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2008 LL Bean
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55 Posts
Discussion Starter #1
Can regular engine idling cause mechanical probs in a Forester ?

(such as Catalytic Converter? ).
 

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2008 LL Bean
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55 Posts
Discussion Starter #5
It turns out that my recent prob turned out to be Catalytic Converter, which will be replaced next week. Vehicle has 43,000 miles......and was just wondering if something I've been doing this winter contributed:

Such as letting the car idle as it warms up (10 minutes, 5 days a week).

In any event.....previous 3 Subies never had a Cat go out. So glad it's covered under warranty.......
 

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99 Forester
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205 Posts
Idling certainly doesn't do anything good. Sometimes the cat just needs a good run to clean it out. Idling followed by short trips may not be getting the cat up to temperature. What were the symptoms that led to the replacement?
 

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2009 Forester X Premium
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3,121 Posts
Not sure about idlings effect on the cats but it is definitely not good to idle that long before you start off.
Why's that? :cool:
Im mean here in the east it gets pretty cold and sometimes it takes 5 or 6 min just to defrost the windows enough to see. I don't believe 10 min to let it warm things up hurts anything. My neighbors Impreza been doing just that for over 10 yrs and 256,000mi and its still kickin.
 

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'09 STI
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4,212 Posts
Well I didn't consider climate in my post but we usually pull out of the garage after 30 secs. or so when it's cold (50's LOL) and then get on the freeway and come to a stop. So it doesn't make much sense to sit for ten minutes before you start off.

I can see a few minutes in colder climates but you actually extend the warm up time in newer cars by idling that long. It may or may not make any difference so it's up to the individual to decide if Subaru's recommendations are applicable to their situation.
 

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2008 LL Bean (4EAT)
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4,909 Posts
Can regular engine idling cause mechanical probs in a Forester? (such as Catalytic Converter? ).
It turns out that my recent prob turned out to be Catalytic Converter, which will be replaced next week. Vehicle has 43,000 miles......and was just wondering if something I've been doing this winter contributed: Such as letting the car idle as it warms up (10 minutes, 5 days a week)...
That is not regular idling. Prolonged idling from a cold start in winter subjects the cold catalytic converter to a rich mixture which it cannot completely catalyze. The practice is not good for the engine or converter or emissions.
See:
Warm up the car by idling › Dr Karl's Great Moments In Science (ABC Science)

I drive off in my 08 LL Bean almost immediately, but use gentle throttle and keep the speed under 45 until the needle is up. In winter temps, the needle rises in a couple of minutes.

Police and service vehicles idle for hours at a time, and it does not harm their converters. But as you suspected, the cumulative hours of cold idling is what caused your converter to fail.
 

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2008 Forester X Premium 5MT
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8,073 Posts
Here is some thing to think about that is pro warming your vehicle up in cold climates.

Most aluminum grades expand/contract at a rate of ~0.0905" over ~39.4"(one meter)(or ~.0023" per inch) for every 212 degrees F difference in temperature OR 0.0000108490" per inch per degree F. The engine block and heads are aluminum and our head bolts are about 10" long.

I will make the comparison from 0 degrees to a full warm engine ~190. Also keep in mind most mechanical fasteners in most standard assemblies have their torque values calculated as if it is being assembled at 70 degrees +/- 2 degrees and clamping loads calculated at operating temperature. So in reality by starting you car in 0 degree temps you already have a decreased clamping load on your head gaskets to provide a seal.

Block/heads

0.0000108490" (rate of aluminum expansion per inch per degree F)
X 10" (same distance as the head bolts)
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0.00010849" (rate of aluminum expansion over 10" per degree F)
X 190 (deg F, difference from 0 to 190 deg)
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0.0206131" (total growth of the aluminum block/heads over the ~10" length of the head blots from 0 degrees to 190 degrees of a up to temp running engine)




Now to be fair you also have to take into account the steel head bolts.

Not exact to the grade of the bolt but a mid grade steel has a average rate of 0.00000645" per inch for every degree F. Depending on the make up(alloy, heat treating..... this rate will change as it will with aluminum) but these numbers will make a decent comparison to get the idea.

Head Bolts

0.00000645" (rate of expansion per inch per degree F)
X 10 (approximate length of head bolts)
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0.0000645" (rate of expansion over 10" pre degree F)
X 190 (deg F, difference from 0 to 190 deg)
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0.012255" (total growth over 10" from 0 degrees to 190 degrees F for the bolts)

Now to make that a direct comparison, keep in mind the engine is designed to run at ~190 degrees so these numbers would be reversed to figure out the loss in the clamping load at zero degrees.

0.012255" (difference in length of the head bolts from 190 to 0 degrees)
-0.0206131" (difference in length of the block/heads from 190 to 0 degrees)
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-0.0083581" The difference between the high expansion rate of the aluminum block heads and the low more stable expansion rate of the steel head bolts.

What that means is at 0 degrees when you start that NA car that has a HG thickness of about .024" is you have lost 0.0083581" of material that would have multiplied the PSI being placed on the head gasket to seal it.

Now this is not the exact math going on in your engine due to a few reasons.

-The rates I used are for non alloys, alloys are typically more temperature stable then a straight up pure metal, Think the gridiron pendulum designed by John Harrison. An alloy is a mix of different metals to achieve the properties you desire. So the real expansion rates will be some what different.

-Also there is no account being taken in for the Crush value of the Head gasket. Which is the number that the head gasket crushes to when fully torqued to spec. IE a new HG may measure 0.028" but has a installed height of 0.024". This 0.004" crush gives you some wiggle room, but not a full 0.004" maybe 0.001-0.002".

-There is also no account being taken for the elastic yield of the head bolts or the elastic yield of the threads in the block.

-These numbers are also going based on a full heat soak, all metal being the same temperature. Which does not happen evenly, it takes time. The engine warms up from the combustion chamber on out. So what that means is even though you coolant temp may be up to temp, the metal at the center of the block halves has no other source of heat transfer(no coolant, flame front as in the cylinder firing...) other then through the block it self or the. in other words the parts of the engine farther away the combustion chamber take longer to warm up and your coolant temp is not a true indicator of those areas.

So even though these numbers are not exact what they do show is that steel and aluminum have different expansion rates. Regardless of the alloy you are not going to over come that as long as the block and the bolts are made of two drastically different metals. That will result in the HG having less of PSI being applied to them to provide a seal as it warms up.

An example of this, my '03 Baja had a HG leak. It was a external coolant leak that would only make it self present if the engine was stone cold below 40 degrees. If it had been driven and parked for a 2-3 hours in 25 degree weather it would not leak. If you let it sit over night it would leak on start up and continue to leak until the coolant temp passed 100 degrees which coincidentally was when that area of the block/heads reached about 40 degrees. Proof of concept.
 

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2015 Forester Premium CVT
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157 Posts
You also have to take into concern the atmospheric pressure, humidity, time of the day, whether the sun is out or not, are the tires hot, is the metal soft, etc., etc.
(Not really...)
 
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