Smoke after engine braking

[dbock]

I have a Legacy 1994 AWD wagon, 185,000 miles.

 

The problem is after a downhill stretch. After 4000' of near-continuous engine braking (we live in a mountain area) the smoke is pretty embarrassing. It lasts for about 5 minutes. Oil consumption is significant all the time, but could mostly be other leaks. The rest of the time there are no visible emissions. I've had this problem for a few years and it has even passed smog between mountain trips. For the last three years I have not lived in a smog-test area, but it seems to be getting worse as the car ages.

 

Here's what I've done recently:

- changed PCV valve (with Subaru part)

- checked all PCV system hoses and air cleaner

- checked spark plugs (no oil on them) and changed them anyway

- measured compression. 145 on all cylinders. Given that I live at 4000', this translates to ~165, i.e. in spec. It does get even higher when I add a little oil to the cylinder (190/160/180/175, variation perhaps due to different amounts of oil added?)

- vacuum test at input manifold (steady at 16 and goes up then down as expected with throttle). Looks normal after correction for altitude.

 

From what I have seen in this forum, this leaves valve guides. Is this a reasonable diagnosis? Anything else to check before pulling the heads off?

 

Assuming it is, there is other work to do. I like the car, but should I hold on to it?

- timing belt (last done 75k ago)

- transmission noise when clutch pedal up. I've been advised this is probably the input shaft bearing. A mechanic said (20k ago) that this might go away eventually as there are two bearings ... is it better to wait until it breaks and get another transmission, or can/should this bearing reasonably be changed sooner?

 

Thanks in advance for any advice.

 

Douglas

[DerFahrer]

I've always been told that an increase in compression after adding oil to the cylinders is a 100% guarantee that your piston rings are worn.

 

Has the engine ever been overheated?

[zstalker]

actually, worn rings would make perfect sense. Engine braking makes a significantly higher vacuum inside the cylinder during the intake stroke (hence the braking effect), but that massive vacuum would pull oil past the worn rings and into the cyliner, where it burns (partially) and comes out as a cloud of blue-grey smoke...not a guarantee, but stands to reason.

~Erik~

[nipper]

Those are not great numbers, especially wet. They are showing that you have worn rings. If it was valves the numbers would not have changed.

 

 

nipper

[dbock]

I've always been told that an increase in compression after adding oil to the cylinders is a 100% guarantee that your piston rings are worn.

 

Has the engine ever been overheated?

 

Not since I bought it at 70,000 miles.

 

I should have been clearer in my previous post. The smoke occurs only once I accelerate away after the engine braking. There is no smoke on the way down. The smoke lasts several minutes. This is what made me wonder if oil has dribbled (and continues to dribble) past the valve stem seals. It seems to me that oil that gets past the rings would be more likely to burn very quickly, i.e. on the first few strokes.

[dbock]

Those are not great numbers, especially wet. They are showing that you have worn rings. If it was valves the numbers would not have changed.

 

 

nipper

 

Thanks for all the replies.

 

What would you expect typical sea-level numbers to be for an engine this age? I understood that when rings go bad they usually show some variation (mine were all in range 145-150 (at 4000 ft = 13% derating)). The variation in wet numbers is probably due to poor technique (teaspoon) when applying oil.

 

I understand that the rings will be worn at this age, but if the valve seals are the weak link (see previous post for my logic) I might get some more miles on that basis without a new engine. Do rings typically last longer than valve stem seals on subarus that have not been overheated?

 

Douglas

[kimokalihi]

Not trying to jack your thread, but I have the same problem with my manual tranny. When the clutch pedal is out, the tranny makes noise but when you put the clutch pedal in, the noise stops.

 

This leads me to think it's got to be a bearing in the tranny. Is this correct?

 

Is this fixable or not? I need to sell the car but everyone is too scared that they'll have to replace the tranny soon.

[nipper]

Thanks for all the replies.

 

What would you expect typical sea-level numbers to be for an engine this age? I understood that when rings go bad they usually show some variation (mine were all in range 145-150 (at 4000 ft = 13% derating)). The variation in wet numbers is probably due to poor technique (teaspoon) when applying oil.

 

I understand that the rings will be worn at this age, but if the valve seals are the weak link (see previous post for my logic) I might get some more miles on that basis without a new engine. Do rings typically last longer than valve stem seals on subarus that have not been overheated?

 

Douglas

 

I have seen subaru engines with over 200,000 miles still hitting 180 psi or better. Normally the rings don't wear. There isnt much technique to doing a wet test.

Valve stem seals are just basically a rubber-type cup. The Rings, well almost everything lasts longer then the valve stem seals. Some engines are worse then others with valve stem seal life.

 

Ring life os affected by heat, oil changes, fuel (detonation) and load (towing).

 

nipper

[Suzam]

Somebody should confirm this, but I think while engine braking the fuel injectors cut off, or at least there is a minimal amount of combustion going on. That being true aren't the pistions are pulling a vacuum and drawing oil from the leaking valve guides?

 

Since there is no combustion in the cylinders and the oil draws passed the valves, it has no where to go but out the exaust valve where it sits until the combustion starts again. At that point wouldn't the excess oil start to burn off and continue until it's gone?

 

That would explain the delay of smoke until you stop the engine braking.

 

[nipper]

Somebody should confirm this, but I think while engine braking the fuel injectors cut off, or at least there is a minimal amount of combustion going on. That being true aren't the pistions are pulling a vacuum and drawing oil from the leaking valve guides?

 

Since there is no combustion in the cylinders and the oil draws passed the valves, it has no where to go but out the exaust valve where it sits until the combustion starts again. At that point wouldn't the excess oil start to burn off and continue until it's gone?

 

That would explain the delay of smoke until you stop the engine braking.

 

 

100 times no

 

think about it. try shutting your car off while moving and see what happens. You loose power steering, power brakes, and air conditioning.

In a manual the mass of the car keeps the engine spinning, but its not producing power, and you have a big chance of a backfire through the catalyst (bad) when the fule injectors would come back on.

 

In an automatic the engine just stops.

 

this is an old wives tail.

nipper

[nipper]

S. That being true aren't the pistions are pulling a vacuum and drawing oil from the leaking valve guides?

 

Since there is no combustion in the cylinders and the oil draws passed the valves, it has no where to go but out the exaust valve where it sits until the combustion starts again. At that point wouldn't the excess oil start to burn off and continue until it's gone?

 

That would explain the delay of smoke until you stop the engine braking.

 

 

Oil burning on deccleration is valve stems or valve stem seals, on acceleration is rings (and he has ring wear by the compression numbers, as you dont usually notice it on acceleration as your, well accelerating).

 

In theory (entertaining that fuel injector off theory) the cylinders are still hot. You have uncontroled burning of oil, which is bad. Also that liquid vapor oil which did not burn goes to the cat, and the cat really does not like that at all.

 

Valve trains wear equally, as opposed to a burned valve which is a differnt situation. if one stem leaks, usually they all leak.

 

http://auto.howstuffworks.com/camshaft.htm

 

there is also a point where the valves overlap, meaning both valves are open. Under your theory, oils can be drawn inot the conmbustion chamber at this time.

 

nipper

[nipper]

PS

 

An even better explination of valve timing

 

nipper

 

EDIT, darn link didnt post now i cant find it again

[uniberp]

I have a Legacy 1994 AWD wagon, 185,000 miles.

 

The problem is after a downhill stretch. After 4000' of near-continuous engine braking (we live in a mountain area) the smoke is pretty embarrassing. It lasts for about 5 minutes. Oil consumption is significant all the time, but could mostly be other leaks. The rest of the time there are no visible emissions. I've had this problem for a few years and it has even passed smog between mountain trips. For the last three years I have not lived in a smog-test area, but it seems to be getting worse as the car ages.

 

Here's what I've done recently:

- changed PCV valve (with Subaru part)

- checked all PCV system hoses and air cleaner

- checked spark plugs (no oil on them) and changed them anyway

- measured compression. 145 on all cylinders. Given that I live at 4000', this translates to ~165, i.e. in spec. It does get even higher when I add a little oil to the cylinder (190/160/180/175, variation perhaps due to different amounts of oil added?)

- vacuum test at input manifold (steady at 16 and goes up then down as expected with throttle). Looks normal after correction for altitude.

 

From what I have seen in this forum, this leaves valve guides. Is this a reasonable diagnosis? Anything else to check before pulling the heads off?

 

Assuming it is, there is other work to do. I like the car, but should I hold on to it?

- timing belt (last done 75k ago)

- transmission noise when clutch pedal up. I've been advised this is probably the input shaft bearing. A mechanic said (20k ago) that this might go away eventually as there are two bearings ... is it better to wait until it breaks and get another transmission, or can/should this bearing reasonably be changed sooner?

 

Thanks in advance for any advice.

 

Douglas

 

Vacuum generated oil from 'wherever' (guides or rings) could be building up in the exhaust piping and burning off when combustion gets hot again.

[Suzam]

Vacuum generated oil from 'wherever' (guides or rings) could be building up in the exhaust piping and burning off when combustion gets hot again.

 

That's where I was heading, just didn't know the mechanics of how the oil wasn't getting burned until the engine braking had stopped.

[Sirlono]

- transmission noise when clutch pedal up. I've been advised this is probably the input shaft bearing. A mechanic said (20k ago) that this might go away eventually as there are two bearings ... is it better to wait until it breaks and get another transmission, or can/should this bearing reasonably be changed sooner?

 

Thanks in advance for any advice.

 

Douglas

 

 

 

I think that the noise you're describing is the throwout bearing. Is it a whistle/chirping noise when you have the clutch engaged? I had mine go bad and decided to fix it along with replacing the clutch. If you fix it make sure that you buy new retaining clips to hold the bearing properly on the tranny input.

 

Daniel

[dbock]

(sorry for the delay in my reply)

 

I don't quite know how to describe the noise, other than "bearing noise". It is noisy when the clutch pedal is *not* depressed, and the noise varies somewhat with gear. A mechanic who listened thought it was the input shaft bearing.

 

-

 

I think that the noise you're describing is the throwout bearing. Is it a whistle/chirping noise when you have the clutch engaged? I had mine go bad and decided to fix it along with replacing the clutch. If you fix it make sure that you buy new retaining clips to hold the bearing properly on the tranny input.

 

Daniel

[frag]

100 times no

 

think about it. try shutting your car off while moving and see what happens. You loose power steering, power brakes, and air conditioning.

In a manual the mass of the car keeps the engine spinning, but its not producing power, and you have a big chance of a backfire through the catalyst (bad) when the fule injectors would come back on.

 

In an automatic the engine just stops.

 

this is an old wives tail.

nipper

 

With everybody's permission and trying not to step on the old wife's tail, with what part of Suzam's description are you in desagreement with? I always thought that's exactly what's happening when an engine is on compression and I dont see how what you are saying contradicts this.

Just trying to learn.

[nipper]

With everybody's permission and trying not to step on the old wife's tail, with what part of Suzam's description are you in desagreement with? I always thought that's exactly what's happening when an engine is on compression and I dont see how what you are saying contradicts this.

Just trying to learn.

 

"Somebody should confirm this, but I think while engine braking the fuel injectors cut off, or at least there is a minimal amount of combustion going on"

 

The injectors dont shut off. There is the same amount of combustion going on as is needed determined by the computer program to make enough power to keep the catalyst hot, the engine running, and the accessories spinning.

 

nipper

[frag]

"Somebody should confirm this, but I think while engine braking the fuel injectors cut off, or at least there is a minimal amount of combustion going on"

 

The injectors dont shut off. There is the same amount of combustion going on as is needed determined by the computer program to make enough power to keep the catalyst hot, the engine running, and the accessories spinning.

 

nipper

 

Is'nt what you're saying the same as «a minimal amount of combustion going on»?

If that was not the case, where would the engine braking come from ?

[nipper]

http://en.wikipedia.org/wiki/Engine_braking

 

 

nipper

[frag]

http://en.wikipedia.org/wiki/Engine_braking

 

 

nipper

 

Without, for the moment, having a look at the thread you're posting, I thing I get the point you're trying to make. There is «minimal combustion» going on, but the braking effect does'nt come from increased vac on the intake stroke cause the intake valves open just the same. The braking effect is due to the compression stroke not being followed by as strong a power stroke (minimal) as is the case when one is pushing on the gas pedal.

Is that kosher to your taste or am I still standing on the old wife's tail ?

[Skip]

Another vote for Suzam and Frag

 

The evidence I present is the non wide band

A/F meters I have on all three of my turbo cars.

 

They all go flat lean (pegged) when descending hills.

 

When at idle the meters tick tock like when cruising.

 

Now Nipper's comment on the ECU triggering the injectors

to flow enough petrol to maintain cat temp does have merit.

 

The lean backfire through the cat is bogus, it's not the

ignition that is turned off but the fuel flow.

 

If the ignition were turned off

the build up of fuel in the cat could cause what he says.

 

If this were the case all my cars would be doing this

after each hill I go down as it is super lean on the decend.

 

There may be a timer circuit involved also.

 

I know the idle switch is because if I just open the throttle

a wee bit, the meters tick tock like normal.

 

I have a Mac Tools automotive oscilloscope.

I should connect it to an injector

and see if I can Myth Bust this one.

 

This is getting off the OP topic, I still vote

valve seals

and possibly some PVC piping issues.

 

(Remembering back to the old EA82 problem)

[nipper]

Lets try this

 

http://en.wikipedia.org/wiki/Manifold_vacuum

 

nipper

[frag]

Lets try this

 

http://en.wikipedia.org/wiki/Manifold_vacuum

 

nipper

 

Thanks. That shows I was part wrong in my last post cause I was overlooking the restrictive effect of the closed throttle plate when the engine is in "compression» mode.

So, if I read correctly, this shows the braking action of the engine is mainly due to the higher vac present at the intake stroke cause of the closed throttle plate. There might also be some braking action at the compression stroke but certainly less cause there would not be much air present in the cylinder to be compressed (some getting past the closed throttle plate and some metered by the IAC).

But this brings us back to Susam's post : oil being sucked in the cylinder, thru the valve guides and/or rings, during the high vac situation of the closed throttle intake stroke and not being completely burned (cause of "minimal combustion» or «extra lean» situation as pointed out by Skip) and finaly appearing as grey smoke at the tail pipe when full combustion is resumed.

I still cannot see anything wrong with this (Susam's) description but it would not be the first time I miss something. :-\

[WAWalker]

Well I came on to this thread about a month to late to offer anything to the OP.

 

But I am a sucker for a good tech discussion.

 

Basicly Suzam was 99% correct rather than the 100 times no.

 

My dad always told me "it is better to remain quite and be thought of as a fool, than to open my mouth and remove all doubt", so I have refrained from comment till I had some data.

 

Pluged in my Select Monitor and went for a drive.

 

Lets just look at what it takes air/fuel wise to "keep and engine running" in an automatic '98 Legacy, at idle, and on decel/engine braking.

 

At idle

 

RPM- 700

MAF- 2.35 grams/second

Fuel injector pulse width- 2.56 ms

 

On decel

 

RPM- 2500-3000+ (manualy shifting down)

MAF- 3.91 g/s

Fuel- .26 ms That is as close to shutting the injectors off as you can get without turning off the key.

 

So minimal combustion on decel/engine braking. YES

Increased vacuum (due to high rpm and closed throttle plate) on worn intake valve guide seals causeing oil to be drawn into cylinders. YES

(The rings are creating the vacuum on the intake stroke when the intake valves are open. The increased vacuum, due to the closed throttle, is acting much more on the intake side of the pistion then it is on the crank case side of the piston. Thus, if the valve guides and seals are worn, that is the path of least resistance for oil to be drawn into the cylinder.)

Cylinders and exhaust loading up with oil that is not burned untill normal combustion resumes. YES

 

Oil cunsumption due to worn rings, is mostly because of blow by, higher crank case pressures than the PCV system can handle. And just plain old poor oil control (rings not scraping oil of the cylinder walls like thay should).

 

Could the rings in this engine contribute to oil comsumption? Sure, but if the smoke stops after normal driving, and it doesn't smoke on hard accel (due to blow by), and with normal and even comprssion readings, a valve job will probably reduce oil comsumption considerably.