Ron_H

It would seem to me to be prudent to have it checked or check it yourself with the Kempf tensioning tool. I think 928 International sells one for a reasonable price. it is simple to use and takes only a few minutes. You'll know quickly how well its doing and can examine it also. And you can keep it in the car for road trips. I once paid $100 to have it checked and tensioned when the warning light came on. Wish I had the tool then because it interrupted a road trip for a day.

The_Remora

Or your waterpump impeller may have let go... not common with newly rebuilt waterpumps, but not unheard of either. If your coolant is not circulating, and there is nothing impeding its flow, then the pump is probably not circulating for either the reason you described or because the impeller busted. Seems you've ruled out what you can by running the motor to see whats happening, now I wouldn't run the car much more until you solve the problem.

Lopez

Update:

The cooling system does not leak, and holds pressure.

The thermostat is new, and not in backwards (I just changed it). I believe that I have all the air out of the system based on checking the bleeder hoses on the water bridge.

The car still heats up (and eventually will overheat) and doesn't seem to be circulating coolant through the radiator

The heater also does not heat up.

What I am unclear on: if the thermostat stays closed, but the water pump is working, shouldn't the heater also heat up as the engine does? If it doesn't, does that not point to a bad water pump?

Either a failed thermostat or a dead water pump would explain the initial problem cropping up. The last thing I can think of to try before pulling the water pump is the trick I've seen in some other threads about overheating where you jam the thermostat open and put it back in. Although this idea does make me nervous (what if it DOES work and now that hot water flows over it, it opens up more and the part you jammed in there gets loose? sounds like a bad idea...)

Anyway - if there's anything else anyone can think of to diagnose the pump before I pull it apart...

Landseer

You could check thermostat operation. Put in a small pot of water, apply heat, measure water temperature when it opens and compare to markings on it. If its old, just buy a new one.

On this car the thermostat opens and closes at the same time. As it opens, the back of it travels toward the rear of car and contacts another rubber coated sealing ring behind it inside the water passageway. Check for its condition. Its sometimes overlooked. Check here for recent post by Dwayne on overheating that has pictures. Dr Bob above mentioned the 3 piece thermostat, not sure if you picked-up on it.

Jamming it open is not the right method on this car to check anything.

There are threads here that talk about bleeding the cooling system. It is possible to check water pump operation by following some of the hose disconnection methods in that body of posts.

Water pump failure can happen several ways. Telltales are fluid on front of oil pan emerging from under the cover. Mine had a skewed & frozen WP pulley and was beginning to shred of belt. Some people here report detatchment of impeller from shaft (can't see this / no telltales). Some folks said its happened on new waterpumps.

Overheating will generate massive amounts of steam at the cap of the reservoir, but the reservoir may be fine as you report.

If its holding fluid when hot, its not a hose (yet) but they all need to be of known condition, else replace. On my car, with seized water pump and good hoses, the steam exited the cap like a geyser. I'm suspicious of your water pump and your back of stat seal, both could be bad. Dwayne had a degraded seal and an air bubble.

mark kibort

removing the thermostat is a great idea! there is no risk, as you are puting something. (a round splin motor drive gear) under the front of the front shaft of the T-stat. it cannot go anywhere. what it will tell you is that it is opening up, and not sticking. I still have issues with my t-stat, even though it has been changed often. the rear seal is important, but not critical

now you are narrowing it down. the fact that the heater doesnt work, is a tell tail pointer that the pump is bad.

if you suspect the pump, do the above with the t-stat, and then pour water from a hose into the lower radiator hose inlet to where it would attach to the engine via another hose that mounts to it (temp) this way, you pour water in and it should be sucked in at a moderate rate during idle. if it just holds its level, it aint moving and this means the pump is bad.

the other way is to use a stethascope and listen to the pump to see if you hear it spinning. however, this is not very useful if you havent listened before.

mk

edit: Yes pinning the t-stat does something. it seals off the top water cross over from the pump. this means , NOW, all the water comes from the lower radiator hose, fed by the radiator (water travels, down to the pump , through the block, up through the heads and out the top of the engine to the top of the radiator. If you pin the t-stat, you make the suction come right from the inlet hose that is fed from the lower radiator hose. Without pinning it, the only sign is the heater not working.
if the pump is freewheeling. (this would be the timing belt is being spared), the car will overheat in about 5 min, and temps will run-away. if the rear seal is bad, still the radiator will be hot top to bottom as all that happens is that the flow is reduced , but still through the radiator. engine hot, over heating, radiator cold, pump bad or t-stat not opening, its that simple.

docmirror

My vote is the water pump shaft spinning free. I've been thinking of a test method without pulling it all down. Here's what I came up with. Go to Home Depot and get a few feet of that 1.5 inch dia clear plastic hose. Replace the upper rad hose with this, and start it up. Let it warm, and keep adding water as needed. Don't put the cap on tight as you don't want any pressure in the system. Watch the water flow when it gets hot. You should see quite a bit of movement from the block to the rad as the t-stat opens.

heinrich

your water pump is toast. don't start the car anymore.

ExKiwi

How about this one?

Remove the thermostat cover and thermostat.

Drape some old towels (carefully) over the engine and leave a gap around the thermostat housing.

Start car and run for a few seconds and stop.

If the towels have coolant on them, the pump is operational. If they don't...

Lopez

Thanks everyone. I figured the heater staying cold was a pretty sure sign. The water pump was installed by the PO less than a month ago - I have the receipt and it's an eBay pump for $109. Not a genuine Porsche part I am guessing

dr bob

Those ebay pumps are a notorious Chinese knock-off. Do a little searching here to find the list of telltale marks on the castings. Did the PO give you the old pump? Would be nice so you have a core to turn in for the new one...

heinrich

Actually the world's very worst pump is the LASO, a German one

Lopez

Yes, I have both pumps - the original one from the engine plus the cheap replacement which is currently on the engine. So that's good, anyway.

mark kibort

you are on the wrong side of the engine. if the t-stat is not opening, there will be no flow, even though the pump could be good. trust me, you have to go to the lower radiator hose and disconnect at the t-stat area. thats where the water enters the pump. or, it enters from behind the t-stat when cold bringing water from the heads and pushes out to the block, short circuiting the radiator and hoses.

just pull the lower radiator hose off the t-stat housing (after wedging the t-stat open) and feed the t-stat housng water through another hose. water should be pushed out the lower radiator hose proving the pump is working.

mk

docmirror

Gee Mark, since there's only one inlet, and outlet to the rad, you're gonna have to help me understand why he won't see water flowing at the top hose, but will see it at the bottom? Or, conversely, 'splain why he will not see water flowing if the pump has failed.

I chose the top hose because it's easier to see, less mess to replace temporarily, and far easier to R&R.

"from the block to the rad as the t-stat opens."

Lopez

Sure - I can still try that, but T-stat open OR closed, shouldn't the heater core get hot water if the pump is running and the heater valve is open?