RD02

2,200 miles on rebuild, decel smoke

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(edited)

Hi everyone,

Follow up on my first engine rebuild, ‘73 2002 auto. Brief summary; head redone with new springs, exhaust valves, guides, seals,stock cam; crank ground and new bearings, rods new bearings top and bottom; cylinders bored and new cast 90mm 9.5 cr pistons with Deves rings. Stock ignition and Solex 2 bbl.

Recent compression test was 155 to 160, leak down was 13% to 15% with air heard from the crank case only. Engine runs good, doesn’t miss a beat. No smoke at all when started or on acceleration.

 

So I’ve searched and read several previous posts about decel smoke and mine is doing the very same as others have posted. Around town driving no problem, but at higher speed and rpm yes. Responses to these posts have varied, some saying the cause is bad set of oil rings, other say bad guides and seals, or loss of brake fluid into the booster which mine doesn’t. 

 

Posters have found faulty oil rings after a rebuild, replaced them and eliminated the smoke. In my case I tend to think it is the oil rings, first because my machinist assembled the head not me, and second, when I set one of the scraper rings into the bore like you would to check end gap it looked like the ring wasn’t perfectly sitting flush against the wall near one end of the ring.

 

So that’s my story. I love driving my car, and on flat ground I can usually avoid the decel smoke by feathering the throttle, but I want it to be right and don’t have a problem doing whatever it takes, HELP!

       Thanks, Robert

 

 

 

Edited by RD02

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Before you get into oil ring/tear down mode...pull the valve cover and get a good flashlight and make sure all the valve stem seals are still on the guides.

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You mention smoke on decell, but it would be great to let us know the color of the smoke.  If the color of smoke is a bluish color then oil is getting into the combustion chamber.  If the smoke is darker (more black) then excess fuel is getting into the combustion chamber.  these are 2 completely separate issues. Let us know, so we can more accurately assist.

Matt

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4 hours ago, andyleonard said:

Before you get into oil ring/tear down mode...pull the valve cover and get a good flashlight and make sure all the valve stem seals are still on the guides.

Okay Andy, will do tomorrow 

 

1 hour ago, Schnellvintage said:

 

You mention smoke on decell, but it would be great to let us know the color of the smoke.

 

Yep Matt, the smoke is blueish.

 

Thanks guys, Robert

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Mine was like the James Bond Aston Martin DB4 in Goldfinger  , so much smoke people were pulling back in case it blew !!

mine was bad oil rings , probably 500 miles , got worse on the drive to San Diego I did to "run in " the engine.

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(edited)
16 hours ago, RD02 said:

Recent compression test was 155 to 160, leak down was 13% to 15% with air heard from the crank case only. Engine runs good, doesn’t miss a beat. No smoke at all when started or on acceleration.

 

Around town driving no problem, but at higher speed and rpm yes. Responses to these posts have varied, some saying the cause is bad set of oil rings, other say bad guides and seals, or loss of brake fluid into the booster which mine doesn’t. 

 

I can usually avoid the decel smoke by feathering the throttle,

 

HELP!

 

 

Your numbers seem more than acceptable but there are several things you do not mention.

 

2,200 miles since the rebuild.  Is there any question about the engine not having been broken in fully? 

 

What type and viscosity oil are you using and how much oil are you consumingLast oil change?

 

Do your spark plugs evidence signs of oil burning?  Notable tail pipe residue? 

 

While your symptoms do not immediately point to ring issues, cylinder wall issues, piston ring land issues have you examined the valve cover vent tube for excessive blow by?  (Not familiar with your intake system, but assuming the tube vents toward the mouth of the carburetor, is it wet or dry?

 

You are running an automatic.  Trans fluid dripping on a hot exhaust may tend to create smoke.  If the system is overfilled and the o-ring on that dip stick tube is compromised, leaks may occur .  .  .   Also, fittings for oil lines to the radiator, and the lines themselves, if your auto is so equipped, can shed a few unwanted drops on a hot exhaust..  Not saying this is your problem, but just something to consider.

Edited by avoirdupois

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What's that cool movie where the wizard mechanic takes a shovel full of baking soda and throws it up into the intakes of a fresh and idling Merlin engine....and proclaims "the rings are now seated?"

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1 hour ago, avoirdupois said:

 

Your numbers seem more than acceptable but there are several things you do not mention.

 

2,200 miles since the rebuild.  Is there any question about the engine not having been broken in fully? 

 

What type and viscosity oil are you using and how much oil are you consumingLast oil change?

 

Do your spark plugs evidence signs of oil burning?  Notable tail pipe residue? 

 

While your symptoms do not immediately point to ring issues, cylinder wall issues, piston ring land issues have you examined the valve cover vent tube for excessive blow by?  (Not familiar with your intake system, but assuming the tube vents toward the mouth of the carburetor, is it wet or dry?

 

You are running an automatic.  Trans fluid dripping on a hot exhaust may tend to create smoke.  If the system is overfilled and the o-ring on that dip stick tube is compromised, leaks may occur .  .  .   Also, fittings for oil lines to the radiator, and the lines themselves, if your auto is so equipped, can shed a few unwanted drops on a hot exhaust..  Not saying this is your problem, but just something to consider.

The product that you are speaking of is “Bon Ami”

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Are you using 10w-30 oil for break-in purposes?  If so, before tearing down the engine, change to 20w-50 and see if the smoke persists.  It's a good idea to change oil after the first thousand or so miles anyway, just to get rid of any small metal particles that were either left in the block during the overhaul, or broke off in the first few miles of operation as the engines new parts became better acquainted with each other.  

 

Also, when you did the rebuild, did you use the valve guides and seals from the E30 version of the M10?  They're a newer and better design, and generally cure the engine's propensity for sucking oil past the seals/guides.

 

mike

 

PS--using Bon Ami to aid breaking new rings in is an old, old trick--but I'm not sure I'd try it with the close clearances of modern engines...

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I am with Sir Mike regarding additives to assist ring break-in as a next-to-last resort.  The Bon Ami recipe generally applied to chrome plated compression rings and cylinder walls that may not have been properly prepared for new rings.  If I read the narrative correctly, this engine is using Deves rings which tend to be very forgiving  as far as break-in.  If the oil control rings are not properly assembled on the piston lands, no amount of abrasive will cure the problem.  Yet that is unlikely because the quantum of smoke seems minimal and there is not any obvious report of smoke on acceleration or steady high speed operation.  This does not discount the possibility that one or more bores has a bottom end taper, which is a reason for attempting to detect if the problem exists for one or more cylinders.

 

If anything, it is more likely that one or more valve stem seals has a slight tear, or needs to be reseated on its guide.  I would not vouch for it, but some tout oils formulated for so-called "high-mileage" engines as an oil control panacea.  Maybe, just maybe, such a product could assist in valve stem function and it probably far less risky than introducing never-scratches powder into the engine.  But whatever the cause, it would be nice to determine if it applies to one or more cylinders.  Spark plug readings can be helpful.

 

As alluded to before, unless you intend to reinvent the oily wheel, there were many reasons for the factory's oil viscosity recommendations mentioned by Mike.  Even with the benefit of hindsight and the advent of better formulations, the recommendations may still hold true today.

 

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I know two machinists who won’t use Deves rings, they are too hard according to them and won’t seat properly.

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I was having oil consuming issues on my newly rebuilt m10 about 4 months ago with ~2300 miles on it. All new seals and everything, Deves rings, and I did what I though to be proper break in (1000 miles of easy driving under 4k rpm and constantly varying rpm, with oil appropriate for break in and a 500 mile oil change). It would blow out a big puff of smoke immediately on acceleration after taking it to 4k plus rpm's and letting off to decelerate. Pulled the engine apart and had the machinist look at it. They said the rings didn't seat properly and that the cylinder walls kept eating away at the rings. 

 

Rebuilt the engine again with the machinist recommended chromoly (i think?) rings. Used Metric Mechanics recommended break in procedures with castrol 20w-50 oil (I still think thats too viscous). Oil change at 30 and 600 miles. Ive been running the car for 2 weeks now and have 1200 miles on the engine. No more oil consumption issues and no more smoke on acceleration. So because I never saw the old Deves rings or how they were installed, nor was the break in appropriate, I cannot say Deves rings are bad. But I probably will not use them again. 

 

There are a number of factors that could be causing your engine to burn a lot of oil: cracked block/head between oil passage and cylinder, bad head gasket b/w oil passage and cylinder, improperly installed piston rings, improperly/not seated piston rings, bad valve stem seals, excessive crankcase pressure causing a lot of blow-by (usually a ring issue and only noticeable if you are venting your crankcase to your intake), etc.

 

Just my experience! Best of luck, and I hope you get it figured out!

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Quote

Did you use a "Torque Plate" when boring the block?

 

Fortunately, a torque plate, while not a bad idea at all, isn't strictly necessary.  It helps.  But the M10 doesn't distort all that much.  For a street build.

 

I have used Deves rings.  They smoked a bit.  Then I did a few track days.

 

Problem solved!

 

heh

 

Several suggestions, none particularly 'smoking gun':

check the intake and exhaust valve stems.  If only one is wet,

you've got a damaged stem seal  It's not hard to do if you're not

paying close attention.  If oil's coming through the stems, often

you'll see traces.  I suspect it's not.

 

As avoir de pois says, rule out the crankcase.  If the rings aren't seating,

blowby can do it too.   Chuffs a bunch of oil up into the head/vent, which is then

sucked into the intake on overrun.  Easy to unhook and rule out.

 

I'd do the track days before the abrasive.

 

t

 

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I would NEVER have a block bored unless a torque plate was used, whether it's a street car, or a race car.....

 

When my Brother built my S14, we bought a torque plate, it also helps that my he owns a automotive machine shop......   :)

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