Strange compression test - 300psi

[Camizzi]

Hello, long time reader first time posting. Have only had my 2000 Xterra for a few months and have already fallen in love. Then the other day I noticed it was running really hot. I pulled over right away to let cool and limped her a few miles to get home. There was no coolant left in radiator. I checked the oil and can see coolant on the dip stick. I proceeded to do a compression test to find that passenger side were all @ 155. Drivers side front 2 were also 155, then I tested the drivers closest to the firewall #6 and it reads 300+. My guage only goes to 300 and it buried the needle. I filled the radiator with water and it leaks out somewhere under the intake a bit more than a steady drip. There is also coolant blowing out of the spark plug hole #6 when I am testing the other plug holes. My father in law thinks its a stuck valve. Wouldnt that mean low compression? I cant find anything on too much compression besides carbon buildup. And it seems that with carbon buildup it only increases compression by a little bit, not double or more. I do not have any idea on where to start. Should I just start tearing it down. I guess I am looking for any advice or knowledge of compression test results like I got. Thanks

[RacerXXL]

Originally Posted by Camizzi

I checked the oil and can see coolant on the dip stick.

I proceeded to do a compression test to find that passenger side were all @ 155. Drivers side front 2 were also 155, then I tested the drivers closest to the firewall #6 and it reads 300+. My guage only goes to 300 and it buried the needle.

There is also coolant blowing out of the spark plug hole #6 when I am testing the other plug holes.

You have a blown head gasket. This is why you are seeing coolant in the oil and coolant blowing out #6 spark plug hole.

As for the weird compression what is harder to compress air or water? If number 6 has coolant/water in it you are trying to compress the liquid as well. It is also reducing combustion chamber volume so you will get a higher reading. Strangely high compression readings can indicate a fluid leak into a cylinder.

You have to pull the heads to replace the blown head gasket, so you will see what is causing the issue when you pull the heads.

[birddog01]

+1 on a blown HG. Coolant in the erl is a giveaway.

[Camizzi]

Thanks, I figured it was HG when I saw the antifreeze in the oil. I was trying to confirm my suspicions with the compression test. I was just expecting to see some low readings. The 300 confused me. I have already started tearing it down. This will be the deepest I've ever been in an engine. I am taking my time labeling and taking pictures for reassembly. I hope it is just the gasket. I just did the knock sensor relocation too. Guess I will put it where it goes now. Since I'm going to be in there I'll replace the timing belt too.

[birddog01]

Originally Posted by Camizzi

Thanks, I figured it was HG when I saw the antifreeze in the oil. I was trying to confirm my suspicions with the compression test. I was just expecting to see some low readings. The 300 confused me. I have already started tearing it down. This will be the deepest I've ever been in an engine. I am taking my time labeling and taking pictures for reassembly. I hope it is just the gasket. I just did the knock sensor relocation too. Guess I will put it where it goes now. Since I'm going to be in there I'll replace the timing belt too.

Going deep your first time is always dicey. You're going in the right direction and she'll let you know if you're doing it wrong.
Just remember to take it slow and don't get frustrated. You may be tempted to give up prematurely.

[k-man]

......I see what you did there.

[Itsme]

The head may be eaten away, due to electrolisis and open to the water jacket. This happens with bi-metal engines, with old worn out antifreeze, or too much water in the mix.
If you are "lucky", it will only be a blown head gasket!