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1999 Forester S
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Discussion Starter #1
99 Forester 175k. Thought it was a misfire issue, but its not. Backfiring in all gears, especially when cold, and randomly at idle. SES light ocassionally flashes, but never stays on, and its clear of codes. Test drive with a friend whose a former service advisor, and hes guessing a belt slip. Thoughts?

Cost through a mechanic? Pretty cheap shop I know of..
 

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2017 Limited
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1,018 Posts
99 Forester 175k. Thought it was a misfire issue, but its not. Backfiring in all gears, especially when cold, and randomly at idle. SES light ocassionally flashes, but never stays on, and its clear of codes. Test drive with a friend whose a former service advisor, and hes guessing a belt slip. Thoughts?

Cost through a mechanic? Pretty cheap shop I know of..
I am guessing it is not a belt slip. But a sensor that has started acting up with the cold weather. With no codes though, it's a crap shoot as to which one. Guessing again, maybe it one that has to do with air temperature, either ambient or something else.

What is the SES light?
 

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1999 Forester S
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56 Posts
Discussion Starter #3
Someone previously mentioned a Throttle Position sensor. What do you think? Sorry-Service Engine Soon aka Check Engine light.
 

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2017 Limited
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1,018 Posts
OK. TPS-throttle position sensor was mentioned. Does the problem go away when the engine warms up? If not maybe it is TPS.

If it does go away, look for a carbon ridge buildup inside the throttle body that blocks off the little air needed on cold starts up to warm up stage.

SES, OK, have not seen that one in a while.
 

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1999 Forester S
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56 Posts
Discussion Starter #5
It gets better when the engine is warmed up, but it doesnt go away completely...worse going uphill with harder acceleration.
 

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2017 Limited
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It gets better when the engine is warmed up, but it doesnt go away completely...worse going uphill with harder acceleration.
OK. Then maybe it's a sensor that detects coolant temperature and adjusts the fuel delivery accordingly.

Just giving you some direction to go. But w/o a code...

BTW, if there is a carbon ridge buildup inside the throttle body at the throttle plate area, it would not throw a code. It is an easy check and fix with an old toothbrush and some carb cleaner spray.
 

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1999 Forester S
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56 Posts
Discussion Starter #7
Yeah, I was super surprised at no codes-makes the search more frustrating.
 

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07 FXT sport
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1,743 Posts
When my throttle position sensor was throwing codes on my wrx, it was running fine (no backfiring), just a weird/higher idle.

This makes me think the tps is not your issue.

dm
 

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2004 Forester XT Premium 4EAT
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30,106 Posts
So the check engine light flashes, or it comes on solid and then goes off again?
 

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1999 Forester S
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Discussion Starter #12
On harder accel, when car is backfiring/lurching, the check engine flashes, then turns off when the bucking/lurching lasts. Usually just a few seconds. No codes :(
 

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2004 Forester XT Premium 4EAT
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I had a cam position sensor fail a couple of months ago with similar symptoms - bucking and lurching at freeway speeds, irregular idle and stalling at slow speeds, flashing CEL - but after flashing a few times the CEL stayed on solid and the code clearly identified the problem. That's odd that you're not getting any codes.
 

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2014 Impreza Ltd CVT
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3,363 Posts
Doesn't the cel code stay in memory even after it goes out??

dm
It should. There are 3 types of codes, current, pending and stored. Many cheaper code readers ONLY read the current codes. Better ones will read the pending and stored as well. So when they state there are NO codes, the Code-Reader they are using may not be capable of reading the other types.

You can have them check the engine timing, they remove the left ( driver's side ) smaller belt cover, the passenger side rubber plug in the front and use the timing cover marks. Granted it takes a small mirror AND bright light to see into the passenger hole to see the timing mark AND a good eye to see the mark on the dampner pulley, if it's still there, but that is the purpose of these, to be able to check timing without removing the covers.

It may take more work to remove the timing covers, but that makes checking the timing marks and belt for slipping much easier.

ALSO is your car an auto or manual? If it's a manual and they didn't replace or properly adjust the belt guide/keeper when the timing belt was last changed, then it is very likely it did jump a tooth or two at the crankshaft. That is the purpose of this guide/keeper.
 
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