Discussion
A friends recently purchased car shows signs of blue smoke from the exhaust. It's much worse after he lifts off and then accelerates again.
With those symptoms I would assume valve guides, but the chap at his local rolling road told him it's more likely to be piston ring/bore related.
The compression pressures are normal and the car performs very well.
Is there a definitive way to ascertain whether it's valve guides or bore related ?
With those symptoms I would assume valve guides, but the chap at his local rolling road told him it's more likely to be piston ring/bore related.
The compression pressures are normal and the car performs very well.
Is there a definitive way to ascertain whether it's valve guides or bore related ?
it could be either as you mention.
if the engine has valve stem seals fitted and it is fairly easy to replace them that's where I would go first as they may be worn or hardened,although new seals may hide out of tolerance clearance of the valve stem to guide but it will give a pointer as to the cause of smoking.
if the engine has valve stem seals fitted and it is fairly easy to replace them that's where I would go first as they may be worn or hardened,although new seals may hide out of tolerance clearance of the valve stem to guide but it will give a pointer as to the cause of smoking.
It's a Jag 4.2 engine and was in regular use until the previous owner sold the car due to illness. It wasn't on the road at the time but had only been idle for a few months.
I was always of the view that smoke when re-accelerating after backing off was down to valve guides.
It seems it's not that clear after all.
I was always of the view that smoke when re-accelerating after backing off was down to valve guides.
It seems it's not that clear after all.
GoodOlBoy said:
It's a Jag 4.2 engine and was in regular use until the previous owner sold the car due to illness. It wasn't on the road at the time but had only been idle for a few months.
I was always of the view that smoke when re-accelerating after backing off was down to valve guides.
It seems it's not that clear after all.
My understanding was always the same as yours, although I am now wondering if its just a myth which I assumed was genuine. Certainly my old Alfa could produce a minor smoke screen after backing off and then booting it. Head off proved the bores were fine and the guides were worn. One example hardly proves anything. I was always of the view that smoke when re-accelerating after backing off was down to valve guides.
It seems it's not that clear after all.
Had a similar issue on a 'new' hipo engine re-build in my Vette - same thing loads of blue smoke when accelerating hard - and when decelerating. Stunk of oil fumes in cabin. Latter was put down to atmospheric venting of crankcase fumes (later changed to pcv system) - the culprits were A. aftermarket dipstick with inaccurate 'full' marking allowing the crankcase to be overfilled. B. aftermarket dipstick tube a poor fit in block and allowing oil vapour/fumes to be pressured out of tube, under engine and straight on to exhaust - creating the blue smoke.
With the car having been stood idle for a while, could be piston rings sticking a little ?
With the compressions being good and car performing well, I'd suggest doing an engine oil and filter change with a good 20w/50 and live with it for a while. May be that the symptoms ease with use........
With the compressions being good and car performing well, I'd suggest doing an engine oil and filter change with a good 20w/50 and live with it for a while. May be that the symptoms ease with use........
mad4amanda said:
do a compression test and note the readings , then do another with a squirt of oil in the cylinder and if the reading improves its likely to be piston bore related if not valve stem seals or guides , what engine is it and has it been left standing at any point ?
The oil test will always "improve" the compression pressure, and cannot discriminate between piston rings and stem seals as a source of oil smoke.The oil test will discriminate between poor ring seal and poor valve seal as a cause of low compression, as it will ghave no effect if tyhe vale is leaking.
It's inappropriate to use it here.
JOhn
off throttle smoke that doesn't appear immediately is usually stem seals, it has to have time to drag down the guide to get worse, so testing from motorway speed and leaving on overrun usually shows it after 5-10 seconds of vacuum pull, on throttle a brief trail while it clears looks a little worse, only my opinion.
leak down test is the other thing to do as mentioned above
leak down test is the other thing to do as mentioned above
SonicShadow said:
Leak down test will help you isolate the problem.
This,is the only practical way to find out,best done with engine warm.Before you do that though check that the pcv system is in good order and or any breathers are not blocked.
As said before high oil level can be a cause also.
There's a breather housing on the front of the cylinder head, Undo the 4 nuts and take it off and check the housing and the pipework is clear; Also check the gauze flame trap is clean (This might be missing, Its sandwiched between the head and the breather housing) Worth checking before any ideas of pulling the engine apart.
GoodOlBoy said:
SonicShadow said:
Leak down test will help you isolate the problem.
How will a leak down test show the condition of the valve guides ?klunkT5 said:
There's a breather housing on the front of the cylinder head, Undo the 4 nuts and take it off and check the housing and the pipework is clear; Also check the gauze flame trap is clean (This might be missing, Its sandwiched between the head and the breather housing) Worth checking before any ideas of pulling the engine apart.
If compression good I bet it's this.Gassing Station | Classic Cars and Yesterday's Heroes | Top of Page | What's New | My Stuff