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Is this possible?

corvetteguy90210

New member
Joined
Apr 30, 2007
Messages
2
Location
Miami, FL
I had a Vette shop replace the intake manifold gasket on my '90 L98. It began leaking oil again immediately and I took it back to shop, they redid the seal free of charge but told me it would likely blow out again due to excessive ring blowby and excessive crankcase pressure forcing the oil to push out the intake manifold seal. They noted that the pcv valve once again has oil residue in it despite having been replaced with the 1st gasket job. I had the shop perform a leakdown test--all valves were well within norm, and also a compression test--1 cyl 200 psi and the rest 205 psi at 101K miles. I'm skeptical--excellent compression, no visible exhaust smoke, not terribly high mileage with regular Mobil 1 oil changes, car runs fine--can there still be excessive blowby, or are they trying to cover for a screwup?
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,547
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
I had a Vette shop replace the intake manifold gasket on my '90 L98. It began leaking oil again immediately and I took it back to shop, they redid the seal free of charge but told me it would likely blow out again due to excessive ring blowby and excessive crankcase pressure forcing the oil to push out the intake manifold seal. They noted that the pcv valve once again has oil residue in it despite having been replaced with the 1st gasket job. I had the shop perform a leakdown test--all valves were well within norm, and also a compression test--1 cyl 200 psi and the rest 205 psi at 101K miles. I'm skeptical--excellent compression, no visible exhaust smoke, not terribly high mileage with regular Mobil 1 oil changes, car runs fine--can there still be excessive blowby, or are they trying to cover for a screwup?
I find it hard to believe it has that much compression, it didn't have that much New!! :upthumbs
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,547
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
What do they have new?

165-175 after the ring's are seated,I have seen a few with as much as 180 but that would be rare!!!

My wifes Jimmy with 198,000 still has 155-160 and I was amazed!! All Lubricants are just better now days, if it's synthetic or not!!:upthumbs
 
S

speedmaster4

Guest
excessive crankcase pressure will push oil out the front crank seal, you'll find oil on the bottom of the front of the oil pan if you're under the car...with time, heat/cooling cycles can loosen the bolts on the intake manifold, causing front/rear oil leaks or anti-freeze leaks on the side.

a tiny dab of RTV at the corners of the intake is req'd to stop oil leaks there on a new gasket install -- sounds like the guys that did yours forgot the RTV.

good idea to check/ tighten your intake manifold bolts once a year...follow the proper sequence when check/tightening as for new install.
 
F

fatboyreyn

Guest
worn valve stem seals causes excessive valve cover pressure. quite possible with 105k
 

SPANISHVETTS

Well-known member
Joined
Nov 5, 2002
Messages
1,143
Location
El puerto de Santa Maria Spain
Corvette
before 60,63&82 now 90 383 Stroker & 91 ZR1-Sold
It is easy enough to check your crankcase pressure. Pull the two hoses off of the valve covers. Plug one and feel the pressure on the other. It should be pulsing without much pressure.

Maybe a dumb question but is the Oil level correct?

If The Right Stuff is not applied properly you will have a leak... Don't ask me how I know.
 

tls630

Well-known member
Joined
Feb 24, 2004
Messages
354
Location
Cape coral Fl.
Corvette
69 roadster 88roadster
I agree with gmjunkie, 101k miles is not a lot in todays world. I had a 4.3 with 380 on it and it now has 450k, never had the heads off. The valve guides leak and it smokes on the first start up of the day, but you never add oil between changes.Todays oil is way superior from the oil a few years ago that you can expect several 100 thousand with a little care. I think your mechanic screwed up and is trying to cover his but!
 

David-F

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Joined
Mar 28, 2007
Messages
53
Location
Woodstock,GA
I would have to agree with most here I suspect the guy doing the job did not clean the front and rear surfaces correctly any oil left on the surface will keep the sealant from adhearing to the metal surfaces. I always use acetone and a clean paper towel for the final wipe to insure there is no film left. GM came out with a bulletin for the LT1&Lt4 on how to improve sealing what they call "The China Wall" at the front and rear block they give a part number for engine adhisive to use and also recomend priming both surfaces just before laying down your bead and setting down the intake on to the engine. It does work better then just applying a bead and putting it together. Als if you have anything other than a very early 90 use 91 gaskets the 91 gaskets have aligment roll pins on the 4 outter gaskets bolt holes, but only the very early 90 L98 didn't have the wider holes in the head to be able to use these gaskets, but all the rest did that is why the 4 outter bolts are longer. If you go to GM they list different gaskets sets for 90 and 91 but again un less your's is an early like within the first few thousand get the 91.

David Fulcher
 
Joined
Jun 20, 2002
Messages
2,509
Location
Tallahassee, FL
Corvette
none right now :(
Intake manifolds are not an area that contain positive pressure on naturally aspirated cars. The oil that leaks from them usually just seeps out. If there were that much pressure 'forcing' the oil out, the oil would shoot out the intake.

If I remember correctly, the PCV valve is on top of the valve cover on the L98. It is impossible for it not to get oil on it in that location, because the oil shoots out of the base of the rocker arms onto the underside of the valve cover, then drips down off the covers onto the rocker tip and fulcrum. When the oil shoots out with the engine revved, it really shoots out. The bottom line is that part of the engine is supposed to be oily.
 

G Winter

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Joined
Aug 21, 2004
Messages
2,177
Location
NW Iowa
Corvette
1990 red convert 6 sp
In all the yrs I have worked on engines, I have never seen enough blowby to blow out the valley gaskets on the intake. The PVC,breather hose or a seal would leak long before the intake would start leaking. Couple things to check if it is only the back. Make sure your oil pressure sending unit isn't leaking and be sure they didn't damage the distributor seal.


Glenn
:w
 

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