Coolant system not depressurizing

2 years ago I purchased a dormant DeLorean that was sitting around for about 7 or 8 years. In the 2 years I have had the car I replaced the entire coolant system and fuel system with new or rebuilt parts. I'm finally reaping the benefits of all my hard work and have a great driving D I use everyday but now I'm finding myself with this odd problem. It seems that the cooling system is not depressurizing itself. Half hour, hour trips around town are fine but if I go somewhere far like a 2 or 3 hour trip and park the car overnight, I find antifreeze on the ground the next day leaking from hoses front and back. When I go to open it pops and there is still a lot of pressure in the system a day later. I found the previous owner had an 18lb cap on the car. I put a 15lb cap on hoping it would release but it didn't and still have the same issue. My biggest fear is blowing a head gasket. The only other thing in question is the tank. I installed an aftermarket stainless steel coolant tank sold by Special T Auto. It feels like the neck is a bit shorter than the stock plastic tank. The cap screws on much tighter on the aftermarket tank making me think that it leaves the cap no room to pop up to relive the pressure. Has anyone else run into this problem?

You made the right move by going back to the 15 pound cap. If you are seeing leaks everywhere in the system then I would first recheck the hose clamps followed by pressure testing the system. Hold the system at 15 pounds and check and correct all leaks. Unfortunately after going over a few basics pressure in the system the next day is not a good sign.

Are you running the bleeder modification off the thermostat housing to a T next to the coolant bottle? Air trapped in the system can also cause pressure to spike as it expands and contracts throughout the system.

I'm in no way stating the tank is the issue due to it's source but did the problem exist prior to the installation of the tank?

Something to keep in mind about cooling systems. When you complete cool down and the system reaches the same temp as outside pressure does not exist. That is why you can remove the cap and not take a coolant bath. Systems that retain pressure usually have engine related concerns where combustion gases are pushing into the cooling system. The most likely point of entry on a PRV is a warped head or popped head gasket. Cracked heads pushing into a coolant passage are not as common but can happen in extreme circumstances.

Check your hose clamps & hope for the best with just a few loose ones, but...

Likely a head gasket. I had this problem, more or less. I would have to keep "burping" the radiator & checking the coolant level frequently. What happens is compression pushes air into the cooling system at the top of the cylinder, which collects in the radiator. Since the air collects up there & expands more than the fluid, it pushes fluid out of the system to make room for the expanded air. The air also is what is holding pressure & is vented when you loosen the cap.

This is seemingly becoming more common. Keep an eye on both your oil & coolant for cross-contamination. It seems the top cylinder leak simply pushes combustion air into the cooling system, but not oil. If you're luck like I was, your coolant will end up smelling like used oil/exhaust gas, but not actually have an oil film/sludge (& vice verse, you won't have coolant in your oil). I drove mine this way & simply kept an eye on it for over a year - even drove to DCS on it (1200 mile round trip).

If you're not crossing fluids, you can just babysit the fluids & levels for a while & save up for an engine top-end rebuild to replace the gaskets. I chose to go with performance cams at the same time "while I was in there".

I agree with everything Justin minus the keep driving it idea. You were uncommonly lucky with your experience. Most of the time damage occurs from letting it go and continuing to drive. I would strongly suggest correcting the issue or parking the vehicle until repairs can be made. Continuing to drive and monitoring is not a good idea.

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Originally Posted by jmettee

Check your hose clamps & hope for the best with just a few loose ones, but...

Likely a head gasket. I had this problem, more or less. I would have to keep "burping" the radiator & checking the coolant level frequently. What happens is compression pushes air into the cooling system at the top of the cylinder, which collects in the radiator. Since the air collects up there & expands more than the fluid, it pushes fluid out of the system to make room for the expanded air. The air also is what is holding pressure & is vented when you loosen the cap.

This is seemingly becoming more common. Keep an eye on both your oil & coolant for cross-contamination. It seems the top cylinder leak simply pushes combustion air into the cooling system, but not oil. If you're luck like I was, your coolant will end up smelling like used oil/exhaust gas, but not actually have an oil film/sludge (& vice verse, you won't have coolant in your oil). I drove mine this way & simply kept an eye on it for over a year - even drove to DCS on it (1200 mile round trip).

If you're not crossing fluids, you can just babysit the fluids & levels for a while & save up for an engine top-end rebuild to replace the gaskets. I chose to go with performance cams at the same time "while I was in there".

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Lets not over complicate this issue and tear off heads just yet!

There are two main reasons to replace your head gaskets.
1) Coolant in your oil
2) Combustion gas in your coolant, this will cause overheating and should be tested for prior to any further action!

If you have run your car on a several hour trip without issue, and have had no history of overheating, I would not lead toward a head gasket issue.

Let me first ask you this simple question. What type of coolant cap are you using?

that is why I said to test it first!

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Originally Posted by DMCTek

Lets not over complicate this issue and tear off heads just yet!

There are two main reasons to replace your head gaskets.
1) Coolant in your oil
2) Combustion gas in your coolant, this will cause overheating and should be tested for prior to any further action!

If you have run your car on a several hour trip without issue, and have had no history of overheating, I would not lead toward a head gasket issue.

Let me first ask you this simple question. What type of coolant cap are you using?

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If the motor was ever overheated in the past (and you don't know) then it is very possible the head gaskets are leaking. Check inside the oil filler and look at the plastic screen. If it is melted or missing that is your first clue. Next, get the motor hot and pressurize the cooling system to 5 psi. If it keeps going up you need to replace the head gaskets. I agree that the sooner the better. Leaking head gaskets don't get better, they get worse and as they get worse all kinds of bad things will happen. At this point all it will probably take is a set of head gaskets. Keep running the motor and you will blow the liner seals. That has a cascade effect of allowing coolant into the oil and the problems multiply.
David Teitelbaum

If you have a blown head gasket, you will loose cooling system pressure very quickly when the engine is not running. If you still have pressure when the engine is cold, then you have to look somewhere else. Wonder if you added some chemicals to your cooling system that is causing a chemical reaction.

Follow the advice given to you by other members and find the problem before you drive your vehicle longer than needed.

Buy one of these kits, use it, and you will quickly know if you have a head gasket leak.

http://www.rxauto.com/products.php?cat=10