325i E46 Low Compression, Overheated

[Liquidarthur]

Good evening,

My 325i recently overheated due to cooling system issues. I didn't notice it getting hot until it was too late. Had it towed home. I suspect it blew the head gasket. I blew compression and got the following numbers:

1 - 179
2 - 150
3 - 85
4 - 60
5 - 119
6 - 176

It has 189K miles. I removed the valve cover and checked all of the head bolts with a torque wrench at 40nm, all of them held, so at least not stripped.

Do ya'll think I should change out the head gasket or look into getting a used ebay engine? Would like to save money and just do the head gasket if possible. Are there any other checks or tests that I should do before I take the head off?

Thank you all,

Liquidarthur

[bmwdirtracer]

The M52TU and the M54 engines do not just "blow head gaskets". IF the head gasket IS blown, then the head is warped, and, very likely, the threads are pulled out of the block.

40Nm is nothing; the final torque is usually around 100 Nm, although of course it's not measured that way.

Why do you suspect the head gasket is blown, anyway? What cooling system maintenance has the car had? It's time for its SECOND complete cooling system replacement, including thermostat, water pump, radiator, expansion tank, water manifold/mounting plate, all hoses, the two coolant pipes under the intake manifold, etc.

First, you need an intact cooling system, then you need to determine whether there are any symptoms of a blown head gasket.

A compression test is not a good test for a blown head gasket, but did you have the fuel delivery disconnected? Did you repeat the test for each cylinder?

You should have absolute evidence that you have a blown head gasket before you pull the head, or buy an engine.

[Liquidarthur]

The M52TU and the M54 engines do not just "blow head gaskets". IF the head gasket IS blown, then the head is warped, and, very likely, the threads are pulled out of the block.

40Nm is nothing; the final torque is usually around 100 Nm, although of course it's not measured that way.

Why do you suspect the head gasket is blown, anyway? What cooling system maintenance has the car had? It's time for its SECOND complete cooling system replacement, including thermostat, water pump, radiator, expansion tank, water manifold/mounting plate, all hoses, the two coolant pipes under the intake manifold, etc.

First, you need an intact cooling system, then you need to determine whether there are any symptoms of a blown head gasket.

A compression test is not a good test for a blown head gasket, but did you have the fuel delivery disconnected? Did you repeat the test for each cylinder?

You should have absolute evidence that you have a blown head gasket before you pull the head, or buy an engine.

The cooling system has had issues for years. I've replaced everything thoughout, including all the items you mention. It seems every time I replaced one cooling system part the leak just moved somewhere else in a never ending game.

I will look into other ways of diagnosing the issue. In the meantime, any thoughts on what is causing the low compression in the middle cylinders?

[02Pilot]

Having had cooling system issues "for years" is why it's best to just replace the whole thing at once. The low compression is most likely caused by a warped head and compromised head gasket, but as Chris said, compression testing is not the best way to determine this. Correct the cooling system issues, then pressure test the cooling system and leakdown test the cylinders.

[Liquidarthur]

Having had cooling system issues "for years" is why it's best to just replace the whole thing at once. The low compression is most likely caused by a warped head and compromised head gasket, but as Chris said, compression testing is not the best way to determine this. Correct the cooling system issues, then pressure test the cooling system and leakdown test the cylinders.

With a leak down test how do you tell if it is the head gasket or the valves leaking?

[02Pilot]

With a leak down test how do you tell if it is the head gasket or the valves leaking?

Bubbles in the coolant for coolant passages. For oil passages, if adjacent cylinders leak significantly more than others, head gasket is likely over valves. You can also feel for air from the intake and exhaust ports if it's the valves, but this requires having the manifolds off.

[bmwdirtracer]

In my experience, you can hear and feel when the air's coming out the valves. Stick your ear in the exhaust, or in the end of any vacuum hose, or onto the intake manifold. Pull #3 and #4 plugs at the same time, and do the leak down test with a piece of tissue in the other spark plug hole....does it flutter?

When you do the leak down test, do it when the engine is at full operating temp, with the coolant reservoir full to the brim, and open. (If it overflows, you have compression gasses in the cooling system.)

Use the maximum air pressure possible; I like 175 psi.

[Liquidarthur]

Thanks for the help.

So, basically, I'm getting a new engine.

That being said, I've dreamed of having the 3.0 since, well, since I got the car. I can get a 3.0 engine for only a few bucks more than the 2.5. I have to do all this work anyway.

My question is, given that this is an automatic, would there be any advantage to the bigger engine? Or would the transmission restrict the power in such a way to make it worthless?

Also, if I go this route, I understand it is easier to get the DME flashed rather than swap it out, because if swapped I would need new keys, key cylinders, maybe a gauge cluster, and other stuff. Can anyone recommend a good place to get it flashed in the Tampa Bay area? Or is there an online service where I can mail it in and get it back flashed? Is this a good idea at all, or should I just throw a next 2.5 in and call it day?