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Discussion Starter #1
Hi All. New member here.
I recently bought a 1998 K3500 pickup with a 5.7L Vortec engine and 140k miles.
I have discovered coolant is getting into the oil, and have decided to take on the fix as a project. First stop is to take off the intake manifold.
My question is this: will it be obvious from the condition of the gasket that the manifold to head joint is the source of the problem? Head gasket would be the next stop.
Pressure test on radiator leaks down, with water disappearing from radiator and appearing in crankcase.
Thanks for any tips!
Dave
 

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take the valve covers off #1......................heads will crack when over heated. most likely the head gasket .. remove heads and check for head distortion.
140K on a 3500 truck 1998 ? that is low miles odd..
 

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Discussion Starter #3
yeah, I think the previous owner let it sit for long periods. it had 120k in 2011 according to Carfax.
You wouldn't suspect intake manifold gasket? Have to take that off anyway to get heads off.
 

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They are known for intake manifold leaks, the 5.7 and the 4.3 both have a long history of leaks.
The problem is with the coolant, Dexcool.
When the coolant level gets low and air (oxygen) gets into the system, there is a chemical reaction that destroys the gasket.

You should be able to see the leak, if it is the gasket.
 

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Ray is right about the coolant. The old DEXCOOL had an issue but the new DEXCOOL is better. I actually saw where the old Dexcool ate thru a head gasket on a mid 90's Corvette 350CI. Intake gaskets will show where the coolant is leaking. Inspect the corner cylinders carefully. Inspect spark plugs for a whitish color to help determine which cylinder is faulty. A cylinder leak down test or compression test on each cylinder can also show up a failed head gasket or cracked head/block.
 

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Discussion Starter #6
Thanks for the tips. My worry is that I will get the manifold off and not be able to tell if the leak was from the gasket. Then put it back together having changed the intake manifold gasket only and find I still have the problem (head gasket). But it sounds from Silverado002 like I should be able to tell from the intake gasket, thanks. I have not yet done a compression test, but the truck runs well, so test may show all good. I am thinking I could have a head gasket bad with a leak from a water passage to an oil passage, but not into a cylinder, right? Then compression test would show good. I will do a compression test anyway before I tear it down.
 

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I am not very informed in this area but.... when I had my tranny rebuilt they found coolant in the tranny fluid and determined it was the rad. Could your pressure test on the rad be pushing the fluid into the crankcase if there is a ruptured internal oil cooling line?

Something to think about anyways.

Al
 

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gm dexcool #1 in the early 1990's did cause plastic materials to breakdown. but your 1998 most likely did not have dexcool #1 .. I have a 1996 Impala and I have no leaks because GM used dexcool #2..today GM has dexcool #3 and I have had no problems with dexcool .. I only use distilled water 35% and 65% dexcool. dexcool #2 which factory GM put in my 1996 I had heater core get gooped up.. so in 1998 I did the complete antifreeze flush .. took out the KS sensors in the block so all the coolant went out. I did my coolant change 2 years ago had to replace the radiator because the plastic high side tank had a crack in it .. the old radiator inside looked like the new radiator ..
I know many in the 1990's did have intake manifold breakdown in the gasket surface area.. but they had early 1990 GM engines most had this issue in the V6 ..
post back the leak location .
 

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I am not very informed in this area but.... when I had my tranny rebuilt they found coolant in the tranny fluid and determined it was the rad. Could your pressure test on the rad be pushing the fluid into the crankcase if there is a ruptured internal oil cooling line?

Something to think about anyways.

Al
Al, that scenario is possible; but only if the truck has engine oil cooling.
With that option, the engine oil is pumped to the rad just like the trans fluid.
 

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Discussion Starter #12
But if I get good compression results, couldn't I still have a bad head gasket? Couldn't the problem be just a path between an oil and water port in the gasket?
 

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a quick check ,, remove the spark plugs set in place the plug cylinder location .then see if the plug has a white color compared to the other plugs should have a tan /lt brn color.. then do the compression test. also remove the valve covers see if the head has a crack.
 

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But if I get good compression results, couldn't I still have a bad head gasket? Couldn't the problem be just a path between an oil and water port in the gasket?
If you have a gasket leak you will not have good compression
 

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Hi All. New member here.
I recently bought a 1998 K3500 pickup with a 5.7L Vortec engine and 140k miles.
I have discovered coolant is getting into the oil, and have decided to take on the fix as a project. First stop is to take off the intake manifold.
My question is this: will it be obvious from the condition of the gasket that the manifold to head joint is the source of the problem? Head gasket would be the next stop.
Pressure test on radiator leaks down, with water disappearing from radiator and appearing in crankcase.
Thanks for any tips!
Dave
Dave , I am a GM Master tech and have a lot of experience with that vehicle, 99% of the time it’s the intake gaskets, the plastic that holds the sealing part in place cracks and it moves under pressure of the cooling system (16lbs) , they leak to the outside and inside and YOU WILL be able to tell if they were leaking when you disassemble , very rarely does this engine have head gasket problems unless it has been severely overheated.
 

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Work ur way down but my bet I’d head gasket/s. My 94’ leaked at the rear of the left head on the ground. Replaced them but the 94 has the old green antifreeze-it was a little over 215,000
 

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Discussion Starter #17
thanks zomojack.. I'm sure hoping intake not head gasket. haven't started on it yet. im going to do a compression check before starting.
 

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.............. they leak to the outside and inside and YOU WILL be able to tell if they were leaking when you disassemble, very rarely does this engine have head gasket problems unless it has been severely overheated.
How can an intake manifold gasket leak cause coolant to get into the crankcase oil?

Dave60, if you do pull the heads off, it‘d be worth it to bring them to a machine shop and check for any warpage.
 
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