Residual Cooling System Pressure After HG Replacement?

DOC4444 · **Posted:** Sun Mar 15, 2015 11:24 am

When the spate of head gasket problems started to really pick up, checking for residual pressure in the cooling system after overnight cool down became one of the main "tests" to determine if a HG leak may be present. However, prior to this concern developing, was anyone checking for residual pressure? I certainly was not and I bet few others were checking this either.

So, around 120K, I started checking for "RP" and started finding it. I even replaced the rad cap with one that has a little lever so you do not have to unscrew the cap. I also pressure tested the new cap to make sure it it was maintaining the full 16 psi it is supposed to.

Several types of commercial tests for cooling system leaks (presence of exhaust gasses, etc.) were performed without reflecting any signs of a problem. Then, when towing a 5000 lb trailer, we finally observed coolant in the overflow, definitive proof of a leaking HG.

So HG was replaced (with ARPs) at about 125K total miles. I checked for residual pressure. There was none. I stopped checking every morning after a week, but still continued randomnly. At about 2K, I found residual pressure again for the first time. I started checking it more frequently. It was inconsistent. However, after about 3K, post HG replacement, it became consistent. We are now at about 8K, post HG replacement and residual pressure is always present.

We have towed a few times and, so far, no coolant has been observed in the overflow.

In case you are wondering, we have had the vehicle since it had 17K miles. To my knowledge, it has never been overheated.

So, I would like to hear, in particular, from the rising numbers of forum members who have replaced their HG and what they have been observing in terms of RP? Or, have you not bothered to check because you thought all of your HG problems were behind you?

(Trying to determine if RP is now, or ever has been, a meaningful test.)

Will look forward to your responses.

Thanks as always,

DOC

olypopper · **Posted:** Sun Mar 15, 2015 3:57 pm

Doc, rising coolant in the overflow or even bubbles appearing in the overflow don't necessarily mean a head gasket problem. And for that matter, neither does RP in the cooling system. My 2001 318 and 2014 6.7 both have RP and certainly don't have HG issues. My plow truck (an 8.0 liter V10) will slightly overheat and the coolant level in the overflow will rise and on a decent overheat even bubble (I'm sure boiling inside the cooling system is causing this)

When you changed your head gasket did you find evidence of failure? Did any combustion gas tests show anything definitive?

DOC4444 · **Posted:** Sun Mar 15, 2015 4:13 pm

HG was leaking on passenger side, between 3 and 4, apparently the most common location. (As noted in my post, all commercial tests were negative.)

DOC

flash7210 · **Posted:** Sun Mar 15, 2015 6:32 pm

Are you currently loosing any coolant?

If not, you dont have a head gasket leak.

DOC4444 · **Posted:** Sun Mar 15, 2015 7:43 pm

As noted in post, no coolant in overflow, so far, even with towing 5K lbs.

However, the point of the post was to try to come to some sort of consensus with the help of the group as to whether morning after residual pressure represents a useful indication of the early stages of a HG problem, or not.

DOC

DOC4444 · **Posted:** Wed Mar 18, 2015 7:25 am

Would really like to hear from more folks who have replaced their head gasket.

DOC

flman · **Posted:** Wed Mar 18, 2015 9:50 am

DOC4444 wrote:

Would really like to hear from more folks who have replaced their head gasket.

DOC

Well, mine came back after a gasket replacement, so I replaced the head as well the second time. Maybe it was a little too warped, maybe it had a leak? I did not bother getting it tested.

gmctd · **Posted:** Wed Mar 18, 2015 10:20 am

For your consideration, some tho'ts on residual cooling system pressure

Firstly: most owners never have reason to check for residual pressure, so little data exists for do it or don't it

Compression pressure is ~450-500psi
Combustion pressure is ~2500psi
Any combustion-chamber pressure leaking past head-gasket always results in combustion detritus in coolant
- this may not be quickly evidenced in remote coolant reservoir systems with no constant circulation thru reservoir
Greater volume of combustion chamber leakage always results in coolant loss
- a minor leak at 450psi will be major leak at 2500psi, over-riding indications of minor leak

Liquids expand when heated, creating pressure in a closed system
Air expands when heated, much greater than any liquid, can create larger volume of pressure in a closed system with remote reservoir

A radiator cap is a 2-way pop-off valve, with calibrated spring-loaded valve for system pressure and a smaller spring-loaded anti-vacuum valve
that functions to open to atmosphere as liquid cools and contracts

This anti-vacuum valve works well in coolant systems with radiator cap mounted directly on radiator because there is minimal air space above
the coolant
- atmospheric pressure opens the valve as liquid cools and system pressure drops below atmospheric pressure

This anti-vacuum valve does not work well in a system with remote pressurized coolant reservoir due to the large volume of air above the
coolant at design-spec'ed levels
- this large volume of air can result in some residual pressure, even over-nite
- use of electric engine heater in cold climes will generate system pressure as coolant boils into steam, can be interpreted as residual
pressure

Lastly: most owners never have reason to check for residual pressure, so little data exists for do it or don't it

dirtmover · **Posted:** Wed Mar 18, 2015 11:29 am

I've owned mine since new, almost 9 years. I've checked many times and there has always been some residual pressure under that cap. It's a sealed system so, unless you open it at exactly the same temperature every time, I'd be concerned to find otherwise. I've topped off exactly twice, once when I replaced the thermostat and the other when I replaced the coolant. There has never been coolant in the overflow section.

I don't buy into the theory that there should be no pressure in the remote tank.

Of course, I could be wrong and have been living with a HG failure for the last 9 years.

Mountainman · **Posted:** Wed Mar 18, 2015 1:07 pm

I have really slow consumption, tiny bubbles, but no pressure to speak of. The cap could be 180k old though. I boiled it over once, there was fluid in the overflow, it pulled it back in, and then there's never been any since??? I feel like my leak is so slow that I could just put in studs, but I might as well pull the head and get it tested. I consume maybe a cup every few hundred miles, or is it just a bad cap. Tiny bubbles should mean that it's the HG, right? I'll get a new cap and test for pressure some more.

racertracer · **Posted:** Wed Mar 18, 2015 2:48 pm

flman wrote:

DOC4444 wrote:

Would really like to hear from more folks who have replaced their head gasket.

DOC

Well, mine came back after a gasket replacement, so I replaced the head as well the second time. Maybe it was a little too warped, maybe it had a leak? I did not bother getting it tested.

flman, did you use ARP studs the first time?

Please confirm what I am reading:

Your HG leak returned after you replaced the head gasket but you did not replace the cylinder head and you did use ARP studs. Is this correct?

flman · **Posted:** Wed Mar 18, 2015 4:21 pm

racertracer wrote:

flman wrote:

DOC4444 wrote:

Would really like to hear from more folks who have replaced their head gasket.

DOC

Well, mine came back after a gasket replacement, so I replaced the head as well the second time. Maybe it was a little too warped, maybe it had a leak? I did not bother getting it tested.

flman, did you use ARP studs the first time?

Please confirm what I am reading:

Your HG leak returned after you replaced the head gasket but you did not replace the cylinder head and you did use ARP studs. Is this correct?

No ARP studs, but the original head was overheated, so I figured to get one from VM Specialists, replace it and use ARP studs. If any one wants the old head, make an offer it is in the attic of my shop. Maybe ARP studs would have made it a permanent fix the first time?

thermorex · **Posted:** Wed Mar 18, 2015 6:50 pm

dirtmover wrote:

Of course, I could be wrong and have been living with a HG failure for the last 9 years.

You should go get a head gasket like right now. And arp studs. Maybe a new head. And most important, panic! Lol.

I think they meant you're not supposed to hear a loud puff when you open the reservoir cap, after the jeep sat for a while and then ran for about a minute, as the engine should not warm up so fast to create coolant warmup. As I know, you open the cap when jeep is not started and cold (to release any existing pressure), twist it back, start the engine, let it run for 30 secs to 1 minute and open it again. Should be no "puff". Am I right?

Residual Cooling System Pressure After HG Replacement?

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