Radiator pressure theory questions
03-03-2014, 11:04 AM
#1
AudiWorld Super User
Thread Starter
Join Date: Feb 2012
Location: Moronville, Tennessee (Middle TN)
Posts: 2,223
Received 83 Likes
on
69 Posts
Radiator pressure theory questions
I have a coolant leak apparently from the driver's side on the bottom of the plastic radiator itself. I can see the spot where there is a crust and it's wet with the pink stuff.
I have wondered; if it's as simple as a pinhole in the plastic, could I leave the pressure cap off in the winter? (Flame suit ON) ...
Main rationale is the temperature difference is so much greater in winter the pressure/vacuum/pressure cycle is surely accelerating the fault process (especially in case it's a crack); and that the system doesn't need the extra capacity (provided by pressurization) in winter as it does in summer.
Evidence is that during summer I replenish from "MIN" to the midline seam in the coolant reservoir maybe once every 5k miles, whilst in winter it's once per thousand. Yes it could be getting worse but it did this last winter too.
Also, where is the vent to atmosphere? If the cap (where I assume it is) were to fail to let air in as the system cools the resulting vacuum would be stressful on the cooloing system components. My concern regarding this potential fault stems from the observation that whenever I open the cap to replenish the coolant there is quite a vacuum within the system (far more so than in my lowly Passat, FWIW); I looked closely at the cap but there does not seem to be any way to disassemble it for cleaning. Hoses are not collapsed or anything, though.
Might just throw that one part at the car. I bet it's $50.
TIA for advice, I've missed you guys!
-Tom
I have wondered; if it's as simple as a pinhole in the plastic, could I leave the pressure cap off in the winter? (Flame suit ON) ...
Main rationale is the temperature difference is so much greater in winter the pressure/vacuum/pressure cycle is surely accelerating the fault process (especially in case it's a crack); and that the system doesn't need the extra capacity (provided by pressurization) in winter as it does in summer.
Evidence is that during summer I replenish from "MIN" to the midline seam in the coolant reservoir maybe once every 5k miles, whilst in winter it's once per thousand. Yes it could be getting worse but it did this last winter too.
Also, where is the vent to atmosphere? If the cap (where I assume it is) were to fail to let air in as the system cools the resulting vacuum would be stressful on the cooloing system components. My concern regarding this potential fault stems from the observation that whenever I open the cap to replenish the coolant there is quite a vacuum within the system (far more so than in my lowly Passat, FWIW); I looked closely at the cap but there does not seem to be any way to disassemble it for cleaning. Hoses are not collapsed or anything, though.
Might just throw that one part at the car. I bet it's $50.
TIA for advice, I've missed you guys!
-Tom
03-03-2014, 03:32 PM
#2
AudiWorld Super User
No plastic at all on my 2004 (4.2L engine) radiator. A Behr 100% aluminum unit. Both original and OEM replacement. Replacement has an additional port that required three new hoses for an additional $200 or so. The extreme cold caused a tiny leak I had for years to catostrophically fail.
03-03-2014, 03:41 PM
#3
AudiWorld Super User
Negative on no cap; ideas
You will likely boil it over w/ no cap, or at least dump a bunch of coolant during the hot engine cycles. Coolant expands a lot if not under pressure, and you can get spot boiling at the internal metal surfaces like around critical head areas.
The vent function is in the cap. If you ever superheat it, the fluid goes out through a port that goes vertically through the overflow bottle to the underside of it. Take a look and you can see it along the outer diameter of the capped area of the bottle.
You can buy the caps on line, either OE or various aftermarket Euro flavors.
Have you looked carefully at the "leak area"? There is a replaceable o-ring in the big larger hose probably down there on a 4.2. There is a similar large hose on my W12 right there. And, in the small world dept, at least the W12 has a second smaller hose down there, just above the large hose. I had a very small leak that seemed to be along the bottom of the motor somehow. I worried about the water cooled alternator and similar oil cooler, having confirmed no issues up by the (W12) water pump or obvious loose clamps that would explain it. I finally figured out it seemed to be that smaller hose buried on top of the larger lower one, and then it was running along the lower edge of the radiator assembly and blowing back onto the motor. That hose had no separately available o ring, so I just sprang for the new one for about $35 when I did my whole coolant flush recently. I caught it only because I went to release the lower hose spring clip and pushed on the upper hose a few times. When I did I saw some drops come out at the smaller coupling for that hose just above.
The vent function is in the cap. If you ever superheat it, the fluid goes out through a port that goes vertically through the overflow bottle to the underside of it. Take a look and you can see it along the outer diameter of the capped area of the bottle.
You can buy the caps on line, either OE or various aftermarket Euro flavors.
Have you looked carefully at the "leak area"? There is a replaceable o-ring in the big larger hose probably down there on a 4.2. There is a similar large hose on my W12 right there. And, in the small world dept, at least the W12 has a second smaller hose down there, just above the large hose. I had a very small leak that seemed to be along the bottom of the motor somehow. I worried about the water cooled alternator and similar oil cooler, having confirmed no issues up by the (W12) water pump or obvious loose clamps that would explain it. I finally figured out it seemed to be that smaller hose buried on top of the larger lower one, and then it was running along the lower edge of the radiator assembly and blowing back onto the motor. That hose had no separately available o ring, so I just sprang for the new one for about $35 when I did my whole coolant flush recently. I caught it only because I went to release the lower hose spring clip and pushed on the upper hose a few times. When I did I saw some drops come out at the smaller coupling for that hose just above.
Last edited by MP4.2+6.0; 03-03-2014 at 03:44 PM.
03-03-2014, 06:11 PM
#4
AudiWorld Super User
Thread Starter
Join Date: Feb 2012
Location: Moronville, Tennessee (Middle TN)
Posts: 2,223
Received 83 Likes
on
69 Posts
You will likely boil it over w/ no cap,<-I respectfully disagree
you can get spot boiling at the internal metal surfaces like around critical head areas.
Good point, this alone is valid enough to justify the cap's use even in the winter
The vent function is in the cap. If you ever superheat it, the fluid goes out through a port that goes vertically through the overflow bottle to the underside of it. Take a look and you can see it along the outer diameter of the capped area of the bottle.
But why doesn't the cap allow air back into the system - why is thre a vacuum in the radiator after cooling?
Have you looked carefully at the "leak area"? Yes I have but thanks; I will look even more carefully next time There is a replaceable o-ring in the big larger hose probably down there on a 4.2. There is a similar large hose on my W12 right there. And, in the small world dept, at least the W12 has a second smaller hose down there, just above the large hose. I had a very small leak that seemed to be along the bottom of the motor somehow. I worried about the water cooled alternator and similar oil cooler, having confirmed no issues up by the (W12) water pump or obvious loose clamps that would explain it. I finally figured out it seemed to be that smaller hose buried on top of the larger lower one, and then it was running along the lower edge of the radiator assembly and blowing back onto the motor. That hose had no separately available o ring, so I just sprang for the new one for about $35 when I did my whole coolant flush recently. I caught it only because I went to release the lower hose spring clip and pushed on the upper hose a few times. When I did I saw some drops come out at the smaller coupling for that hose just above.
you can get spot boiling at the internal metal surfaces like around critical head areas.
Good point, this alone is valid enough to justify the cap's use even in the winter
The vent function is in the cap. If you ever superheat it, the fluid goes out through a port that goes vertically through the overflow bottle to the underside of it. Take a look and you can see it along the outer diameter of the capped area of the bottle.
But why doesn't the cap allow air back into the system - why is thre a vacuum in the radiator after cooling?
Have you looked carefully at the "leak area"? Yes I have but thanks; I will look even more carefully next time There is a replaceable o-ring in the big larger hose probably down there on a 4.2. There is a similar large hose on my W12 right there. And, in the small world dept, at least the W12 has a second smaller hose down there, just above the large hose. I had a very small leak that seemed to be along the bottom of the motor somehow. I worried about the water cooled alternator and similar oil cooler, having confirmed no issues up by the (W12) water pump or obvious loose clamps that would explain it. I finally figured out it seemed to be that smaller hose buried on top of the larger lower one, and then it was running along the lower edge of the radiator assembly and blowing back onto the motor. That hose had no separately available o ring, so I just sprang for the new one for about $35 when I did my whole coolant flush recently. I caught it only because I went to release the lower hose spring clip and pushed on the upper hose a few times. When I did I saw some drops come out at the smaller coupling for that hose just above.
-Tom
Thread
Thread Starter
Forum
Replies
Last Post
Keeno
A8 / S8 (D3 Platform) Discussion
0
02-10-2008 08:14 AM
djillusion
S4 / RS4 (B5 Platform) Discussion
8
01-05-2007 12:53 PM
ArTT Vandelay
Motorsport Discussion
5
10-11-2004 10:28 AM
DanosS4
S4 / RS4 (B5 Platform) Discussion
4
12-04-2002 09:00 AM