My 1991 xj6 with 150k runs and drives perfectly and it has never over heated. But it builds up pressure in the expansion tank every time that I drive it. Enough to release fluid out the new cap. It seems like the head gasket but there is no oil in the antifreeze and no antifreeze in the oil. It also has perfect compression. I have completely flushed the system and changed the thermostat. Any other ideas before it blows another radiator hose?

 

Carl, I would tell you to try starting the car with the radiator cap removed (engine cold) and then let the engine come up to temp. At this point you will install the radiator cap. This will cause the expanding air due to being heated up to be minimized, hopefully preventing your over pressure condition.

 

The pressure builds the most after the thermostat opens and increases with RPM.

 

Carl, one of the things that I would recommend is letting the car come up to temp and checking the fluid level. With the engine up at normal temp, the level in the overflow bottle should be somewhere between the "MAX" line and 10mm above the "MAX" line as stated in the JTIS manual. Level should be checked 1 minute after turning the car off.

The other thing to consider is that the overflow bottle cap may be bad. IT has an internal pressure relief valve if I recall to prevent over pressurization of the system. With age, it may be failing towards a higher higher pressure leading to your issues of blowing out hoses. The only other thing that I can think of is that you have some sort of restriction in your coolant system and that it is causing fluid to be shifted around inside your car, therefore back up into your overflow bottle. If you know what hoses are expanding, then that will help you narrow down where the blockage is as the expanding hoses are telling you what parts are upstream of the blockage. If you are seeing hoses collapse slightly, then they are downstream of the blockage.

I wish I could put my hands on the car. I think it would be easier to diagnose where the problem is.

 

The cap is new. But blockage does make sense. I guess I will try to get the tube off the block to check since the rest of the system seems to flush out just fine. When I put a hose on the overflow tank water comes out both end of the radiator just fine. I should have mentioned that this all started when I added a can of Barrs leak radiator stop leak to the system. Thaks agaain and I wish you could get your hands on it too.

 

Carl, well, I hate to say this, but it looks like you may have done this to yourself. The radiator stop leak products are hard on the engine and it is very likely caused the blockage. As for where the blockage is, Who knows. My guess at the moment would be somewhere like the head. The heat tends to set the product.

You can try flushing the motor once again, but I don't think that is going to help you. I am not a big advocator of "stop leak" products. In most cases, they tend to cause more harm than good. Only in emergencies would I recommend it (stuck in the desert and need to get out of the heat, stuck in deep snow and need to get out, etc). I don't even recommend this stuff for tires. Again, emergency use only and you plan on replacing said item shortly thereafter.

You may want to contact the manufacturer and see if there is some product that they can recommend that would remove the stop leak compound. Just keep a thought on what it may do to other parts of the car (ie, rubber products in the water pump, tin in the head gasket, etc).

 

the thing about the blockage is I would be getting atnifreeze backed up from the water pump. But all the excessive pressure is relived in gas (or air) form fron the expansion tank.

 

carl, I would believe what you are experiencing is the fact that due to the restriction, you are developing a hot spot in the engine. This is allowing the coolant to expand into steam. Due to steam occupying more space then water, that would push the coolant into the expansion bottle. Because the expansion bottle is sealed (up to a certain pressure), as you push the coolant into the bottle, it will pressurize it.

 

Run a compresion test again, with the coolant cap off and the reservoir filled to the top. When you test the cylinder which has a compression leak into the cooling system, you will see the the coolant level jump up and overflow the reservoir every time that piston comes up on compression stroke.

The comparison of compression readings you first took does not verify that the head gasket is intact. This is the simplest way to accurately diagnose the problem, and I am guessing it's the one you have.