Compression test. Not the results I expected…

Furious_George

Furious_George

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Joined
May 7, 2022
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Location
Montana
I have exhaust gas in my radiator according to an exhaust leak tester. Engine doesn’t overheat. The reservoir seemed to be higher than normal tho. Warmed up the engine and did a compression test. I was expecting the head gaskets to be damaged and a bigger difference in the numbers. Any ideas what is going on? Could it be a tiny leak in the head gasket where it shows 178?
2011 Rubicon 3.8L

Drivers side
180
180
180-
Passengers side
180
179
178
 
I don't see a cylinder with low enough compression that would mean a bad head gasket.

Does the reservoir tank level go down when the engine cools? Is the radiator still full when it's cooled down?
 
BLACKJKU said:
I don't see a cylinder with low enough compression that would mean a bad head gasket.

Does the reservoir tank level go down when the engine cools? Is the radiator still full when it's cooled down?
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The radiator was a tiny bit low. Maybe a quart or so. I topped it off and it seems to be staying full. I just don’t understand where the exhaust gas is coming from.
 
One thing I forgot to mention is it’s supercharged. 9psi max boost. Would the boost under load be pushing through the gasket and that’s why the compression test seems fine?
 
Only way for exhaust gas to get in the radiator is from the combustion cylinder.

Boost is pushing through what gasket?
 
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BLACKJKU said:
Only way for exhaust gas to get in the radiator is from the combustion cylinder.

Boost is pushing through what gasket?
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The head gasket. I’m just really confused. I’ve blown a head gasket in this vehicle in the past. It would literally blow the radiator cap off and drench my engine when I would floor it. It’s not doing that this time.
 
Would it be possible that you still have some of the coolant still in it from the last time the head gasket blew.

Your not testing the coolant for exhaust gasses?
 
BLACKJKU said:
Would it be possible that you still have some of the coolant still in it from the last time the head gasket blew.

Your not testing the coolant for exhaust gasses?
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I flushed it a couple times since then. That was like 40k miles ago when it blew the first time. I tested the coolant just now and it says there’s exhaust gas. I’m testing with that blue liquid that turns yellow if exhaust fumes are present.
 
I guess you could have a pin hole that just started in the head gasket and it hasn't gotten big enough for the compression to drop alot in a cylinder.

Something has to be pushing out the coolant in the radiator.
 
I’m thinking you’re right about the pin hole size leak. It doesn’t overheat at all. And the reservoir isn’t bubbling. I’m just dreading ripping the top end apart again lol
 
I think if it was mine and I had to put new head gaskets on the supercharger would stay off.
 
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BLACKJKU said:
I think if it was mine and I had to put new head gaskets on the supercharger would stay off.
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Supercharger for sale 😜 j/k

8178F8AA-608F-4280-8698-EE5FC0FC89ED.jpeg
 
Furious_George said:
I’m thinking you’re right about the pin hole size leak. It doesn’t overheat at all. And the reservoir isn’t bubbling. I’m just dreading ripping the top end apart again lol
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Have you thought about stronger head bolts? Like from ARP? Maybe see if someone makes a copper gasket too?
 
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