mark kibort

I have a set up at hope to simulate full load dyno operation, for tuning and checking WOT system function as well as rear brake bedding for race pads. usually, it is a very easy process for most tests, but WOT is a little more tricky as you are putting the entire power of the engine at the control of the rear brakes. it takes a little time to get full equilibrium, and that amount of time can be 5-10 seconds , but the WOT period is only a few seconds. (normal dyno runs are about 6-8 seconds). without the dyn fans, temp can go up pretty rapidly, especially with multple back to back runs. we were trying hard to get confirmation of 3000rpm WOT and 5000rpmWOT, and by the time we got it, the engine was hot......... i did one more because the camera didnt focus on the harmonic balance timing marks as we were checking timing at WOT, so we did it again. the red overheat light came on just as we finished and i let it idle for a bit with the fans on . (both fans turned on as i was only using one) the temp got down to the below center mark and i felt the hoses and they were soft, just as they were at Thunderhill a few years ago during an overheat due to air in the system to start.. they were soft and had major air pockets in both hoses. water level had raised..... so,I looked under the car and there was a pool of water. .............i opened the cap.. not under pressure, as i mentioned hoses were soft and full of air , and started to birp the system slowly. (as you squeeze the hoses, the air escapes into the water bottle). when they were solid , i took note of the pressure (still at mid line) and turned off the car... the car lost about a pint of water, and i replaced it. in the am, felt the hoses......still solid, and started and drove the car. no issues. no leaks.......cap was tested, pressure high with hoses rock hard at operating temp.

the moral of the story here is that, when racing, max efficiency for cooling, you need to burp out the air of the lines. the highest point is NOT the water bottle it is the top of the water bridge, unless the water is filled more than half way, and even then, it doesnt allow for the air in the lines to completely burp out. fluid still moves with air, but if you have overheating issues, be sure to do that simple process. take cap off water expansion tank, squeeze both lines repeatedly and then replace cap............done.............. especially before a race or DE day , IF the car is cold and there is no pressure in the hoses..

the theory is that the water boils first locally, and the volume increases in some areas, which pushes the pressure up over the pressure rating of the cap, and then pushes air, and then water out the cap overflow tube. as the engine cools, the water returns to the lines and areas of the engine where the local boiling occurred , but not alll the volume is replaced with water. those are the air bubbles...... the overflow level will be higher than you started (if you caught it early enough) and then all you do is do the process while hot if there is no pressure in the lines.... replace cap and you are good to go. this iis exactly what happened to the engine at the thunderhill race a few years ago. it wasnt air flow, it wasnt headgaskets, it wasnt anything but a standard overheat that was caused by air in the system before the race. it happened one more tim at laguna when we had those 115degree temps , for which at thunderhill is barely possible to survive, but with the slower speeds at laguna, there was no chance. i had to back off for a few laps to regain normal temps and give up 2 positions in the race. no red lights of loss of fluid though. I cut more air holes in the nose and again, i have not seen any overheating, even on some pretty hot race days this season.

there was a lot of discussion of the system normally taking care of air bubbles and there not being a need for burping. My experience and that of porsche motorsports, who owned this car before me, and has not just one water bleeder but two of them, on the water bridge for (what i suspect as ) the purpose of letting air pockets out of the cooling system. (this was a modification to the holbert water bridge by porsche which i dont even use as i have a better way)

Here is a picture of the porsche after-manufact. fluid bleeders:

andy-gts

wheres the pic.....??

GregBBRD

The thing about theories....one tiny part that is not true or there is an exception to....makes the entire theory wrong.

Let's start with this...define one of your parameters:

#1. With the thermostat already open, how many feet per second do you think the water is flowing at 5,000rpms?

#2. At that flow rate, how fast is "boiling water" mixing with water just below the boiling point?

#3. Please define "local boiling", using #1 and #2 above. Is one entire side of the engine "local"?

Hey_Allen

Has anyone considered running a bleeder vapor return line from the top of the water bridge, something like the LS engines run from the high points on the heads?

I know it's not "how it was designed" but might solve a number of the air bubble at the thermostat issues I see so frequently in the 928 forums.


I freely admit, I'm new to the Porsche world and don't have all the answers, nor have I found all the threads yet. I am slowly digging further into the history of this section, and learning as I go.

Speedtoys

" I see so frequently"

What, 1 thread all summer?


AND..the fix was a stuck thermostat, not a bubble?

Hey_Allen

I don't know the exact timeline for the threads, but I've been reading back through the threads here and on Pelican Parts, and searching the topic as well (due to the car I was just given being parked 10 years ago, for overheating...) and see numerous threads on overheating, and a good few talking about different methods of burping the cooling system.


That all said, I'm glad to find them, and hopeful that the car I just picked up yesterday might have been parked for something so simply resolved.
Numerous other problems are likely after letting a CIS car sit for 10 years, but if that's the source of the overheating, rather than a warped head, blown head gasket, or spun cylinder sleeve, I'll count myself lucky!

Rob Edwards

There is already a bleeder line from the top of the water bridge back to the reservoir- 999 181 709 50 from 78-82, 928 106 275 02 from 83-86, and 928 106 275 03 from 87-95, and The 928 cooling system self bleeds in all model years, there's no need for burping. Drill a 2mm hole at 12 o'clock in the thermostat disc if you like, but even that's not necessary.

Speedtoys

True, but how _many_ were actual air pocket problems...

mark kibort

T-stat is working perfectly. it over heated. This is easy to force, and was exactly to the level i saw at the track. i dont know if it is "local heating" thats an assumption . what i do know based on the data is that the engine over heats and pushes water out the overflow due to over pressure through the cap, as designed.

as it continues to run, the systems contracts, the reservoir is now full and the cooling system is at normal temp, with air in the lines. (and they are now, not under pressure , as the air to replace the contracting water volume)
if the entire water system is equally boiling, then there wouldnt be local boiling, due to the high rate of flow.... i have to imagine water near or on the heads starts to boil first, no??

The bottom line, the system over heats, water will be pushed out via boiling and expansion, the water level rises to near the top of overflow and as it cools, that's where it remains. air in the lines, perfectly normal temps, (middle range) and no pressure in the system. It all follows GasLaw. PV/NRT

"If the theory doesnt fit the facts.........you change the facts" (Einstein)

mark kibort

when i had air in the system starting from cold, for some reason.. the t-stat wasnt getting enough flow through it , the engine would start to overheat. (on the street, i would pull over and the radiator was cold) i then turn off the engine, and restart after a few seconds, and immediately the car would return to normal operating condition temps. ever since ive been making sure system is burped.. i havent had this issue in a few years.

mark kibort

It may have some function , but it doesnt solve the problem. infact, if it worked so well, Porsche Motorsports wouldnt have modified the top of the bridge with a bleeder on both upper engine water inlets and outlets.
the fact is, generally, i do believe over time, the engine will self bleed to a point.... again, i just over heated, pushed out the water , ended up with air in the system and it wasnt going anywhere and didnt go anywhere. but, by opening the expansion valve, and burping the system.... squeezing both hoses simultaneously, pushes the air to the overflow container...... 5 or 6 successive squeezes, drops the overflow level by about a quart if there is a lot of air. i even tried jacking up the rear of the car to see if the "bleeder line" could solve the issue and it cant............a least for 10 -15mins of running. the air gets trapped and there is no way for it to escape. however, over heat cycles, it seems to get better with each heat cycle, where the the entire cooling system expands and contracts.

Rob Edwards

60,000+ Porsche 928s were built over 18 years without bleeders on the top of the water crossover, and none of them overheated unless there was _something else_ wrong in the cooling system. We've already discussed this in the past, do I need to re-post the pics of the reservoir being the high point in the system?

When's the last time you pressure tested your cooling system? Or replaced your $8 reservoir cap?

mark kibort

Here are the bleeders that were added by Porsche motorsports. the obviously thought that the stock system was not adequate in getting the air out of the system if you do some measurements, you see that the bottom of the reservoir, is HIGHER than the top of the water bridge. this is why the bleeder hose is not as effective as it could be.

mark kibort

it's adequate. and if you have plenty of heat cycles and are not racing with air in the system, from a prior boil over, or first fill up, you will have a problem on the tract. there is a reason why i run below centerline during all races.... its bled and its functioning properly. however, if i do boil over, like i did on that 115 degree day, the system ends up with air in it and it doesnt solve itself until bled or having several heat cycles.

Ive already measured the heights.... just did right now... the lowest part of the reserior is 22.5". the height of the "Y" leading to the reservoir is 20-21"... the height of the water bridge of the outflowing hose from the pump is 23" and the height, of the lower radiator hose meeting the housing )top is 22". i just measured. this means the water bridge is the highest point, so, yes you can air pockets that cant clear easily in the main hoses.

I pressured tested the system and the cap is in spec too. also tested and is not cheap.PORSCHE CAP functioning to spec. this is not my first cooling rodeo. the system hold pressure fine, no leaks. why do you think racing at 90degree track temps, dosnt bother my system with no water loss, and very cool temps. you can see the videos. the temps are always way down during races on 70 degree days, and barely to mid during 80 to 90 degree days.

Rob Edwards

22 minutes ago, in post #11 you said you overheated, now you're saying your temps are way down on hot days. Which is it? It's like arguing with Jello.

You were right, I was wrong. Radiator caps are $9, not $8. Very expensive.

http://shop.928intl.com/Radiator-Cap...28-106-257-03/