Coolant Flow Path Mystery
09-20-2003, 04:22 PM
#1
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Important Fan Speed / Hot Running Information!
Since this old thread has been dredged up from >1yr. ago, I guess it deserves an update...
So that you don't have to read the entire thread to find the most important information:
*********************************************************
I found ~7V at the fans at ~190F (where they should have ramped up to ~9V). There is a significant difference in the amount of air being moved between these two voltages.
Cleaning the ground point which is located nearest the ballast resistors corrected the problem. (Follow Wally's advice & check all your grounds!)
*********************************************************
Since then I have removed the flap assembly, added a pusher fan and am running a 33% solution of antifreeze with WaterWetter. The pusher fan is independently controlled with a thermostat bulb wedged between the tubes near the outlet fitting of the radiator. This fan is one of the 30A units from the earliest cars; it is 3" thick with a 16" diameter and moves a *lot* of air.
The temp gauge behaves much better, and the engine temperature stays below 200F as measured with my infra-red. Airflow seems to be a big factor in what I was observing, but I still feel that the car should not need so much help to maintain its cool. I plan on swapping the radiator out, measuring the coolant flow, and maybe even pulling the water pump to check the impeller and cavity.
*********************************************************
So that you don't have to read the entire thread to find the most important information:
*********************************************************
I found ~7V at the fans at ~190F (where they should have ramped up to ~9V). There is a significant difference in the amount of air being moved between these two voltages.
Cleaning the ground point which is located nearest the ballast resistors corrected the problem. (Follow Wally's advice & check all your grounds!)
*********************************************************
Since then I have removed the flap assembly, added a pusher fan and am running a 33% solution of antifreeze with WaterWetter. The pusher fan is independently controlled with a thermostat bulb wedged between the tubes near the outlet fitting of the radiator. This fan is one of the 30A units from the earliest cars; it is 3" thick with a 16" diameter and moves a *lot* of air.
The temp gauge behaves much better, and the engine temperature stays below 200F as measured with my infra-red. Airflow seems to be a big factor in what I was observing, but I still feel that the car should not need so much help to maintain its cool. I plan on swapping the radiator out, measuring the coolant flow, and maybe even pulling the water pump to check the impeller and cavity.
*********************************************************
Last edited by Old & New; 10-05-2004 at 10:49 AM.
09-20-2003, 05:51 PM
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Wrong!
The coolant flows from the water pump through the block. It RETURNS to the cross pipe behind the thermostat and, either goes directly into the UPPER radiator hose or is diverted back through the water pump without going through the radiator. It flows from the LOWER radiator hose through the thermostat housing and down into the water pump.
The coolant flows from the water pump through the block. It RETURNS to the cross pipe behind the thermostat and, either goes directly into the UPPER radiator hose or is diverted back through the water pump without going through the radiator. It flows from the LOWER radiator hose through the thermostat housing and down into the water pump.
09-20-2003, 07:41 PM
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How about if I look at an official 928 publication instead? (Porsche 928 Service Information, 4582.21)
It says:
"The coolant is forced by the pump (4) into the cylinder block thruogh ports (16), flows around the cylinders and into the cylinder heads through channels (17). After passing through the cylinder head the coolant goes to the thermostat housing (7), in which the thermostat (6) is located.
On a cold engine the thermostat closes the return from the radiator and opens the bypass passage to the pump (4). The coolant recirculates inside the engine.
Once the coolant has reached a temperature of 83 deg C, the thermostat begins to open the radiator return passage (19) to the pump. The bypass passage to the pump is closed more and ore. The coolant now passes through the radiator.
If the heater is turned on, coolant goes from the right cylinder head to the heater core and from there back into the filler line (22) or expansion tank (13). The filler line goes directly from the expansion tank to the thermostat housing."
The diagram clearly shows the coolant entering the radiator through the upper hose, and returning to the thermostat housing through the lower hose. This top-to-bottom flow is the standard flow through the radiator for virtually all cars with vertical tubes.
It isn't possible to make a centrifigual flow pump flow backwards by changine the impeller design or putting one on backwards. Flow is always in the center of the impeller, out at the outer edges.
Your problem may be due to a sticky thermostat. Instead of opening slowly and smoothly, it might be that it hangs shut until the expanding wax in the bulb forces the valve open with a snap.
Wally Plumley
928 Specialists
www.928gt.com
It says:
"The coolant is forced by the pump (4) into the cylinder block thruogh ports (16), flows around the cylinders and into the cylinder heads through channels (17). After passing through the cylinder head the coolant goes to the thermostat housing (7), in which the thermostat (6) is located.
On a cold engine the thermostat closes the return from the radiator and opens the bypass passage to the pump (4). The coolant recirculates inside the engine.
Once the coolant has reached a temperature of 83 deg C, the thermostat begins to open the radiator return passage (19) to the pump. The bypass passage to the pump is closed more and ore. The coolant now passes through the radiator.
If the heater is turned on, coolant goes from the right cylinder head to the heater core and from there back into the filler line (22) or expansion tank (13). The filler line goes directly from the expansion tank to the thermostat housing."
The diagram clearly shows the coolant entering the radiator through the upper hose, and returning to the thermostat housing through the lower hose. This top-to-bottom flow is the standard flow through the radiator for virtually all cars with vertical tubes.
It isn't possible to make a centrifigual flow pump flow backwards by changine the impeller design or putting one on backwards. Flow is always in the center of the impeller, out at the outer edges.
Your problem may be due to a sticky thermostat. Instead of opening slowly and smoothly, it might be that it hangs shut until the expanding wax in the bulb forces the valve open with a snap.
Wally Plumley
928 Specialists
www.928gt.com
09-20-2003, 10:35 PM
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Turbochargers also have this "radial inflow" characteristic on their turbine section....gasses flow from the circumference, past the impellor "buckets" and then into the center toward the exhaust.
Do the IR thing; I wonder about your gauge.
N!
Do the IR thing; I wonder about your gauge.
N!
09-20-2003, 11:27 PM
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The IR says the hoses are 205F as the car just sits idling with an ambient temp of 70F. Not good, me thinks.
The car alway hovered just under the second white line, and now it hovers just over the second white line. The addition of the supercharger obstructed the radiator a bit & the fans probably move a little less air, but the problem is apparent at highway speed. It hovered just under the red arc the other day when I was on a long stretch at 80mph at 80F. It seems like it is very marginal & I wouldn't trust it to not boil over on a hot day, given the way it acts now.
Next I will probably pull the thermostat (and plug the bypass passage) and do some flow testing before I go after the pump.
The temp gauge on my 81 always stabilized at one spot and never moved much, ever.
The car alway hovered just under the second white line, and now it hovers just over the second white line. The addition of the supercharger obstructed the radiator a bit & the fans probably move a little less air, but the problem is apparent at highway speed. It hovered just under the red arc the other day when I was on a long stretch at 80mph at 80F. It seems like it is very marginal & I wouldn't trust it to not boil over on a hot day, given the way it acts now.
Next I will probably pull the thermostat (and plug the bypass passage) and do some flow testing before I go after the pump.
The temp gauge on my 81 always stabilized at one spot and never moved much, ever.
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09-21-2003, 01:04 AM
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Make sure that the rear seal for the thermostat is in place and in good condition. The thermostat must seal against the rear seal. Make sure that the thermostat moves far enough to seal when it is hot.
09-21-2003, 02:15 AM
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Ernest,
Yes. See above. The addition of the s/c slightly exacerbated the problem, but the gauge sat on the high side before the s/c.
Wally,
Yes, I just installed the new thermostat and seal that I got from your company. The thermostat I took out tested good, and the seal wasn't bad, either.
Anyone have any IR readings of their radiator hoses? Mine read 205F when the needle was slightly above the second line.
Yes. See above. The addition of the s/c slightly exacerbated the problem, but the gauge sat on the high side before the s/c.
Wally,
Yes, I just installed the new thermostat and seal that I got from your company. The thermostat I took out tested good, and the seal wasn't bad, either.
Anyone have any IR readings of their radiator hoses? Mine read 205F when the needle was slightly above the second line.
Last edited by Old & New; 10-04-2004 at 11:28 AM.
09-21-2003, 02:42 AM
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The sensor on the driver's side of the car on the cross pipe behind the thermostat housing is the gauge sensor. The pipe near it should agree with your dash gauge. The temperature on your gauge is reading about 195 to 200 degrees F with 180 to 185 degrees F being in the "comfortable" range.
The cooling system depends on:
Adequate coolant flow with a good water pump, a properly bled cooling system, and a normally functioning thermostat.
Adequate radiator cooling
Adequate air flow through the nose under the spoiler, through the A/C condensor, and over a properly functioning radiator.
Adequate oil cooling with a properly functioning oil thermostat.
In my case, I have thoroughly checked every system except the oil cooler and I'll be doing that this week.
I, and a number of others have a similar problem and, so far, no cause or solution has been found.
The cooling system depends on:
Adequate coolant flow with a good water pump, a properly bled cooling system, and a normally functioning thermostat.
Adequate radiator cooling
Adequate air flow through the nose under the spoiler, through the A/C condensor, and over a properly functioning radiator.
Adequate oil cooling with a properly functioning oil thermostat.
In my case, I have thoroughly checked every system except the oil cooler and I'll be doing that this week.
I, and a number of others have a similar problem and, so far, no cause or solution has been found.
09-21-2003, 11:01 AM
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The real kicker here is that the system is very robust & should have plenty of excess capacity available. I would have trusted taking my 81 through the desert... This one is always on the edge. I'll get together with my buddy & do some comparisons, and maybe figure out what the difference is.
- Recent radiator clean and unobstructed by debris; flaps tied open
- No corrosion or deposits
- Water pump replaced 12K miles ago
- new 75F thermostat and rear seal
- Recent radiator clean and unobstructed by debris; flaps tied open
- No corrosion or deposits
- Water pump replaced 12K miles ago
- new 75F thermostat and rear seal
Last edited by Old & New; 09-21-2003 at 10:20 PM.
09-21-2003, 01:03 PM
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agreed, it is probably the thermistat or a loose water pump impellor,
however it is possible to be something else, to test the thermistat just take it out and try to boil it in a pot see if it opens, however also take out the thermistat and BLOCK the return line to the waterpump, so it is not recirculating the water but forcing it to ALWAYS go through the rad, some people might not agree with doing this so try at your own risk
however it is possible to be something else, to test the thermistat just take it out and try to boil it in a pot see if it opens, however also take out the thermistat and BLOCK the return line to the waterpump, so it is not recirculating the water but forcing it to ALWAYS go through the rad, some people might not agree with doing this so try at your own risk