Coolant system pulling vacuum

[geogecko87]

The car is a 1997 Z3, 1.9l with 160k miles. I repaced the entire cooling system and have bled all the air out several times. The heater works fine, even at idle and the running temperature gets up to 99-101 C in Florida summertime driving and never overheats (gauge never goes above middle mark). There is no white smoke from the exhaust, no oil in the coolant or coolant in the oil, and the engine runs fine. The cooling system does not loose any coolant over several weeks of casual driving.

Now for the strange part. Twice now, after the car has set for a day or more after driving, I have noticed that the coolant expansion level seems a little high and the upper radiator hose is collapsed. When I loosen the expansion fill cap, there is a sucking sound, and the coolant level drops to normal and the hose expands. It appears that the system is loosing air when hot and then pulling a vacuum as it cools. But where is the leak? Should I be concerned?

[Mr Bingley]

It sounds like you are overfilling the expansion tank when cold. Not familiar with the 1.9 system, but on my 3.2 the fill line when cold is significantly lower in the tank than I would expect based on other cars--less than half way up the tank (tank is taller than it is wide). You might also replace the cap as a matter of course.

[cyberman]

It sounds like you are overfilling the expansion tank when cold. Not familiar with the 1.9 system, but on my 3.2 the fill line when cold is significantly lower in the tank than I would expect based on other cars--less than half way up the tank (tank is taller than it is wide). You might also replace the cap as a matter of course.

I have a 96 with the same engine. The fill mark (max) is about halfway up the reservoir tank. In the garage, I have to use a flashlight to verify the level because it is so far down.

[geogecko87]

That's where I try to keep my level. The level rises 1-1.5 cm when the upper hose collapses and then returns to the halfway mark when the vacuum is released.

[cyberman]

That's where I try to keep my level. The level rises 1-1.5 cm when the upper hose collapses and then returns to the halfway mark when the vacuum is released.

The system should allow coolant to freely pass back and forth between the radiator and the reservoir. I can't find much researching this because the words "vacuum" and "cooling system" bring up stuff about vacuuming the AC system. There is a thread on a Subaru forum I found here: https://www.ultimatesubaru.org/forum...system-vacuum/ where the OP has a similar problem. The main thing I get from this is that it is possible you have a very small head gasket leak that's introducing a small amount of air into the cooling system. I know you don't see any other signs, such as coolant in the oil, but a tiny leak may be just enough. I'm hoping that isn't the issue.

BTW: the cooling system gauge is somewhat useless as it's designed to stay centered until the temperature is very high. The did this to keep drivers from freaking out. I hooked up a cheap bluetooth engine monitor to the OBDII port inside the car so I can read the coolant temp more accurately on my Android phone. I picked one that never stops drawing power, so I had to put in a cable with a power switch. One day I'll purchase a new one that has auto-shutoff. I use a $5 app called Torque Pro.

[geogecko87]

I also use bluetooth OBDII through Torque Pro, which is how I know my temps stay between 99 and 101 C while driving. I also thought of a small headgasket leak raising the system pressure above 2 bar and blowing excess pressure past the cap. I hope that's not the case. I guess I could buy one of those gas analyzers that pulls air from the cooling system through a liquid reagent to check. I hypothesized that additional gas pressure added to the system would only raise the pressure to 2 bar (the cap setting) and leave excess pressure in the system on cooling, not a vacuum. Obviously I might be wrong.

In the meantime, I purchased a genuine BMW cap to replace the Mahle one, and discovered that the BMW one has a built in vacuum relief valve which the Mahle seemed to lack.

Cyberman, if you are using a Torque Pro on a 1.9l, what driving temps do you see?

[cyberman]

Cyberman, if you are using a Torque Pro on a 1.9l, what driving temps do you see?

I see 100.3 to 104 (when it's really hot). Never more even when outside temps are 95 or more. I'm also wondering if you've got a small amount of air in the system. It took me a better part of a day using the bleeder screw with the front of the car elevated to get all the air out when I did the cooling system replacement (everything). I had the heater valve open, everything recommended, but still had to go through an ungodly number of heat-up, bleed, and cool-down cycles. Even then, I kept having to add a small amount of coolant every day or two. But the lack of the vacuum relief valve may be a clue and cure the issue.

I have a small loss constantly now, perhaps 2-3 marks on the coolant reservoir every thousand miles or so. I've put dye tracer in the coolant and can't find a leak. I'm hoping it's not a pinhole under the dash. Then again, maybe it's normal.

[geogecko87]

Your temps sound like mine, even in 100 F Florida weather (sitting still it can reach 105 before the fan kicks in and brings it immediately back down), so maybe I'm not far off. I'll try bleeding a few more times. BTW I used one of those titration headgasket leak detectors on it - twice - for 3 mins each time and the liquid never changed, so if there is a leak it's either very small or episodic. Thanks for your help.

[cyberman]

Let us know if the new cap helps.

[geogecko87]

Thus far the new cap seems to have solved the problem. I'll reopen the thread if the problem returns.

[Hoyt Clagwell]

Thus far the new cap seems to have solved the problem. I'll reopen the thread if the problem returns.

Good luck!

[cyberman]

Thanks for the update, and, hopefully, closing the case and thread.