A while back I posted a message asking for advice on replacing the valve
cover gaskets on my 1985 GMC S15 (2.8L V6) which had been leaking oil. Well
the valve cover change (new Edlebrock covers, Carquest rubber gaskets) has
been done and was sucessful, at least as far as I can tell. The problem now
is that the VC gaskets apparently were not the only source of the oil leak.
Apparently there is another leak on the top of the motor, on the back. This
presents a problem for me since I cannot even see this part of the engine -
its about 6" from the firewall and is covered with hoses, the distributor,
etc. I did manage to look a little with a mirror and also get my hand back
there and feel around and as far as I can tell there is a seam that appears
to be the source of the oil. My question for the experts is what gasket
could this be? I was thinking maybe the junction of the head and the block,
could this be a possibility? All I've ever heard about head gasket leaks say
that this means a coolant/oil exchange but I'm not showing any signs of
this, just the oil leak. I've never seen the motor out of the vehicle so I
don't know whats back there. Its not the distributor shaft, I can see that
good. Could the manifold gasket failure or head gasket failure cause an oil
leak back there? I've had a compression test done and compression is good on
all cylinders, although not perfect. Please give me some advice as to where
this leak could be, also if anyone has a close-up photo of the top back of
this engine or one similar please email it to me so I can see what I'm
looking for. The manual photo is not very helpful.
I had a similar vehicle which required my replacing the intake manifold
gaskets, the front and rear portion of which consists only of a bead of
silicone gasket sealer. Common source of leaks. Mine leaked in the
rear of the engine like yours.
What is the evidence of a leak? Are you seeing oil underneath the vehicle?
The 2.8 had a history of rear main seal problems. As old as yours is I'd
almost hang my hat on that.
The leak that I see (or have felt) is definately coming from the top rear of
the motor, and runs down onto the front of the transmission, and onto the
hot exhaust pipes below, where it sizzles and smokes like all get-out. It
actually pools up in various pockets behind the manifold area. I can wipe it
clean, but after a few minutes of running, it's pooled up again. This has
caused a huge oil slick down the back of the motor and onto the transmission
and oil pan. As for the rear seals, I cannot say for sure that they are not
leaking also, as the entire back and underside of the motor seems to be
covered in fairly fresh oil. Another thing that makes me suspect the
manifold gasket is the fact that this engine overheated once years ago and I
beleive the leak began shortly afterwards and has gotten gradually worse.
How can I identify a rear seal problem? Would a rear seal leak come from the
junction of the engine block to the transmission or is there another area to
You may have an intake gasket leak, but it was/is very common
for these engines to leak from the distributor shaft o-ring seal.
The o-ring seal sits right above an oil gallery and is exposed to
full line pressure. You might want to start by removing the
distributor and checked the seal. If it hasn't been changed for
a while, you will probably find something that is hard and
cracks apart when you remove it. You can get the new updated
seal from GM, it is a brown/tan color. This new one does not
seem to deteriorate like the old seals did.
Thanks for the tip. I did see the place where the shaft goes in the block
and all appeared dry in that area, but if and when they do the manifold
gasket I'll get them to change out the distributor seal as well.
Probably the area between the intake and the heads on the rear of the
engine. The 2.8 uses part of the intake as part of the valve cover seat.
That is the seam you found. You may as well plan on pulling the VCs,
intake and resealing the top end. I hope the engine is in very good
shape other than the leak. If not you may want to consider replacing it.
The 3.4 will bolt in and give you some extra performance.
I just called a local machine shop where I was quoted $375 for a manifold
gasket R&R job. That actually sounds a little low to me, since I have paid
almost double that just to replace the VC gaskets (before doing it myself a
second time). The engine still has good compression readings on all
cylinders and as far as I know is in good shape. A brand new 3.4 would be
I looked back at my receipt and it wasn't nearly as bad as I'd thought. The
$700 included many other things including a compression check ($90)
replacement of both front brake rotors ($70) and replacement of many vaccum
hoses, not to mention the cost of all the parts.
The actual labor charged for the valve cover replacement was $240.
Now the proposed $375 for the manifold gasket replacement seems to fall
right in line where it should.
the intake manifold is sealed with a bead of rtv sealant its probably
old an leaking. I had a 85 blazer with same eng I can relate to
little space issues. recommend you put new rtv back there to do this
you must take out
intake manifold whih may be more than you want to do. If its not
leaking bad let it be and just check your oil
every so often
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