At 01:41 AM 1/9/00 +1100, Neil Cotty wrote:
>....New steering rack in the A. and ....
>
>I have a very annoying exhaust leak that I'm finding impossible to track
down! With the engine idling hot, I very clearly see exhaust gases escaping
from the region below the rear header pipe. It is not coming from where the
manifold meets with the head, this I have eliminated. It also isn't coming
from where the header joins the top pipe where the three studs are.
Note: Plain exhaust gases are generally invisible. Smoke in the engine
bay, especially when idling, is often attributable to oil on the exhaust
system, often on the iron exhaust manifold. A leaky valve cover gasket can
dump oil on the manifold, more when running at road speed. Leaky tappet
covers can do the same, but usually only if leaking very badly. There is
only a very rare chance that you could have oil leaking from the left rear
corner of the head gasket. But I save the best for last.
You may not have noticed that four of the manifold studs (not the end ones)
screw into the head directly in line with the head studs. These holes
actually intersect in the head. When the engine is running the flow back
oil from the rocker shaft can get past the nut on top of the rocker
pedestal and flow down around the head stud. With blind threaded holes in
the block and a good head gasket this oil fills up the space around the
head stud. The oil will flow then into the threads around the manifold
studs where it can escape past the threads and run out past the lockwasher
and end up on the hot exhaust manifold, where it smokes when the engine is
running. You would need to remove the carbs and heat shield to get to
them, but the solution is to remove those four manifold studs (one at a
time), clean them well with solvent, apply thread sealant and reinstall them.
>.... I got on my local motorway at approx 70mph and there was massive
plumes of smoke coming out from behind the car. ....
Loose valve guides or bad piston rings can allow oil into the combustion
chamber and out into the exhaust. During easy running this can coat the
windscreen of the car following you with a fine speckeling of oil droplets.
During hard running it can cause visible smoking out the exhaust as the
exhaust pipe gets hot enough to burn off (some of) the oil as it passes
through the pipe.
Bad rings can also cause lots of blow by through the crankcase at high
throttle settings (even if you have good compression test readings), which
can blow lots of oil out the vent pipe on the front tappet cover. At about
one quart of oil per tank of fuel (sometimes worse) this coats the
underside of the car with oil, including the exhaust pipe which can then
smoke when hot. Smoke getting into the interior of the car indicates leaks
through the lower body, possibly around the battery cover.
>.... When I back off the accelerator, say going down a hill, I get a
decent crack or two from the exhaust..... does this indicate a leak somewhere?
Generally, yes, an air leak, probably somewhere around the exhaust
manifold. The fuel system goes rich on decelleration (engine braking)
dumping some unburned fuel into the exhaust system. If there is a leak
somewhere in the exhaust the introduced air can combine with the raw fuel
and heat to cause backfire in the exhaust.
>.... I am POSITIVE it isn't manifold to head, all gaskets are brand new etc.
An old adage says that if you don't know for sure what it is, then you
can't say for sure what it isn't. I once installed a manifold gasket and
accidentally got about 1/4 inch of one end folded over double thickness,
causing quite a leak.. It made enough noise when running that I thought it
had a broken exhaust manifold, and that was accompanied by regular backfire
on overrun and occasional popping at constant throttle on the highway.
>.... all I can see is smoke. Is anyone out there an MGA pyschic????
Yes, but I wouldn't want to make a profession out of it.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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