High Cooling System Pressure

[Nissan_51X]

So about a month ago I was driving through town and I noticed my car was running funny. I pulled into a parking lot and popped the hood. I inspected all the usual stuff and noticed the upper and lower radiator hoses were very firm. I opened up the radiator cap and it hissed and pissed steam. With the cap open I could see the radiator was low on water. No Bueno. So I walked to a nearby store and got some distilled water. After filling the radiator I drove home pulling over every so often checking the water. It seemed normal all the home. Maintaining the level. When got home I let it cool down to room temp, about 60 deg. I tested my cooling system with a cooling system testing tool. Pressurizing the system via the radiator it held at 16psi with the engine cold. Now the funny part, when I ran the car up to operating temp the pressure in the system ramped up to almost 18-20psi. I pulled the plugs and none seemed to be out of the ordinary, nice and brown. I had thought maybe I had a cooling passage leaking into a combustion chamber but doesnt seem to be that. The reason it concerns me is that seems like high pressure and I worry about the force on the hoses. I have a pretty stock L20b and stock 510 auto radiator in the car and am running pretty reasonable timing and a 32/36 carb. Has anybody have any idea what may be going on? Ive been driving it very cautiously the last couple of weeks and have had a few times where when I pull the cap lever it hisses pretty loud, sometimes water come out the overflow hose. But it seems to maintain level. I have since installed a water temp gauge and it seems to run just fine at 160 deg, with a 180 deg Thermostat. Anyway, this all seems strange to me. It is definitely a new development. Any suggestions or knowledge is much appreciated.

[Ash510]

I would try a new rad cap. if it is a 16lbs cap it should not let pressure build past that. it should let it drain to an overflow can after 16lbs

[datzenmike]

Sounds like a head gasket starting to go. Water doesn't always get in with the compression pushing out.

You might idle it with the cap off and watch for bubbles but the hole may be too small and idle pressures not enough. Any chance the coolant smells like gas?

If worried about the pressure loosen the cap to let the bubbles out as you drive and keep a watch on the level. If it is the gasket it could go for weeks or only days so be prepared for it.

[James]

IIRC Stock Radiator cap was at 7LBS.

[vgwagon]

When testing with a pressure tester you have to remember that it doesn't bleed pressure off, so a reading creeping up to 18-20 psi is normal.
The stock pressure if I remember correctly is ~13psi, the higher the pressure the hotter the coolant can run without boiling over.
Try a 13psi cap and make a overflow setup if you want the level to stay full. Originally these vented to the ground and full level was below the cap opening.
Releasing the rad cap when the system is pressurized isn't the best idea so be careful.
A cheap overflow setup is a pop bottle so you can make that happen easily.
Doubtful your head gasket is a problem right now and the temps the gauge show are good.....carry on carrying on.

[okayfine]

Try a 13psi cap and make a overflow setup if you want the level to stay full. Originally these vented to the ground and full level was below the cap opening.
Releasing the rad cap when the system is pressurized isn't the best idea so be careful.

This. I'm not seeing a problem, as described. Your cooling system is under pressure via the engine, so you will get hissing and steam, etc., if you pop the cap before it has cooled. Physics.

You mention your new temp gauge, but have you calibrated it to know that what it is telling you is actually what's going on? If so, 160° is actually quite low and you're running way below peak efficiency.

Lastly, your first line you say the car was running "funny" but never mention it again. What was "funny" about it before you dug in to the cooling system? Is that the real issue, if there is one, or...?

[Nissan_51X]

Thanks all.

Datzenmike: I havent smelled gas in the cooling system, and there isnt any sign of a petroleum slick floating on top or anything. When I let it idle I also didnt notice any bubbles. If they were there would they float on the surface or look like cavitation though out the coolant?

Vgwagon: The higher the pressure the higher the boiling point, something like 2 deg for every psi. Cant remember. I will be looking into an over flow setup just for piece of mind.

Okayfine: Im not sure if what I am experiencing is a problem, I just was concerned as my last two 510's never did anything like this, the hoses always were under pressure but if I gave them a good squeeze it would bypass the cap and let coolant and or air out of the bypass hose to the ground. This engine isnt doing that. The radiator and cap Im using now are the same as my they were with my L16.

As far as the calibration of the temp gauge all I can say is I used it to test a thermostat awhile back. I took a pot of water put it on the stove, suspended the thermostat in the middle of the water, hung the gauge probe next to it, and let it heat up. The gauge slowly climbed as the water heated up, and right at about 140 deg the 'stat started moving and was fully opened right as the gauge hit 170-180. I have it mounted into the front of the 'stat housing in the original sender location. It is mounted into a reducer which puts it a bit further out, which I think may be giving me the lower reading. I havent tested that theory yet.

What would be the signs of running below peak efficiency? I seem to have plenty of power still, havent noticed a change in fuel economy and my plugs still looks really good.

Lastly, about the running "funny". I was just cruising along and the engine seemed very strained at idle. Almost as if it had an A/C pump kick on. That was when I pulled over to inspect and noticed the unusually high, to me, cooling system pressure. The part that really bothered me was the absence of coolant. When I went to top it off it took almost an entire 3 liters. That and I dont neglect my vehicles, I generally keep up with fluid levels and general maintenance very well. So it was just concerning.


As far as I can tell, there seems to be no apparent problem, the car isnt running "funny" anymore, and the fluid level seems to be stable and the pressure consistent. I am going to replace the cap, and see what that does. Possibly run a catch can. I have an aluminum radiator that is ready to go in and Ill change the hoses at the same time. Ill do a flush just because it couldnt hurt. Thanks guys. If you think of anything else Im all ears. Thanks again.

[datzenmike]

There's a coolant catch can and then there's a coolant recovery system which is very different. It'd mostly the type of rad cap you get. Coolant recovery allows the ejected coolant to siphon back into the rad when it cools.

[Nissan_51X]

thats right. the catch can is just a reservoir that holds fluid that gets spit out. Usually has the inlet on the top, and is vented to the atmosphere. And the recovery system is under pressure, and has the inlet at the bottom of the container so when the radiator and cooling system cool, the fluid is then replaced into the radiator. Correct?

[datzenmike]

I don't think the coolant recovery is under pressure. Any container will do

[thisismatt]

I don't think the coolant recovery is under pressure. Any container will do

Right, it's just that the hose goes to the bottom of the container and should always be submerged so that it can suck coolant back into the radiator when the system cools.

[Nissan_51X]

I pulled the head last week. My pcv hose to the carb was full of milkshake. It didnt appear anywhere else though. Not in the water, or oil. I did encounter some milkshake on the fuel pump arm, and in the the very back right hand corner of the head. As I had suspected the number one cylinder had a blown fire ring. Right into the water jacket. Cleaned everything up and check the flatness and put it all back together. Its much better now. Pulls much better, and idles much smoother. Thanks everyone.