Backfire out the exhaust, or backfire into the intake manifold?
A cylinder leakdown test could confirm valve condition.
^^ Like he said!
To further amplify what
@Schurkey said, the beauty of taking the additional step
of a leakdown test
prior to teardown is that you have a much sharper picture
of what you are wading into.
For example, if you are putting 100psi (at TDC on the compression stroke)
into the #3 cylinder and the hiss of leaking air is clearly heard when listening at
the tailpipe, but quiet at the throttle body, then you know that it's a
bad/bent/burnt exhaust valve and/or seat.
And if the exhaust is quiet but there's pronounced hissing at the throttle body,
then obviously the reverse is true. And how much/how little hissing is heard
at the oil fill opening will tell you just how good/how bad the ring seal is in the
wounded hole.
****
Speaking of which, how much (if any) coolant is being consumed? And how many
miles per quart of oil consumed are you experiencing? My reason for asking is
that if there's
no coolant consumption & the the oil consumption is below the
threshold of pain then fixing whatever is indicated by the leakdown test makes
good sense.
NOTE: Before I added the leakdown tester to a normal compression test in my
troubleshooting toolkit I thought that it was a bit of a luxury to perform. Not
anymore -- with the price of everything these days I like to hedge my repair bets
as much as possible with hard diagnostic data *before* I (or whoever I'm helping)
decide to start down the repair path.
Let us know any more info that you've got, and if you can elaborate a bit more
on
when it went from good to bad then that will help us further refine our educated
guesses. :0)
****
For what it's worth here's a photo of a cylinder head that I troubleshot down to
a burned exhaust valve prior to removal thanks to the
*loud* hissing out of
the tailpipe prior to disassembly. By the way, thanks to the reading of the spark
plugs I already knew which hole was suspect.
Troubleshooting, Sesame Street style -- "One of these things is not like the others."
You must be registered for see images attach
Next, the compression test was good in 3 cylinders, but near-zero in the hole
where the discolored spark plug was installed. I then performed a leakdown test
on the bad cylinder, which confirmed my suspicion that the exhaust valve(s)
as the root cause.
Given at the time that this was my only vehicle, based on this info I decided to
minimize the time it took to go from 3 back to 4 cylinder power and ordered a rebuilt
replacement head, swapped that in, and then after setting the valve lash I
confirmed the fix with a follow-on leakdown test
before firing it back up -- it was
excellent, all 4 cylinders now matched! It fired up flawlessly, and no further
issues for the remainder of the time that I got to drive the car.
To summarize, I definitely also vote for a leakdown down prior to teardown so
that you are going in with as much info as you can reasonably gather.
Forewarned is forearmed! :0)