine" bulb.

My worst fears is that the new knock sensor will report knock to the ECU, even though I cannot hear it. Or that the distributor (do they have distributors) or ECU is whacked out (not likely, still runs). The engine seems strong, even with the retard. My girlfriend is an 'exuberant' driver, so I was grateful it was a little pokey. We're crazy about the car.

When he calls (I hate to badger him since he took it as emergency), I'm going to ask him to pull the plugs, if just to make sure they come out (not 'fused' to the head). I want him to do a compression or leak-down test, also.

Right now I'm entertaining in worst fears and feeling stupid for not seeing the obvious in front of my face. I'm trailing the previous owners (smilingly) and hope to tell whoever did that how bad a thing that was to do. Instructionallly.




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