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Blue smoke when letting off the gas

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Mitchman2 Avatar
Mitchman2 Mitchell Andrus
Mills River, NC, USA   USA
One of our members has a short-lived plume of blue smoke after letting off the accelerator. Before I talk to him I thought I'd come up with a cause and solution so he can mull over a plan of action.

Anyone care to forward a cause and/or cure?



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copernicus Avatar
copernicus Nick Kopernik
Finally Snowing in, CT, USA   USA
Blue smoke is typically the result of burning oil; in my case it ended up being worn valve guides. Letting off the accelerator is basically deceleration, so more likely its valves/guides vs. rings, but it could be either or all. Time for some compression testing.

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DevonMG Avatar
DevonMG John Russell
Okehampton, Devon, UK   GBR
If the engine is not that old and worn then the most likely explanation is worn/perished valve stem seals, which are letting oil into the cylinders on the overrun when the highest vacuum is created (if there's no fuel being ignited then the piston creates a vacuum behind it as it travels down the bore on the power stroke). https://mgaguru.com/mgtech/engine/ch202.htm

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Don MGA 1600 Avatar
Don MGA 1600 Silver Member Don Tremblay
Rutland, Rutland, USA   USA
1960 MG MGA "My Oldest Friend"
1962 Jaguar E-Type "Rich Bitch"
Mitch,

Blue smoke is oil. Black smoke is fuel. Letting off the gas with the throttle plates closed results in the highest vacuum within the engine and valve train. As such, you are either sucking oil down the valve guides due to worn seals or the crankcase is not venting.

Don

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bobs77vet Avatar
bobs77vet bob K.
northern Va, VA, USA   USA
my money says is its coming from the top end....valve guides/seals

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pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
Worn guides or defect in the crankcase venting system.

Adrian



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tvrgeek Avatar
tvrgeek Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Valve guides. If crankcase vent, it might blow some on acceleration too. My experience, yours may vary.



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jonathan in mass jonathan eagle
weston, MA, USA   USA
When I had my head rebuilt, it had the standard seals and valve deflector. Brand new and no worn valves.
The smoke also shows up after the car will sit and upon startup. Thats a real tip off. I would ask if this is the case.
I replaced the valve seals with the new recommended seals and no more smoke and less oil consumption.
I used the rope method. Whole thing took me about 1-2 hours.
Jonathan

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mgformechanic Avatar
mgformechanic John Arrayet
Montreal North, QC, Canada   CAN
if the head was rebuilt and the guides were redone in brass, they have higher clearances; if the engine does not have time to warm up on a short drive oil will accumulate in the cylinder when shut off. a hot engine will not do this.

run at least 10 w 30 as opposed to the thinner oils. Also if you are overfilling the oil pan, this will aggravate the symptoms; good to check the seals anyway. Had 5 MGAs, one with a 5 main bearing B.

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