I removed the upper intake manifold from my 97 Bonneville today to
replace the leaking gasket, and I'm a little concerned about all the
gunk I found under the manifold. This is the first time I've taken an
intake manifold off (I knew it would be a learning experience), so I'm
not sure if what I'm seeing is normal.
I have posted a pic at http://www.thezac.com/car /
Q1: Could someone comment on the sediment? The inside of the injector
holes don't look too great either.
Q2: I have read that engine damage can occur from a leaking intake
manifold gasket. What kind of damage?
Q3: I heard that sometimes the intake manifold itself on these engines
can get "burned out" in a certain spot - where? Is it obvious if it
I hope I'm not pushing my luck - I'm DIY'er whose still learning...
Normal. Don't waste your time trying to clean it, it will gunk
up again in short order. DO clean the gasket surfaces however...
Worst case would be a connecting rod hanging out of the engine
block. Spark plugs, O2 sensors and the catalytic convertors can
and do suffer damage also. Pull a couple of spark plugs, if you
see darkish deposits, change them out and avoid the inevitable
Here is a picture of the bottom of the upper plenum, throttle
The larger round hole is where the EGR pipe extends up into the
upper plenum, the two smaller holes are coolant passages.
Here is a picture of the throttle body end of the upper plenum:
There are three oblong shaped holes visible between the two lower
studs, the two outside oval holes are adjacent to the EGR hole in
the first picture. The two outer oblong holes connect to the two
smaller round (coolant) holes in the first picture. Probe from
either to the other ( EGR to coolant passage) with a small
screwdriver or awl, you'll find soft spots and/or outright holes.
Gone on long enough, the whole lower part of the throttle body
end around where the o-ring fits around the coolant passages will
be soft and crumbly.
I always do the lower intake gaskets when I find a leaking upper,
I have yet to pull one apart where the lower gaskets aren't
disintegrating also. Your picture shows fluid pooled in the
lower intake, I can't tell if it is oil or coolant. If you DIDN'T
find raw coolant laying in the lower manifold when you removed
the upper, then I'd suspect that the lower gaskets are leaking
and that this is where you coolant loss is. If it is oil pooled,
the lower manifold bolt threads weren't/aren't sealed. Use thread
sealer/locker on the lower bolts and pay close attention to the
torque specs for the UPPER plenum bolts. You HAVE to use an inch
pound torque wrench because the values are quite low, otherwise
you'll crush the new upper plenum.
Pay close attention to the O-rings in the PCV cavity, if you
forget the O-ring around the base of the PCV valve, you'll set
MAP sensor codes.
Watch the connections to the purge solenoid, if you reverse them,
you'll set EVAP codes. Connect them exactly as shown on the
underhood vacuum diagram.
If you decide to do the lower intake gaskets, you'll also want to
replace the plastic elbow that connects between the right side of
the lower to the alternator bracket, hell, I'd do the lower just
to replace this elbow since they too are a weak point and prone
What type of thread sealant should I use for the lower manifold bolts?
The GM parts guy told me they use a silicone liquid sealant. I bought
some Permatex high-temp thread sealant, but it doesn't say anything
about being silicone-based. The guy at NAPA (parts store - don't know
if they're national) told me I could use plummer's thread tape.
I dont know if its normal or not but I am sure its one of the reason why
GM is in the condition its in now, I had the gasket already replaced on my
3.1. Its been a common problem for years and GM has not addressed it.
3.1, 3.4, 3.8 and I just found out yesterday that it can also be a problem
on the 5.7 V8!. I had mine replaced at the dealer at about 100k miles on
my 97 for about $800. Its why I dont think my next vehicle will be a GM. The
problem has been going on for years with no hint of a recall.
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