Pressure under the coolant cap

racertracer · **Posted:** Thu Apr 10, 2014 7:31 am

Well, after my long evening commute, I performed the normal under hood check up, for oil level and coolant cap pressure. when I opened up the coolant cap it let out an explosion of pressure. Dang it.

I replaced the cap and started the engine to let it run for one minute, so that I could perform the under cap pressure test and got nothing.

This is disturbing.

I am thinking head gasket leak again.

If this is correct then this will be my third time on 2 separate CRD's.

72,000 miles, Hayden clutch and nylon fan installed.

tjkj2002 · **Posted:** Thu Apr 10, 2014 7:38 am

You should have pressure in the cooling system after a long drive,your cooling system is pressurized hence why it states " Never Open When Hot".A blown gasket would over-pressurize the system blowing past the caps rated blow-off pressure.

racertracer · **Posted:** Thu Apr 10, 2014 7:47 am

I've never experienced pressure under the cap in this vehicle, no matter how long the trip was. I've been told a little pressure is OK, but this was an explosive kind of release of pressure.

I will keep checking it.

tjkj2002 · **Posted:** Thu Apr 10, 2014 8:29 am

racertracer wrote:

I've never experienced pressure under the cap in this vehicle, no matter how long the trip was. I've been told a little pressure is OK, but this was an explosive kind of release of pressure.

I will keep checking it.

Look at the cap,it will have a psi on it(16psi is rather standard),that is basically the system operating pressure when hot.Pressurized system allows for a much higher boiling point.When at pressure the coolant is compressed as it wants to expand but can't,suddenly release that pressure and it the coolant will expand very quickly spraying everywhere with force.

If you do not have any pressure in the system with a fully warm engine then your cap is not functioning(or you have a leak) and your cooling system boiling point is severely reduced.

flash7210 · **Posted:** Thu Apr 10, 2014 9:25 am

I agree with tjkj.

A fully warmed up engine should have pressure under the radiator cap.

Last summer I did a cooling system pressure test. I put my pressure tester on the coolant
tank and let the engine idle till it got up to operating temperature. The highest temp I
recorded was 205 F and the pressure tester showed about 12 psi. At 205 F is when the
electric cooling fan turned on. The fan stayed on till the temp got down to about 185 F and
the pressure went down to about 6 psi.
I consider this a good test.

I also did a cold pressure test (engine not running) by using the pressure tester to pump up
the coolant tank to about 16 psi. Pressure held steady indicating no leaks in the system.

These tests gave me confidence that my cooling system was in good shape.

thermorex · **Posted:** Thu Apr 10, 2014 9:44 am

It's normal to have pressure in the cooling system after engine runs for a while and warms up. I think most coolant caps have a warning on them not to open when engine is hot.

racertracer · **Posted:** Thu Apr 10, 2014 1:40 pm

Well, there is definitely a considerable amount of pressure under the cap.

Even if it is a small amount that is accumulated after the first one minute run of the engine.

I have never before experienced any amount of accumulated pressure under the cap with this CRD, no matter how long of a drive I went on and one of the trips was over 350 miles long one way.

It's a head gasket problem... I've lived it before 2 other times and I continually denied believing it ...... telling myself the exact same thing you all are mentioning now.

What I am experiencing is the initial stages of a head gasket leak, avoiding doing the repair soon can cause other problems like a warped or cracked head.

I again performed the one minute pressure under cap test and it proved positive.

My diesel mechanic says "no pressure under the cap... period.... it's a diesel, a small amount of pressure after a long drive is OK but even then it's questionable."

Chalk another one up.

72,000 miles, Hayden fan clutch w/nylon fan, replaced coolant at 37,000 miles with new OEM coolant, OEM thermostat installed and it works properly, with GDE hot Tune. Temperature gauge never went above the half way mark, no indication of over heating.

RJM · **Posted:** Thu Apr 10, 2014 2:25 pm

Your diesel mechanic doesn't know what he's talking about. I have measured between 8-11psi, its quite normal.

grywlfbg · **Posted:** Thu Apr 10, 2014 2:28 pm

As the others have said I wouldn't panic yet. If you open the hood and the overflow part of the reservoir is full, and is STILL FULL the next morning then it's time to start worrying. That means the system is being pressurized beyond the rating of the cap and venting into the overflow.

Mine started doing this for the 3rd time last weekend - 108k miles. I'm done w/ this rig. Picking up an 07 WK CRD next weekend.

arasb · **Posted:** Thu Apr 10, 2014 2:42 pm

The only test that would be a clear indicator of a leaking head gasket would be if after a long drive, you wait until the engine is completely cool, like in the morning, then check the pressure cap. If there is pressure in the system when your coolant is cold you have a leak. I have always had pressure when warm, even now with a new head gasket and ARP bolts.

tjkj2002 · **Posted:** Thu Apr 10, 2014 3:14 pm

racertracer wrote:

My diesel mechanic says "no pressure under the cap... period.... it's a diesel, a small amount of pressure after a long drive is OK but even then it's questionable."

.

I'd run far away from him,he is a complete idiot and does not know anything about vehicles.

Even if you release the pressure when hot and screw the cap on and leave it for 1min and take the cap off again it can build pressure again,granted way less, and is normal since the coolant is still hot and will still try and expand.

If your system was designed to run no pressure in the system you would not have a normal pressure cap,you'd have a vent only.If your that worried go down to Napa and buy yourself a Block test kit and by the time you reach home safely remove the cap and start the engine and test away.If the blue fluid turns yellow then you have combustion gasses in the cooling system,if it stays blue your good.I'm pretty sure you have spent a lot of $$$ with a idiot to replace head gaskets that actually did not fail and your going to do it again.

flman · **Posted:** Thu Apr 10, 2014 4:06 pm

What is there to worry about 16 PSI cap, both of mine have pressure when they are hot, this is something all engines have had for 100 years.

OTOH, if the rad hose is firm and the system has pressure first thing in the morning when it is cold, like mine did when it had a HG leak, then you can start worrying.

Maybe it was the engine flush that did it in?? :frankie:

RJM · **Posted:** Thu Apr 10, 2014 5:21 pm

I believe that some head gasket failures are due to low PH. Hoat coolant is lower in PH than most other types of antifreeze right out of the bottle, coupled with using distilled water which is also lower PH than tap water (Tap water 7.0-7.4 normally distilled can be anywhere from 6.2 to 6.

This can be a destructive mix! most manufactures want the PH above 8.5+ for their engines. I always use distilled water but adjust the PH using calcium or sodium bi-carb to bring the distilled water up to at least 7.5-8.0 before it goes in the rad. Improper antifreeze chemistry can start chewing away at metals in short order especially when dis-similar ones are present in the engine. Essentially acting like a battery causing electrolysis. :2cents:

What's up with the smile where the eight is supposed to be??? can't change it, go figure.

racertracer · **Posted:** Thu Apr 10, 2014 6:59 pm

Lots of pressure under the cap after the ride home this afternoon. Released the pressure and performed the one minute engine run cap remove test and again found pressure under the cap. I will be doing the emissions gas in the blue bottle test, to positively confirm it's a Head Gasket leak.

jlgail4309 · **Posted:** Thu Apr 10, 2014 10:42 pm

Mine does the same thing when I pull my travel trailer. Head gasket

flman · **Posted:** Fri Apr 11, 2014 6:46 am

racertracer wrote:

Lots of pressure under the cap after the ride home this afternoon. Released the pressure and performed the one minute engine run cap remove test and again found pressure under the cap. I will be doing the emissions gas in the blue bottle test, to positively confirm it's a Head Gasket leak.

You are really convinced this is a zero pressure system? :banghead:

dirtmover · **Posted:** Fri Apr 11, 2014 8:58 am

racertracer wrote:

My diesel mechanic says "no pressure under the cap... period.... it's a diesel, a small amount of pressure after a long drive is OK but even then it's questionable."

Your mechanic is wrong. This is a pressurized system. Since it's a closed system it can't be any other unless Chrysler/VM have found a magic coolant formulation that doesn't expand with temperature.

The only thing open to question here is what degree of pressurization can be attributed to possible HG leakage versus normal coolant expansion.

racertracer · **Posted:** Fri Apr 11, 2014 1:19 pm

He's right... I've never before experienced pressure under the cap on this vehicle, no matter for how many miles I drove it.

Just recently it began to build up pressure.

Any ideas of what I am doing wrong.

grywlfbg · **Posted:** Fri Apr 11, 2014 2:58 pm

I would just watch the coolant reservoir level and overflow reservoir level. With my 2 head gasket/head failures I would start loosing coolant from the reservoir and the overflow would be full. Over time (few hundred miles) it would loose coolant faster and faster. At the end I was driving behind my wife (after having brought her a jug of water) and when she accelerated I could see the coolant hitting the ground.

So Just carry a jug of water w/ you and if you get a low coolant light, just top it off after it cools off. Then you know you have a few hundred miles to get it fixed.

flman · **Posted:** Fri Apr 11, 2014 4:05 pm

racertracer wrote:

He's right... I've never before experienced pressure under the cap on this vehicle, no matter for how many miles I drove it.

Just recently it began to build up pressure.

Any ideas of what I am doing wrong.

Maybe you left the cap loose previously? :jester:

Pressure under the coolant cap

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