Thank you. This gives me some insight. So, in the morning when I leave for work it smokes a lot when I first get on it for my acceleration up to 50 but it does clear even while I’m still on the throttle. I’ve also tested having the engine in first gear while I’m driving holding 3,000rpm but the load isn’t huge and it smokes. If I floor it and the transmission downshifts, it doesn’t really smoke. It does smoke worse on hard decel though. There is no smoke at idle and cylinder compression is within 10 percent variance from 170-180 for all cylinders with no outliers. Having this information, what do you think? I also have videos.
Greetings IndexOof0,
When you read the following keep in mind that we aren't dealing with a scenario where the valves are 100% responsible
while the rings are 0% responsible, or vice versa. And we've all been around engines where the valves were 50% and
the rings were 50% -- in other words,
everything is equally worn out. :-(
In English, there may be some overlap between failure modes. For example, let's say that wear in the valve area is causing
80% of the blue smoke, while the rings are contributing 20%. So we have to factor in that in the real world interpreting
the symptoms will be a matter of degree.
****
EDIT: High compression is an indicator that the rings are in good condition, as well as the valve face to valve seat sealing.
And since oil is running down the valve stems, this excess oiling is on the
other side of the valve that's seeing pressure
during the compression test.
So it's possible to have good valve to valve seat sealing, and if you add good rings to the mix then you have good compression
readings -- while at the same time also having poor oil control in the valve seal/valve stem/valve guide area. (!)
After reading the above I'm going to say that the majority of your oil burning is during periods of high intake manifold
vacuum. And the highest probability of where oil is being pulled into the intake stream would be down the intake
valve guides. Either the valve stems are so loose inside the valve guides that the seals can't perform their job while
the stems are jumping around -or- the guides aren't worn but the seals were damaged during installation. (A third
possibility is a higher lift aftermarket cam creating an interference situation at max lift while the valve spring is fully
depressed.)
NOTE: Any way that excess oil can be sucked into the intake manifold during periods of high vacuum
is a
possibility. Usually if the intake manifold is leaking between the lifter valley and the bottoms of the
intake ports, one or more affected cylinders are running rough at high vacuum (low MAP) readings. On
the other hand, I've seen heads with bad intake seals run smoothly while metering oil onto the backside
of the intake valves. (BTW, these are general statements and not absolute rules.)
Anyway, when you drive your truck tomorrow, watch for:
* Puffs of blue smoke on initial start up. Definitely points to a valve guide/valve stem seal problem.
Why it occurs: Oil drains down valve stems from valve spring retainer and pools onto backside of
intake valves. When the engine is first started, this oil is washed off by the air/fuel mixture, goes
into the cylinder, is burned, and puffs of blue smoke are visible. NOTE: You may want to stand
behind the truck and have a helper turn the key for you. BTW, this is not a rare sight on a high
mileage SBC, especially the older ones where the umbrella seals would harden, crack, and fall away.
* Wait until the exhaust clears up, and then take off. In order to give yourself the clearest picture
of what's going on, try to drive as smoothly as possible. That is, watch the exhaust while doing
a long, steady acceleration. I would expect that the longer you accelerate (for example climbing a
steady grade on a highway) the cleaner the exhaust will look.
Conversely, if you were to slow down via engine braking (downshifting, automatic or standard) I
would expect more blue smoke than if you were to put the transmission in neutral and use only
your brakes to slow the vehicle. And remember, during engine braking with a closed throttle
a fuel cutoff may occur, so there won't be any smoke initially, followed by billowing smoke when
the fueling restarts.
Q: You mentioned recent replacement of the valve stem seals? If you have already described
how it went, could you point me to where you wrote this? And if you haven't, please describe
how you went about it, and what snags, if any, did you run into? Is all the associated
valvetrain factory original, or are you running a higher lift aftermarket cam?
At any rate, with careful observation we should be able to figure out where the oil consumption
is coming from, giving you enough data in order to make an informed decision of what
it's going to take to keep your GMT400 on the road...where it belongs. :0)
Safe travels --