i have noticed my radiator hoses are very stiff after driving a bit. it has been doing this for a while now (over a year). my cooling system has always been maintained extremely well. i have never had the gauge over 100c and the coolant is very clean. it doesn't lose coolant either it's just seems after a drive there is a lot of pressure. when i release the cap, it only smells sweet like antifreeze. i have the cap marked "100" and im guessing it is original if not, it's at least 15 years old. the springs seems very strong. i'm about to get a lower pressure cap because i don't want to blow something up like the rad tank or a heater hose. what are the symptoms of a blown HG or cracked head on these engines? I really hope that this is maybe normal. I have put lots of miles on like this driving quite hard and it hasn't gotten worse. the pressure goes away after the engine cools down. what do you think?

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

I personally think you should leave everything be. Unless somethings out of spec. dont fix it. There are many ways to test. If excessive pressure exists, it points you to combustion pressure. Which would displace the coolant in the head. The plugs may indicate overheat. If there was combustion pressure present in the coolant you might see a series of trapped airbubbles travelling into the remote tank. But most definitively, theres a tester and a test sample fluid that is used at a good parts store. Usually one with a machine shop. You put the test fluid in the tester and sample the rising heat over the rad. cap area. cap removed. If theres a discoloration, the color change can be read according to the instructions. You can also pull out your thermostat and run the procedures for diag on the part. Look at it for opening. Measure the temps its experiencing during the test and compare it to spec. You can monitor cooling fan operation and compare it to spec., leakdown test. cap pressure test. and on and on. steam from the exhaust? after its warmed up? leaves blocking the rad. fins? no fan during a/c operation, discolored oil? water droplets in the oil cap. creamy discolored deposit underside of oil cap? rough run? greenish spark plugs. coolant fouled oxy sensor. these are all indicators . There are probably many more. See startek info for your vehicle specs. Since this is a question that can be best answered by experienced judgement, ask a technician to run it through the tests and see if anything fails.

You get my vote on a lower pressure cap. I ran a 7# on my signature car for years, and now that I converted it to NPG, I run it with the cap loose at 0 PSI.

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'83 300D, 126K miles.

Pressure when hot? No pressure when cooled off?

Guess what?

That's normal.

A physical property of water is that it expands when heated.
Hence the pressure in the system after you run the car.

As the car cools down, the water volume (pressure) decreases, ideally back to ambient pressure (no pressure differential on either side of the cap).

As long as the pressure does not exceed the rated pressure on the cap, no coolant will be lost. This is how a system in good working order will operate.

Generally, a compression leak into the coolant system will be noted by pressure remaining in the system even after an overnight cooloff. If you have a pressure differential on a completely cooled system, this is most likely a combustion leak into the coolant system, pressurizing the system beyond it's original atmospheric pressure (at rest). Conversely, a coolant leak into the combustion chamber is noted by white exhaust and a sweet smelling exhaust.

It's also a bad idea to remove the pressure by loosening the cap. While this may help a terminally ill system, it hinders effective heat transfer within a good system. Water's boiling point is raised as the pressure increases. This fact helps keep subnucleate boiling reduced, greatly increasing the heat transfer to the water. Additionally, growth in the water's volume (remember, heat causes this to happen) will cause a loss of water as it is ejected from the (formerly closed) system.

Hope this helps explain what you are seeing on your car.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

Before you buy a new cap, test the one you have. If it's 15 years old, it's probably already down to a fraction of its rated pressure. No need to pay for a 4 psi cap if you already have one!!!

(Which is not to say that I recommend a lower-rated cap in the first place.)

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Take precautions when removing the radiator cap or it's overflow container equivelant with any engine up to temperature. There have been many burns inflicted over the years this way.

At a minumin always use a cover rag over the cap and partial release only first. Much better and less chance of a scalding to let the system cool off first.

Your coolant system by your description seems normal and this is the way a good tight system should operate.

The cap definitely holds pressure. I don't think it ever releases on it's own. On the 300sd I sold, it could be 96° outside and had just gotten off the toll way but the hoses were pretty soft. I literally cannot pinch my upper rad hose at all until I pop the cap. Is it possible that because my car has mostly all new hoses that they aren't porus and letting pressure out? I know that sounds silly... If also seems like if drive easy, it doesn't build as much pressure in a short drive. If I drive harder where im in full boost, it builds pretty quickly. Again, I go on 3+ hr drives on the highway often and that's keeping up with the Dallas flow while not overheating or anything like that. I did notice that if I top the expansion tank off, it will always make it's way back down to right under the line. This is in months though... I think the pressure slowly pushes it through the little gaps it can find. I noticed it dripping from the drain plug on the rad last night...

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect

https://www.youtube.com/watch?v=06ecZNqgPDM&feature=youtube_gdata_player

Here is a video of the pressure to give you an idea

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect