No. 2 cylinder losing compression. Bad valve or leaky head gasket?

[JasinC19]

Hey guys,

I'm doing my best to diagnose my mother in-law's '99 v6 mustang.

Symptoms are an intermittent miss under load, sometimes gets pretty bad while driving.

We have ruled out plugs/wires/etc, no codes.

Leakdown test shows that #2 cylinder won't hold compression.

Her mechanic says it's a stuck/burnt valve and wants to take heads off and have them rebuilt.

I want to make sure it's not "just" a head gasket.

I don't see any oil/coolant mixing and no visible external leaks. Is there any way I can be sure of what the real problem is before dropping $1400?

Thanks!

 

[wmburns]

So did you run a leak down test or a compression test? I asked because a leak down test should tell you exactly where the leak is. Try repeating the leak down test. Note where the escaping air is heard from.

If leaking through the head gasket, likely there will be bubbles in the coolant.

If leaking through an exhaust value, the air will be heard escaping through the tail pipe.

A bad head gasket can also show itself during a compression test. Look for lower compression in cylinders that are next to each other. So if a problem is suspected in #2, #1 and #3 may have lower compression as well.

 

[my00stang]

If it was a bad head casket u would have blue smoke come out of your exhaust and u would be able to see it because you would be mixing coolant oil together so it would look slugy. Also you would be burning coolant because of this you would be constantly runing low on coolant and would be running hot. From what you have explained sounds like one of the valves is getting stuck opened and the heads will have to come off to fix this.

 

[mach1yellow]

to eliminate a head gasket as a cause do a block test, kit is avalible at local part store. if it fails the test it is most likley a head gasket or a crack somwere. doing a block test your looking for exhaust fumes in the cooling system

 

[DavidEwers]

What does the spark plug look like?