Compresssion test to detect cracked head or head gasket?

Wouldn't a compression test on a hot engine show if a head or head gasket was cracked?

If you pulled all 6 plugs I would think having good compression across all cylinders would be a pretty certain sign of no head crack or faulty head gasket.

Would you have to open the coolant cap (carefully HOT) to remove pressure there as well to test properly?

The vehicle in question is new to me, visually no crack can be seen in head, oil pressure is low, haven't had long enough to determine if coolant is being lost. Was just thinking that the compression test should indicate yeas or no...am I right?

unless it's really bad, no, a compression test won't.

A leak down test should, though. This puts outside air pressure into the cylinder. If there is a leak in the gasket or a crack, it should bubble into the cooling system.

It is possible to have a cracked head or head gasket leak that allows coolant to mix with the crankcase oil (with no effect on combustion chamber). So a compression test is only partly conclusive.

NAPA sells a kit that you can use to check for combustion gas in your coolant for about $50. You pull air from the top of the radiator into a test chamber and if you have combustion gas in the rad, the test chemical will change colors. Of course, if you see bubbles in the coolant, then you already know.

Take a careful look at your plugs. If one is wet looking (from coolant), then that's another indication of head gasket or head cracking issues.

Engines with blown head gaskets or heads leaking coolant into the combustion chamber will typically put white smoke out of the exhaust.

Thanks guys!


...darn

may try the air compressor and look for bubbles or just be patient and wait for the Blackstone oil test kit to arrive.

Actually a compression check can, but the leakdown usually confirms final results. On my '00, the compression was as follows:

1=110 2=115 3=90 4=100 5=115 6=115

Many will ask just how does it run good and get no error codes for spark misfire, well I am not getting it at all. Jeep has been getting an average of around 18mpg, but when I first got it I was getting 20mpg. On a recent trip from STL to Louisville, KY, I did 70mph and was getting 17+mpg. Bottom line, anything can be possible. Can you still have a blown head gasket or cracked, sure. Will you do what I am about to do and that is to change the head?? Maybe maybe not. But a compression test when you are losing coolant and cannot find it leaking externally, means its leaking internally, and thus needs to be addressed. I have not gotten milkshaky oil yet, but with the coolant being lost, I am concerned as to what's going on with the rest of the engine. My mpgs have gone down by 2, and thus I need that back, plus with those above numbers, that makes me think that the gasket might be failed across more than 1 cylinder. I will be in the middle of getting pictures and doing the writeup on this in the upcoming days, and I will keep posted as I go as to how the fix is progressing. Hope that answers your questions regarding this issue.

Jeff

IGeeky1 said:

Thanks guys!


...darn

may try the air compressor and look for bubbles or just be patient and wait for the Blackstone oil test kit to arrive.

Click to expand...

I bought that combustion leak tester as well, and it said i had no combustion leaks.

Still was loosing coolant, so i pulled the valve cover. Totally milkshaked. Turned out the head in my '99 was cracked.

I guess those leak testers don't always work. I honestly don't even know.

Yeah, ihscoutlover, I'm there with you, time for a head swap.

There are two directions the coolant can leak internally: into the crankcase (oil) or into the combustion chamber and out the exhaust. The combustion leak tester only checks for combustion gas in the cooling system. It is not intended to detect coolant leaks into the oil. If you have coolant in your oil, it will get milky as others have pointed out. The oil will float on top of the water/coolant, so you can also crack the oil pan drain plug and look for water.

IGeekly1 - Just curious, why do you think you have a head/head gasket issue? What symptoms does your vehicle have ?

The 2001 XJ is a brand new vehicle to me, ran fine while test driving. Oil was very dark and appeared to need changed badly but I saw no sign of milkshake. Coolant looked a bit old too out nothing out of normal really. I heard some engine ticking but pretty much have heard that from nearly every older wrngler or cherokee (of acquaintances)I've ever heard so did not concern me much.

Then I drove it longer...oil pressure dropped to the point of popping on check gauges light at stop light. So I drain and change oil and filter...still no sign I could see of water/coolant, it just was dark dark.

Changed plugs, they looked used but not bad to me.

Read about the 0331 block plague of 2000/2001 and pull valve cover. Cleaned off area between cylinders 3 and 4 real well and saw nothing

https://www.dropbox.com/sc/uplopvpoc8kbttp/kqVaZJ6kJ8

Made cardboard oil damn to see if I was getting oil. Things were real gunky/sticky and black. Perhaps ran too thick of oil or additives or perhaps bars leak or something?

https://www.dropbox.com/sc/qcsl6ae7n63by7v/vAinhoqJbq

Rockers and pivots did not look like they should for an engine with 93k miles on it. Intake rocker (closest to firewall) rocker on cylinder four could be wiggled by hand making clatter. I fed the lifter some mmo, cleaned things up a bit and put cover back on.

https://www.dropbox.com/sc/u9l0yl13jvgwnmq/ExV9fpjx-t

Since then I've flushed the coolant and added fresh to get a better baseline. I've felt as if the oil level has risen...and the coolant has gone down...but just a bit. I guess I need to drive more miles to really know but then was hoping to put as few on it as possible prior to alleviating the problem.

In the end this will be a vehicle that only get about 6k miles per year put on it, mostly 15 minute to 30 minute jaunts with a dozen 45 minute jaunts pulling a 2000lb boat/trailer pre year. So, while I know the new head will likely not cure the low oil pressure problem, I may decide to let it ride a bit since under most of the oil, which seems to come to temp about 20 minutes after the coolant, will just be getting up to emp on most trips.

At any rate... so I figured I'm just about to rip the head off and if I go to the trouble of disconnecting the exhaust headers, I may as well find a good junk yard head for $200 and put it on. Should kill the issue of having scraped up rocker/pivots as well.

The low oil pressure is obviously a big concern. The bottom end of the engine could be excessively worn, causing low oil pressure. It is also possible that the pressure sensor is not working properly. Since there's lots of gunk, indicating the PO was not too good about changing the oil, it's possible that gunk is preventing the sensor from working properly. I bought an old Ford once with a 460 V8 that would set off the low oil pressure warning light at stoplights. I ran a can of GUNK motor flush (radiator specialty company) in the motor (idle only!) and the sensor started working properly. It's also possible that the sensor needs to be replaced. You can also a quart of diesel fuel or transmission fluid instead of GUNK to clean engines internally. Some folks use MMO. Just drain a quart of oil, add cleaner, and let the engine run at slow idle for 5 minutes, then change the oil. I would start with a good flush of the oil system.

Digger87xj said:

unless it's really bad, no, a compression test won't.

A leak down test should, though. This puts outside air pressure into the cylinder. If there is a leak in the gasket or a crack, it should bubble into the cooling system.

Click to expand...

I had an unusual head gasket leak that passed most of the tests. It was leaking between two adjacent cylinders. Only test that caught it was compression. Engine had a rough idle. Coolant and oil tests were negative for head cracks and negative for head gasket leaks.

Another engine was negative on ALL tests, except for miss fires and fouled spark plugs, and somewhat low compression. But on examination of the pulled heads the heads had fine hairline cracks.

Low is a relative term, How low is the oil pressure? What oil are you using. Have you verified the oil pressure with a mechanical gauge?

IGeeky1 said:

Wouldn't a compression test on a hot engine show if a head or head gasket was cracked?

If you pulled all 6 plugs I would think having good compression across all cylinders would be a pretty certain sign of no head crack or faulty head gasket.

Would you have to open the coolant cap (carefully HOT) to remove pressure there as well to test properly?

The vehicle in question is new to me, visually no crack can be seen in head, oil pressure is low, haven't had long enough to determine if coolant is being lost. Was just thinking that the compression test should indicate yeas or no...am I right?

Click to expand...

I had a 2000 XJ with 240k that had 5 lbs oil pressure at warm idle. It was good when cold.
We pulled the pan and found the oil pump intake screen clogged with gunk. Since the pan was off, we put in a Melling high volume oil pump, purchased from Amazon. Problem solved. It was probably just the debris on the pick up filter. It may be worth checking, given the gunk found up top.

The typical crack on the 0331 head (2000+) is between #3 and #4 under the valve cover.