Jon Schepps

I've been having my share of coolant overflow issues for the past couple of years, for which the "fix" has been to replace the headgasket. Now its doing it again, first track event after installing a new hg.
The symptoms are classic - engine temperature is normal, no coolant is lost during low-load driving, but after periods of on-boost driving, coolant pees out the overflow pipe. No overheating occurs (unless a LOT of coolant is lost).

For the purpose of this discussion, let's assume the head, block, studs, hg, pressure cap, bleeding process, a/f mixture, timing, etc., etc. are all good.

Here's the question: Suppose the coolant flow from the engine water pump to the turbo was blocked by constriction or a stuck turbo thermostat, such that the turbo was not getting water pumped over to it. Could this cause the water in the turbo (which it will still get via the expansion tank, through the turbo's electric water pump) to boil? If so, this boiling could eventually build up system pressure and cause dumping out the overflow pipe. In this scenario, the engine coolant temperature, measured over at the cyl head, will likely be normal, since that coolant is circulating properly. Only after sufficient coolant loss will the engine temperature start to rise.

Does this sound plausible? Has anyone ever attributed coolant overflow to blocked turbo cooling?

Jon.

hosrom_951

Yes, there might be something stuck in the system or a blockage.

Jon Schepps

hosrom_951,

Not sure what you meant by your answer. My question started with the condition that there IS blockage, and asked if this would cause coolant boiling by the turbo and subsequent overflow out the dump tube.

Are you saying that it would? If so, is this based on direct experience or intuition?

Thanks,
Jon

Songzzz

I dont think anything is stuck in there. My money is on a leaky head gasket. Although it could be the head has a hair-line crack.

Are you using stock head studs? Did you re-torque after initial heat cycling? How's the mating surface for both the cylinder head and block?

1. Try pressurizing the water system at 15psi then look thru the sparkplug hole to see if coolant gets into the combustion chamber.

2. If you have good vacuum (i.e. 18 to 20), what you can do is let the car idle for a good 15 minutes and see if you have white smoke at the tail pipe.

Jon Schepps

Songzzz and others,

Hmmm, I guess I didn't pose the question properly. I'm NOT looking to diagnose a problem. Let's consider it a hypothetical case as follows:

I have an engine that is in PERFECT condition EXCEPT there is no coolant flow through the turbo. Would this cause coolant boiling in the turbo, system pressure increase and subsequent loss out the overflow pipe?

PorscheDoc

Lets go with no then.

msaturbo

I had the same experience. My problem was the turbo's water coolant pump went bad and stopped pumping water thru the turbo after the engine was turned off.

I also found that the pressure cap on the coolant expansion tank was not holding pressure and helped cause the overflow.

msaturbo

Jason_86_951

How long does your turbo water pump run after the car is shut off??
It should run at least 30 seconds, and if the turbo temp sensor calls for it longer.
I'm betting that your temp sensor is faulty, which would still allow for the water pump to run for 30 seconds, but thats not nearly enough to cool a hot turbo after a spirited run.

I'm having the same issue, and did this as a little test.
After finding out that the temp sensor was faulty, I took the car for a good run, and let the car idle for a time then shut off.
I watched the pump run for it's 30 seconds, then it shut off because the temp sensor did NOT tell it to run longer, then I watched the heat from the turbo start to boil over the overflow tank, I quickly grounded the sensor wire and let the pump run for a additional 3-4 minutes. NO BOILOVER.

I also second the bad cap analysis. Lower pressure, also lowers the boiling point. Doesn't help your cause at all.
HTH
Jason

Jon Schepps

masturbo, jason-

Aha... I think we're getting somewhere. Again, I want to assume that EVERYTHING else is perfect - rad cap, turbo electric pump, turbo temp. sensor/thermoswitch, pump timer/relay.

In your cases, water was circulating through the turbo while the car was running, but when you shut it down, and the electric pump fails (jason) or the temp. sensor/thermoswitch fails (masturbo), then the water in the turbo will boil, pressurize the expansion tank and force an overflow.

This is more relevant to the question I posed, although in my case the scenario is somewhat reversed... that is, in my otherwise perfect engine, the electric pump and associated temp. sensor and timer do what they're supposed to do, BUT, when the engine is running, there is no coolant flow from the engine wp through the turbo. Based on your observations, it sounds like the result would be the same - boiling and overflow.

However, there is one other difference. In my "otherwise hypothetically perfect" engine - even if there is no coolant flow from the engine wp to the turbo, the electric pump would still turn on when the thermoswitch tripped, and it would circulate water. However, this water only circulates from the expansion tank to the turbo and back to the expansion tank, and therfore may not supply the cooling needed to bring it under boiling point, as you've both seen.
This also assumes that the electric pump is enabled when the engine is running (is it?).

I think we're on to something.

Thanks,
Jon.

Jon Schepps

Jason,

To specifically answer your questions-
Pump runs for 20+ seconds after engine shutoff, or longer if the thermoswitch tells it to. So pump, timer, thermoswitch are fine.

Rad cap is new, could be, but not likely to be faulty.

I'm simply trying to determine if this SINGLE POINT of failure (blocked coolant flow between engine wp and turbo) would cause system pressurization and overflow. Sounds like it could.

Thanks,
Jon.

Peckster

So what is the point of your question?

I had no symtoms like you describe when my turbo pump failed, and it was probably out of action for a while.

So I too would say no.

Head gasket. Hah!

Jason_86_951

Is it the right pressure rating???
There are 2 different ones IIRC, ours takes the higher pressure one.
If you have the wrong one, it could lead to boiling over.
I think ours is a 1.5 BAR, anyone know for sure?

Jon Schepps

Peckster,
The point is, I have already exhausted all the typical causes of this symptom, and I was therefore trying to bypass all of the usual diagnoses, such as:
head gasktet -> replaced it 3 times
stretched studs -> installed Raceware
head warpage -> its been skimmed and tested
block warpage -> its been measured
rad cap -> replaced
turbo pump -> replaced
water pump -> replaced
etc.
etc.
etc.

By posing the question as I did, I was trying to avoid having people second guess other problems, and focus on THIS failure mode. I have never seen information anywhere that suggests that failure to supply coolant through the turbo, as might be caused by a stuck t-thermostat in the water pump, or some other constriction, would produce symptoms identical to a failing head gasket. I am suggesting that it could. If I'm right, then this could help others who have experienced otherwise inexplicable head gasket failures (because its not the head gasket).


If your turbo pump fails, that merely means you may potentially overheat the turbo AFTER the engine is shut down. Others have apparently seen the coolant boil when this happens. Whether this causes an overflow is therefore only a matter of degree - i.e. how long the boiling persists.

So I don't understand the basis of your conclusion that it must be a bad head gasket, Hah!

Jon.

Jon Schepps

Jason,

Yes, the turbo uses a higher pressure cap, because they potentially run hotter than the NA's. This does not mean however, that at NORMAL operating temperature, the lower-pressure cap will fail. And I've experienced this coolant loss at well within the normal temperature range on the gauge. My car usually runs just above the lower normal range mark.

Jon.

Flight_951

When the engine is shut down the temperature will rise quite a bit even if the pump is working. If you're certain the cap is correct, I'd make sure the coolant is not much above the Minimum line on the expansion tank when cold. I'm saying this because you said you are losing coolant from the overflow without a high temp on the dash gauge.