TAREK

My '84 is overheating and it seems to be due to coolant flow. Coolant rubber line out of the block is blazing hot while radiator return line is barely warm to the touch

checked Thermostat - passes the boiling water test trhe same as the brand new one
Took hoses out and filled with water from a hose while running. Water pump seems to be operating fine. Did the same thing on the '85. Water flow seems similar

Forced water into the radiator inlet. "Almost" the same flow came out of the other side

Pretty dry around engine gaskets and coolant path - no leaks or wet spots that I could see

Checked flow from coolant reservoir to block - all pass

the only thing I did not do, is put everything back together the way it was and see if it doesn't not overheat anymore by some miracle (sometimes it happens)

Any help would be greatly appreciated. Thanks in advance

FlyingDog

Do you have a plastic impeller on your WP? It could have seperated from the shaft.

TAREK

Don't know if the impeller was plastic or steel. But either way, wouldn't I have no coolant pumping out of the block in that case?

Fabio421

How do you know you are overheating? Coolant blowing off, guage reads higher than normal etc? This may not apply to you but I have had other vehicles "overheat" due to a cracked radiator cap. Check the bottom side of the radiator cap ( spring loaded "seal") and make sure it isn't cracked. I know this sounds like BS but I have seen it many times with non Porsche vehicles.
By the way. My offer of a helping hand still stands. Good luck.

FlyingDog

If the seals and bearings are still good, you won't have any leaks and the timing belt won't be effected. The water pump will spin, it just won't pump.

TAREK

Thanks Fabio. I appreciate the advice. I will put it back together and install a radiator cap from the other cars. (The one on it doesn't look bad, but I'll try anyway). I know it's overheating because after the first 15 minutes or so at idle, it climbs to RED very quickly, consistently with increased temperature on the radiator inlet hose.
Thanks for the offer to lend a hand. I do have a long list of projects for you, sometime in the fall. I try to do the least challenging tasks at home, until they turn into big projects. Hopefully this one will be resolved soon or I would have to tow it to you. How far is your shop from Clearwater Beach?

TAREK

but the water pump is actually pumping coolant out

FlyingDog

Is it pumping once it reaches that 15 minute point you mentioned? The plastic impeller is pressed and pinned. The pin could have broken or walked out. The impeller could hold while cold, then start slipping as the temp climbs.

Fabio's radiator cap idea is a possibility that I saw somebody else mention before. Abother possibility I've seen mentioned is putting the thermostat seal on the wrong side.

TAREK

I'm testing the water pump with Thermostat and all hoses removed. And it pumps fine at high temps. Reason for the initial 15 minutes is just warm-up, followed by an open Thermostat but then.... no flow just as you need it...hence the fast climb in temperature. I sound like I understand what's happening, don't I? now if only I can figure out what is obstructing the flow

I still don't understand why a cracked radiator cap would cause this. I drove my other 928 for years without a radiator cap with no issues, but I'm willing to believe anything that fixes my car

FlyingDog

I'm out of ideas. You've gone beyond my limited knowledge.

ErnestSw

Tarek,
You need two things for proper cooling, coolant flow and air flow. It sounds like your cooling fans are not cooling the radiator properly at idle. If the car does NOT overheat on the road then it's air flow. Check to see whether your fan is working and check the AC condenser in front of (behind?) the radiator and the radiator itself for anything that could block air flow.
Also, PET shows a sealing ring behind your thermostat. If this ring needs replacing it could prevent blockage of the bypass and prevent flow to the radiator.

TAREK

Thanks Ernest. The overheating problem is on the road as well as at idle. Both fans are operating as intended. No debris that impede air flow. The mechanical fan performance is always questionable, of course. This car always ran cool, even in hot Florida heat with AC on. Sat for a month and now this problem. Thermostat, inner ring, and outer seal are brand new (and installed correctly). I'm starting to think that although the water pump is actually pumping, it may not be able to hold pressure under load when the system is closed up. I wonder if that is one of the possibilities someone has experienced before

SharkSkin

Do you have an IR thermometer? See if the radiator is heating evenly top to bottom and has a good temp gradient across the width. If there are cold spots then it's clogged and should be rodded out or replaced. My radiator was 50°F colder on the bottom than the top, and it was because the cores were full of Alumiseal or bars leak or some such crap.

heinrich

My S4 many years ago would overheat and spew coolant. I replaced the rad cap and the temp sensors and she never did again. Also, did you replace the inner seal for the therm? the one that is hard to remove (sorry if i was too lazy to read and missed it)

Fabio421

Quote "I'm testing the water pump with Thermostat and all hoses removed. And it pumps fine at high temps. Reason for the initial 15 minutes is just warm-up, followed by an open Thermostat but then.... no flow just as you need it...hence the fast climb in temperature. I sound like I understand what's happening, don't I? now if only I can figure out what is obstructing the flow"

You mention an "open " thermostat. I have read elsewhere on this board that the thermostat actually closes off a port that bypasses the radiator. So be sure that your thermostat is working "backwards". I'm sure you have already checked this but sometimes simple things are overlooked because they are simple. I have an infrared temp. gun if you need to use it for any reason.
Another thing to consider, with all hoses removed you cannot properly test your water pump. In theory at least, you will see a much higher impeller load while trying to pump water through the small orifices at the radiator. Just another theoretical to think of. LOL