Mrmerlin

I had a chance to look at more than a few sharks this weekend at Sharktoberfest, it was a blast! I noticed a coolant line routing that to me defies logic,(I only saw 2 cars that had the line routed over the top hose) this has to do with the bleeder hose attached to the front of the water bridge, (the small one that goes to the coolant tank), the same as the the small line that goes to the top of the radiator. It seems most cars have this hose routed under the top radiator hose, my guess is that when a timing belt is done the big hoses are removed and the radiator is removed, when the car is put back together this small hose seems to hang low and the big top hose is put back on without lifting the small hose over the top rad hose, now the purpose of the self bleeding part of the 928 engine is to remove any and and all air pockets, as such it only makes sense to have the highest line in the system routed the highest possible way.With the line routed under the top hose the line can fill with coolant and possible not let out all of the air in the system. Anyway its easy to fix remove the small line from the front of the water bridge and stick a bolt in the end so nothing leaks out, pull it out from under the top hose and rout it over the top hose , remove the bolt then install the hose and tighten the clamp, now there should be a gradual run to the coolant tank and the air should be be removed from the top of the water bridge, the last thing that should be done is to turn on the heater this will help to fill the heater core and also purge out the air that rides in the back of the head albeit only for the right cylinder head, this in conjunction with the front bleeder hose will help the coolant system remove as much air as is possible from the coolant system and reduce the chance of airpockets, So check your hose routing, it might be interesting to see if any of you notice a temperature reduction on the gauge as the air pocket will quite possibly be near to the temp sender near where the bleeder line is connected, Stan

WICruiser

Another thing I better check as I did all of the above (i.e. replaced TB/WP, hoses, etc.) but I don't recall how I routed the hose and my car does tend to run warm (if you believe the temperature gauge).

dr bob

Stan opened this discussion at the Shartoberfest event when all the hoods were open. I had all the 'before' pictures from the first TB project, and that vent hose was routed below the heater valve return hose on my car from the factory. Hmmmm...

The ideal vapor purge line is a continuous slope up from the water bridge to the pressure switch abd reservoir. But the pressure switch is higher than the port in the reservoir. Hmmmm... I've had OK luck with leaving the vent line where it is, under the heater valve return line from the reservoir. I open the heater valve, rev the engine some to get good flow through the water bridge and the thermostat area, and expect good water flow through the reservoir after that. My too sense.

Nicole

Can you post a diagram of which hose and where? If it's the hose I think it is, then Porsche routed it below the upper radiator hose. I have pictures of press cars from the time to proof it... no hoses above the upper radiator hose.

dr bob

The hose in question is the little recirc line from the water bridge to the coolant reservoir. It can be run above or below the heater circuit return hose, which is the larger hose that ports out next to the recirc line. In service, with the thermostat closed and the heater valve open, coolant flows from the rear of the right side cyl head, through the open heater valve to the heater core. Then it flows across the rear of the engine bay to the reservoir, and returns through the larger hose there to the bridge at the front of the engine. The little recirc line is there to allow any vapor or air trapped in the water bridge to return to the reservoir. In a perfect world, the small hose would slope continuously from the engine to the top of the reservoir. It can't, because it needs to go up over the top of the shock tower before it goes down a little and through the pressure switch to the reservoir. High spot is therefore forward of the reservoir.

Stan's point is that you can minimize the dip right at the water bridge by routing the vent line over rather than under the heater circuit return hose.

Mrmerlin

Thanks Bob for your descriptive explanation, I am thinking that you mean to rout the small line over the top radiator hose , anyway thats what i have done,

Charley B

Good catch Stan. Whether of not a given year was routed above the larger hose, it sounds like the most effective way to do it. I'll be checking mine tonight.
Glad you made it to Sharktoberfest; work kept me away at the last minute.

dr bob

I did not mean to run it over the upper radiator hose. it CAN run on top of the smaller hose that drains the reservoir when the heater valve is open. Running the hose over the big radiato hose doesn't really buy anything, since it would leave you with a high spot where it goes over the big hose, drop down slightly to route under the edge of the fender, then go down again to the port on the reservoir. Remember that this hose plays a part only when the heater valve is turned off/closed. At that point only, pressure in the reservoir is equal to the top of the engine brifge, and any vapor bubbles would, theoretically, migrate up from the water bridge to the reservoir. Reality is that you need to open the heater valve and rev the engine enough to force the vapors in the water bridge to circulate to the reservoir through the block and the heater core. The vent hose would work perfectly if the engine was lower and there wasn't a high point over the shock tower.

Tom in Austin

A photo would sure help here ...

dr bob

Pictures...

First shows the front of the coolant bridge with the small vent line running next to the slightly larger return line from the reservoir. Stan's proposal would have the small vent line rerouted so it passes over the top of the upper radiator hose rather than underneath as I show it here. The disadvantage to running it over the upper hose is that it would have a high spot there over the upper hose, then route under the intake air tube, back up over the shock tower (shown in the second picture) to where it connects at the top of the reservoir in the vapor space there. The return line from the reservoir connects in the liquid space at the bottom of the reservoir, by the way

To get air/vapors to flow from the water bridge to the reservoir when tthere is no pressure difference between the reservoir and the water bridge (heater valve is closed) the vent line must have a continuous slope up from the water bridge to the top of the reservoir, with no high or low spots that might trap vapor or liquid respectively. In the factory position as shown, the line drops immediately forward of the water bridge to route under the upper radiator hose, so it's function is doomed to failure from the beginning. Moving the hose to route over the upper radiator hose would cure that problem, but present a new problem with the low spot now moved to where the vent line dips under the fresh air intake hose. There's another high spot at the shock tower, but that drains liquid to the reservoir so it's not so bad.

The point that needs to be made here is that the vent line is intended to help move air that accumulates in the water bridge to the top of the reservoir. Generally, this is a situation that only happens when the coolant level drops too low, perhaps following a cooling system service or from a leak. Once the air is moved out of the water bridge, it should not just reappear there. The instruction to open the heater valve as part of the purge process following service is very important. While the heater valve is open and the thermostat is closed, coolant flows rapidly through the heater core and the reservoir. Coolant in the reservoir is degassed, with vapors and air rising to the top and liquid returned to the water bridge through that return line. Purge with he engine RPM's raised some, so that air and vapors in the water bridge are pushed through the engine by the increased velocity of the coolant in the bridge. This is the accelerated version of what most owners experience when they report that they have to top up the coolant level a few times after a flush and refill. You can possibly accelereat teh process even more by removing the vent line at the bridge and only restore it when liquid flows from the nipple. Ditto the fitting on the top of the radiator core. Again, once the two high spots and the heater core are degassed, these vent lines don't do a lot for us.

The pictures:

Hilton

Interestingly, I went and had a look in my own engine bay, and it appears that this isn't an issue for RHD cars (our coolant reservoir is on the US driver's side). The line runs a pretty good slope all the way from the bridge up to the reservoir.

Mrmerlin

as a side note when the coolant cools the level will drop and then an airspace will develop in the highest portion of the the engine( the water bridge) the vent line will then come back into play. if you have any doubt about this just try removing the temperature sensor located on the top of the water bridge when the engine is cold, there wont be any coolant flowing out. I still think that for what its worth the line should be routed across the top of the top rad hose, thats least restricted routing , even though it may have a few dips they wont be as much as circulating the bleeder line under the top rad hose....water/air flows thru this line when the water pump is pumping, regardless of the pressure in the system, you can check this by removing the resivour cap and watching the coolant come out of both bleeder hoses

dr bob

I disagree. Where would the air come from to get into the water bridge? The seam (normal liquid level) in the reservoir on my car is above the top of the water bridge. On the slope of the line-- it's eithet continuous or it isn't. If it isn't, with a low spot then a high spot on the way to the reservoir, the hose will be liquid locked, preventing vapor flow to the reservoir.

-----------------------

The discussion is really moot. Anyone that feels the need can route their vent hoses however they wish. In my somewaht educated opinion, it doesn't buy you anything. In Stan's opinion, it might. There's no harm trying it. I'm just not sure what the real benefit might be or how it could be measured.

SharkSkin

Stan, I remember talking with you about this last weekend -- have you found an air bubble in your coolant bridge?

Mrmerlin

no but the coolant level will drop slightly after engine shut down so the possibility exists for an airpocket, and i have removed a few temp senders and no coolant leaked out even though the systems were full. temp sensor is on the top of the water bridge