Residual pressure: valid symptom of bad head gasket? - PeachParts Mercedes-Benz Forum

DieselAddict · #1 09-30-2007, 04:14 PM

I'm starting to question the validity of this symptom. My '96 E300D has had minor residual pressure in the cooling system as long as I can remember and I always attributed it to a leaky head gasket. Recently I went 2 weeks without releasing the pressure and it built up to about 3 psi cold, surely a sign of a leaky head gasket, right? Maybe not. The hot pressure is still about the same, around 7 psi and the system is leak-free based on what my pressure tester is telling me. Could it be just water that evaporates and gets trapped in the reservoir? Interestingly, my 240D used to do the opposite and it would build some vacuum in the system after cooling down. My Jetta seems to have no pressure at all or a very slight vacuum when cold. The vacuum doesn't make a whole lot of sense either. Water vapor pressure is all I can think of. The big test for my E300D will be when I do my first oil analysis. Hopefully there's no coolant in the oil.

kerry · #2 09-30-2007, 04:57 PM

There's also a product that tests for the presence of exhaust gas residue in the coolant.

Ken300D · #3 09-30-2007, 08:24 PM

I do not feel it is a valid test and feel sorry for the people who tear down their engine based on it.

If you see bubbles, that's a sign. But residual pressure is not necessarily due to a bad head gasket.

Ken300D

DieselAddict · #4 10-01-2007, 03:59 AM

Kerry, you're correct there's such a product and maybe one day I'll get it. The trick might be finding one that tests for diesel exhaust and not just gasser exhaust.

Ken, I think it is a valid test when you have a rock-hard hose in the morning after just one drive, but that's not even close to my case. I almost decided once to tear down my engine because of this but I'm glad I haven't done so yet. Do you have any other ideas what causes the residual pressure?

patbob · #5 10-01-2007, 04:53 PM

I think we can rule out a decrease in volume as a cause of the increase in pressure

.

The engine is either pumping gas in, sucking it in, or there's a chemical reaction going on that is producing gas. If it's not exhaust gasses, then is there any way to pump in intake air (I can't think of any)? Or a way to suck in air someplace (water pump intake somehow?)? If you haven't changed the coolant in a number of years, perhaps it's anti-electrolytic properties have been exhausted and it's time for a change?

TMAllison · #6 10-01-2007, 05:19 PM

Why not compression test it?

rwthomas1 · #7 10-01-2007, 06:53 PM

I had one of the rare cases of a 617 with a bad headgasket. It would push just a bit of coolant out of the overflow tube after running hard. The giveaway was that the coolant actually turned dark from the soot over time. Its pretty obvious when you see it. The strange thing is the engine ran great otherwise and did so for 25K before I tore it down. I say test the coolant and do a compression test. Just to be really sure. RT

DieselAddict · #8 10-02-2007, 02:57 AM

I'm not sure compression testing will tell me much. This is a very minor and gradual pressure build up in the cooling system, meaning I can take the car and run it WOT several times and the upper radiator hose is still squeezable and the next morning there will be no pressure, assuming I let it out the day before. It takes several days of driving to actually start building up any noticeable pressure. The coolant is fresh and there has never been any visible soot or oil contamination. I'm thinking this may be caused by water evaporating out of the coolant mix and not fully condensing when it cools down again, but I don't know how plausible this idea is. Like I said earlier in the past I always thought it's a leaky head gasket, but what puzzles me now is why the hot pressure remains about the same regardless of whether I periodically release the pressure, i.e. it never seems to go over 7 psi or so when hot, and the system seems to hold pressure well and doesn't leak. If the head gasket really was letting combustion pressure into the cooling system, wouldn't the cooling system eventually pressurize to the cap's limit, which is about 20 psi?

Ken300D · #9 10-02-2007, 12:09 PM

What you are describing is actually the rare case of a vintage Mercedes cooling system in good condition.

The rest of them slowly leak down when not running.

It's water vapor.

Ken300D

DieselAddict · #10 10-02-2007, 03:07 PM

But mine isn't really vintage. Or are you saying that mine is actually working as it should and the rest of them that don't have pressure are all leaking?

patbob · #11 10-02-2007, 08:18 PM

Quote:

Originally Posted by DieselAddict

This is a very minor and gradual pressure build up in the cooling system, meaning I can take the car and run it WOT several times and the upper radiator hose is still squeezable and the next morning there will be no pressure, assuming I let it out the day before. It takes several days of driving to actually start building up any noticeable pressure. The coolant is fresh and there has never been any visible soot or oil contamination.

I'm just curious.. what kind/brand of coolant? Also, does the pressure still build up if you don't drive the car for a few days (or maybe weeks, since everything won't be getting hot)?

If pressure builds up, even slowly, without driving the car, then that's not normal (or shouldn't be) in my book.

JimmyL · #12 10-02-2007, 11:32 PM

Heck, just go ahead and get your oil analysis done a bit early. I would have to know what that info told me......

DieselAddict · #13 10-03-2007, 02:33 AM

patbob, I use Zerex G05 coolant. The pressure doesn't build up while the car is just sitting, that certainly wouldn't make any sense. It builds up as the engine warms up as it should, but then as the engine cools down the pressure doesn't go down all the way to where it was originally. After just one day of driving there's no noticeable residual pressure, but give it a few days of driving and it will accumulate a little bit.

JimmyL, I'll probably have the analysis done in the next couple months. I'm not really in a hurry as the car has been driven like this for a long time.

patbob · #14 10-03-2007, 02:51 PM

Quote:

Originally Posted by DieselAddict

patbob, I use Zerex G05 coolant. The pressure doesn't build up while the car is just sitting, that certainly wouldn't make any sense. It builds up as the engine warms up as it should, but then as the engine cools down the pressure doesn't go down all the way to where it was originally. After just one day of driving there's no noticeable residual pressure, but give it a few days of driving and it will accumulate a little bit.

Well, I dunno. I'm only harping on the chemical reaction theme because it's the most damaging thing I can think of that could cause this if the head gasket isn't leaking. If it were a chemical reaction that's producing the gas, then it'd probably be happening even when the car was sitting, albeit perhaps very, very slowly -- most chemical reactions are accelerated by heat. Exhausted coolant anticorrosion packages, or using a coolant that doesn't have one, could both cause it, neither of which are your case.

Not head gasket, not chemical reaction, doesn't sound like much to worry about then.

vstech · #15 10-03-2007, 02:57 PM

these cars have no overflow resivoir. so if the pressures drop from cooling below the cap resistance, some air will enter. air does not compress, so you can get a very small pressure build up over time. solution? change the cap. make sure you have a MB cap.