Hello folks,

I guess this summer is taxing all the 928 cooling systems. Seems like there are a lot of inquiries. I'm having a little trouble myself. My car, which is an 89 s4, is losing coolant from the expansion tank. The interesting part is, the temperatures at the coolant bridge are roughly 175 to 185 degrees. The temperature indicator in the cabin is either needled at the first white line or halfway to the second white line. This is where it typically runs although most often just above the first white line.
So basically the way it seems, I have elevated system pressures but not the expected coolant temps. I thought perhaps I had combustion gases entering the system. I did a check for this and it came up negative. The car always had a very small coolant leak, adding maybe 1/2 cup of coolant every two months. I couldn't find the leak. I thought perhaps the leak was so small that it was cooking off the hot engine before it could drip to the ground. I pressurized the system cold with sheets underneath and still couldn't find a leak. I don't know if it's related, frankly I don't think so.
One curious thing I noticed; when the car is up to temperature and the expansion tank is almost filled, I rev the car to 1500 rpm and I can hear a mechanical "tock" in the thermostat housing. Nowhere else in the engine so it's not like detonation. The only way to describe it as a mechanical intermittent "tock...tock"...... maybe it's a sticking thermostat. Has anyone ever heard their thermostat function? I used a stethoscope with a steel pickup.
All hoses, water pump, expansion tank, were replaced during an engine rebuild roughly 3 years ago, maybe 15,000 miles since then.
I'm going to go boil some 928 thermostats on the stove to see how they behave while operating. In the meantime, if anybody has any experience related to this sort of thing, please jump in.

Sorry for the long post,
thanks,
Coleman

 

check for leaks in the expansion tank, if you are not having overheating issues.
the clicking noise from the water thermostat would be noise from the water pump. water could be leaking from there and evaporating. if it is truely coming from the expansion tank, put a little water bottle on the exit tube and see if it is coming from there. just duct tape it to the little overflow tube and tie wrap it to the firewall , so it doesnt hit the headers. probably could be done very easily from underneath the car. that will at least tell you where the water is going or coming from. if not from there, it has to be from the hoses, pump or going into the engine combustion chambers.

 

Thanks Mark.......
I put a coolant catch can at the expansion tank and it does fill.......so the coolant is coming from the tank overflow. The noise, you said, may be the water pump. All I would expect to hear is a bearing noise not the random mechanical clicking..........have you heard that type sound from this area before? Also why the elevated pressures with a normal temp?

Thanks,
Coleman

 

The expansion tank should be filled to the half-way mark when hot.

When was the last time you replaced the cap on the expansion tank? They do go bad over time.

 

Thanks Randy......
I always thought the seam in the middle of the tank was the cold fill line....then an inch or so of expansion when hot. In my case it is overflowing when hot.

Thanks,
Coleman

 

Coleman--

Your symptoms may not be good. High cooling system pressure, the presence of gasses in the system and the high level when hot are clasic indicators of a blown headgasket.

Before we jump to that conclusion though, what is your coolant blend? The reservoir cap needs to hold correct pressure, else it will leak and cause the noise you describe. Also, there's a critical rubber seal that fits in the water bridge behind the thermostat. That seal is the rear seat for the thermostat, allowing the thermostat to completely block coolant recirculation when the coolant is up to temperature.

Ultimately, the ticking is likely from vapor bubbles, either steam from a system that won't hold pressure, or combustion gasses passing through the pump. So, replace the reservoir cap, verify the coolant blend is at least 25-30% but not more than about 50%, add a bottle of Redline Water Wetter to the mix. If that thermostat seal wasn't replaced when the hoses and other bits were last changed, do that now with a new thermostat and front housing seal. Oh-- more than a few folks have installed the front thermostat seal, the big o-ring one, between the thermostat and the water bridge. Correct sequence is rear seal, then thermostat, then front seal (the o-ring), then the thermostat housing/lower radiator hose nozzle. If the o-ring goes in before the thermostat, ther will be a space in front of that rear seal where coolant will recirculate rather than pass through the radiator.

 

as long as you are a little over half way ALL the time, you are ok. I have found that under filling the overfill tank has issues with flow in the system.

the cap is a non issue usually, you can check to see if it is holding pressure. its only 14psi before designed release , so its pretty easy to see when its warm, if you have a moderate seal there.

It sure sounds like head gasket to me, but does it happen when you really beat on the engine , or just street cruising?

 

Bob,
The mix has always been 50/50 here in hot humid Va. I'm quite confident when I rebuilt the engine a new thermostat "base" seal was installed and in proper sequence. I've fitted a new cap at 13 lbs. The presence of combustion gases was not indicated when I did the "block test" at the expansion tank. I agree....all symptoms imply a head gasket or worse.....but the test doesn't substantiate this. I suppose I should at least pull the thermostat to verify it's proper function.....and gasket orientation. I'll add the water wetter after...if things are unchanged. Also, what thermostats are most folks using? I have an extra 83 and 75 .

Appreciate the help,
Coleman

 

All,
When I think about it..........would coolant leaking into a cylinder or vise-versa be obvious in a plug read? If I'm looking at a head gasket, which cylinder bank?

 

I had a blown headgasket on my 81. It turned out to be an external leak though. The valley was filling just enough to overflow down the back of the bellhousing onto the CAT. There it was being boiled away, leaving white streaks onlong the CAT (that should have been my clue). If you have an internal leak the plug for that cylinder is usually clean. Mine were all dirty but in the end I was filling the expansion tank 1-2 times a month.

Good luck,

 

If you have coolant leaking intonthe cylinder it will usually show up as an extremely clean looking piston and plug, but it is quite likely that if your had gasket is failing that you will not leak coolant into the cyclinder as there is only a max ~30 psi (15 for the rad cap and 15 if the piston draws a pure vac on the intake stroke) differential between the combuston chamber and the coolant. It is much more probable that the the combustion gasses will push past the head gasket on the power stroke (since the pressure differential is much higher), and cause coolant system over pressure and displace coolant out the over flow. I still wouldn't jump to the conclusion that the head gasket has failed just yet, especially if you ran a block test and the results came back negative.

 

He said he checked for combustion gases and the test was negative.

Anyway, a leak at the rate of 1/2 cup over 2 months is going to be very hard to find. That's the equivalent of 2-3 drops per hour (1/2 cup = ~120ml; ~20 drops per ml). I'd consider that negligible. Double check all the hose clamps and feel around the ends of all the hoses for wetness or dried mineral desposits.

 

Great observatrions , both...Tom and Simon.
Tom..... I'll try and "black light" it this evening.
Simon....I'll read the plugs as a matter of course and post the results.

However, I'm still stymied as to why I have such substantial buildup of pressure under moderate load with a negative result in the "block test".....It seems only combustion pressures could have such a rapid onset.

 

I had a leak on the GTS that ended up being two small leaks. One was one of the hoses under the coolant reservoir. (I replaced both while I was there) The second didn't manifest itself until I got the first one fixed and was the pressure sensor on the engine side of the passenger wheel well. Both were small leaks, but allowed pressure and ultimately coolant to escape.

I had similar symptoms to you and similar coolant leakage.

 

I missed that you had elevated system pressure - how did you determine that? If so, the overflow bottle cap would relieve the pressure at just below 1 bar and therefore would be the source of the loss (which would go our via the overflow hose)?