Coolant System acting up. Got logic?

[3D914]

OK, I tested the thermostat and it opened a little at 180 def F - close enough. But since this doesn't seem to match with the water pump, I'll need to get another.

So if I'm reading Clarks page correctly, I need the old style thermostat (944 106 129 05) and old style o-ring (944 106 929 05) since my oring groove is the 4.5mm - correct?

I also had a thin .5mm spacer that went between the thermostat and the snap ring. It appears to be 900-234-160-00 from pelicans catalog. Without the spacer the snap ring and thermo with outer seal stack up to 4mm. That leaves a .5mm gap. Is that acceptable?

[mxfrank]

Just an explanation of how a too-thick gasket can cause overheating problems. Your thermostat is double acting, meaning that it's actually two valves in one. Here is a photo of some double acting thermostats:

http://www.coolcatcorp.com/faqs/gatesthermo.jpg

These happen to be Jaguar thermostats, but the idea is exactly the same for Porsche. Notice that there's a flat plate at the bottom of the thermostat. When the thermostat is hot, this plate moves back along with the main poppet valve. When it's fully retracted, the main poppet is open, while the plate blocks off the bypass passage. Flow is thus either through the radiator or through the bypass (the bypass leads back into the engine cooling galleries). As a result, water is either circulating through the radiator, or circulating around the block. This type of design is called "constant flow".

If your gasket is too thick, then the bypass poppet may not be fully seating, allowing some coolant to always circulate around the block rather than going through the radiator. It's unlikely that this will cause the engine to boil over, but it can result in warm running conditions.

[John_AZ]

Originally Posted by 3D914

I checked for details on my rebuilt water pump and thermostat.
Water Pump
This is an updated late model (non-turbo) pump. It does not have the extra port for the turbo, but it does have the new guard rail (as did the one it replaced). The diameter of the roller the belt drives on is approx 58mm.
Keep ya posted.

Originally Posted by 3D914

So if I'm reading Clarks page correctly, I need the old style thermostat (944 106 129 05) and old style o-ring (944 106 929 05) since my oring groove is the 4.5mm - correct?

I also had a thin .5mm spacer that went between the thermostat and the snap ring. It appears to be 900-234-160-00 from pelicans catalog. Without the spacer the snap ring and thermo with outer seal stack up to 4mm. That leaves a .5mm gap. Is that acceptable?

I think you have it right.

Quote from the Clarks forum link:

tooter884
Senior Technician
--------------------------------------------------------------------------------

Thanks Clark for the clarification. I obviously have the "old style" updated water pump, which I was not even aware existed -- hence, the confusion. To add further confusion, this is a non-turbo version updated pump that doesn't require the block off plate since there is no hole machined for the turbo-related cooling requirement. This pump apparently requires the old-style thermostat, and it definitely requires the old-style sealing ring due to the 4.5 mm distant snap ring groove. I can confirm the thermostat conclusion when I remove the old water pump. There aren't many easy or crystal clear refurb tasks on this 1983. So thanks again Clark for your time and effort in producing this truly informative and educational site. Would never get this gem back on the road without you and the other contributors to this site.
(END quote)

Clarks says the shim washer is used on the 7.5 mm distance updated WP. Clarks is not sure (in the forum) if it is used on the 4.5mm distance updated WPs but he included the shim washer in his inventory of parts required in his shop manual.

Pelican says it is used on all updated WPs. YMMV ???

GL
John

[mxfrank]

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Let's clear up a few fine points of cooling system design. Every car has some mechanism to allow coolant circulation around the block when the engine is cool and the thermostat is closed. This is called bypass, because the water bypasses the radiator. Water leaking past the thermostat into the radiator isn't "bypass". Nor is it necessarily a bad thing, unless the leak is so large that the motor doesn't warm up. In fact, many thermostats have some mechanism for controlled leakage, in order to allow air to bleed out of the block. For example, again, a Jaguar thermostat:

http://www.coolcatcorp.com/faqs/gatesairbleed.jpg

Why might the car overheat when the thermostat is removed? Because the thermostat in a Porsche, as with most European cars, controls two passageways...a bypass passage, which leads back around the block, and a main passage, which leads out into the radiator. With no thermostat, a high percentage of coolant will circulate around the block without ever passing through the radiator.

'murican V8's typically have a more primitive bypass system. They will use a single action thermostat which only controls the main passage to the radiator. Bypass flow is controlled by differential pressure: the thermostat restricts the exit passage, creating a venturi effect. As a result, the pressure at the entry to the upper hose is very low, which is to say you have suction drawing coolant into the radiator. The bypass passage is always open, but there is usually a baffle cast into the manifold which makes flow through an open thermostat the preferred path. There is always some misrouted coolant with this system, but if you remove the thermostat without installing a restrictor, the bypass passage may then have the lowest pressure in the top end, and water will take the path of least resistance around the block.

Finally the "slow radiator theory". If you could move water through the radiator at supersonic speeds, you would actually have better cooling. The reason is that heat exchangers work best where the temperature differential is greatest. If the coolant moves so slowly that exit water is at ambient temperature, the cool end of the radiator would provide no cooling. If you could move the water through so fast that exit water was barely cooler than inlet water, you would maximize heat exchange across the entire surface of the radiator. There are two reasons that cars aren't designed this way: the water pump would draw too much power, and there is a limit to how fast you can move coolant without cavitation effects.

The thermostat has one other purpose in American cars. While an open thermostat passage has low pressure due to venturi effects, pressure behind the thermostat is elevated. Higher pressure in the head raises the boiling point, preventing local boiling.

[3D914]

mx, thanks for the detailed explanation. Not surpisingly I was recently dealing with cooling problems in our Ford Windstar - and I was getting totally confused between the two systems. Age does that to I guess. Now I see more clearly the differences. Thanks.

John, Thanks for the clarification. I read tha posting twice and still missed the point that you quoted about the spacer being used on both types. I miss tha old days of reading things on paper. Where's my saw, I need to cut down another tree!

I'll order the parts from Pelican this afternoon, including a new $16 cap. Wow!

Enjoy!

[3D914]

OK - so it continues.

I picked up the correct thermostat, spacer, and snap ring & new cap from Pelicans. Installed them, burped the system several times over a few days. I did replace the incorrect bleed screw with a correct one.

Now it worse than before. In less than 5 miles in 70's degree temps, it gets to 5/7 (97 C) mark and stays there. Both fans are running and it never cools down.

Once I shut off the car the single fan runs for a while then shuts off. I turn on the key to the On position to look at the gauge - expecting it to show at 80 C. Not! Still sitting on 97 C mark.
This just doesn't make sense, unless there's some kind of coolant flow blockage in the block itself. What else makes sense?

Still puzzled.

[3D914]

Also, someone mentioned that perhaps the water pump does not have enough flow. Is there a procedure to test the output of the pump?

TIA,

[John_AZ]

Your radiator fan switch should drop the temp when the car sits after running. When you restart, does the temp drop back to the 70s?

If the temp does not drop back to the 70s, it could be the radiator. Did you flush the radiator with a hose both ways when you had the coolant drained?

A shop would have a laser temp gun to check if you have a blockage. Amazon has a DIY gun for $15. It would show hot spots that may be causing blockage.

Amazon.com: Temperature Gun Infrared Thermometer w/ Laser Sight: Home Improvement

I do not know what else to check. Maybe the WP impeller is bad???

GL
John

[djnolan]

Maybe the gauge or sensor is reading wrong? Other than the fans running, are there other indications of high coolant temps, such as boiling over, or predetonation? Are the heater and bypass hoses connected correctly? is there a way to flow test the radiator? some random thoughts...

[John_AZ]

More ideas.

When the car gets hot, do both radiator hoses feel hot? If both feel hot water is flowing into the radiator and the thermostat has opened. If both are not hot, the radiator may be blocked.

Here is another thread.
overheating.

Clarks has a troubleshooting post as well:
Cooling System Troubleshooting

GL
John

[3D914]

John,

Radiator fan switch does NOT appear to drop the temps measured on the block. Once I shut off the car the single fan runs for a while then shuts off. I turn on the key to the On position to look at the gauge - expecting it to show at 80 C. It's still sitting on 97 C mark.

When the radiator was out for the engine swap I back flushed it and it was fine. It did sit in the garage for many months before it went in so it's possible, though not likely, something got inside and is causing blockage. When running both upper and lower hoses get hot, though the upper is much hotter than lower.

Nolan,
I checked both the sensor and the gauge and they appear to be within specs. There haven't been any signs of booiling over as I've tried not to push it to that point - I just put this engine in.

My plans this weekend are to pick up a pressure-test radiator cap and a thermal gun, drain the coolant from the radiator. Flush it and pressure test it. It should hold 15psi right?

[3D914]

One thought. If my timing were off by one tooth in either direction - could this result in the same symptoms?

I triple checked everything as I had the engine on the stand where everything was easy to see, but I suppose anything is possible.

[nota]

you know it is very common for a hot car to get hotter coolant after shut off
it is called heat soak as the block/head ect tranfers heat to the coolant
but the coolant is not moving so sensor reading can go way up after shut down

[djnolan]

if it were an old car with a points distributor I might suspect incorrect spark timing causing the overheating. if incorrect valve timing it might run very rough, but I would be guessing as to how it would effect temperature and power.

On a lot of other cars, you could remove the radiator cap and look down into the radiator to watch the coolant flow as the engine ran. This would confirm no blockage in the rad or thermostat. I can't think of a way to confirm coolant flow on a 944 except by trial and error.

Back to the Clarks guide, did you change the thermostat again? maybe the tstat isn't fully opening? are you sure you are bleeding correctly? is there a way to check the impeller without removing the wp?

As for testing the radiator I was thinking to flow test to see if water was able to flow thru it under pressure. It would require rigging up some type of adapter to attach a garden hose to it. maybe a mouse built a nest in it?

[mikepellegrini]

Did you ever get this resolved?

[Dawgz83948]

Originally Posted by mxfrank

'murican V8's typically have a more primitive bypass system. They will use a single action thermostat which only controls the main passage to the radiator. Bypass flow is controlled by differential pressure: the thermostat restricts the exit passage, creating a venturi effect. As a result, the pressure at the entry to the upper hose is very low, which is to say you have suction drawing coolant into the radiator. The bypass passage is always open, but there is usually a baffle cast into the manifold which makes flow through an open thermostat the preferred path. There is always some misrouted coolant with this system, but if you remove the thermostat without installing a restrictor, the bypass passage may then have the lowest pressure in the top end, and water will take the path of least resistance around the block.

Finally the "slow radiator theory". If you could move water through the radiator at supersonic speeds, you would actually have better cooling. The reason is that heat exchangers work best where the temperature differential is greatest. If the coolant moves so slowly that exit water is at ambient temperature, the cool end of the radiator would provide no cooling. If you could move the water through so fast that exit water was barely cooler than inlet water, you would maximize heat exchange across the entire surface of the radiator. There are two reasons that cars aren't designed this way: the water pump would draw too much power, and there is a limit to how fast you can move coolant without cavitation effects.

The thermostat has one other purpose in American cars. While an open thermostat passage has low pressure due to venturi effects, pressure behind the thermostat is elevated. Higher pressure in the head raises the boiling point, preventing local boiling.

Your incorrect about American V8's, at least the one I have in my 944.

[mikez944]

As was mentioned earlier, the head gasket needs to be a suspect. Your symptoms are exactly what I had with my BMW 535i. the giveaway was the pressure in the cooling system getting so high that it eventually ruptured a hose. I spent weeks replacing the thermostat, water pump, fan switches, sending units and bleeding the system. Looking back it should have been pretty obvious when, no matter how much I bled the system, there was still air coming out of the bleeder.

[3D914]

The problem did get resolved, but not by me. I sold the car to the gentleman I bought the engine from. He wanted to turn it into a track car. He ended up replacing the radiator, temp gauge, and all temp sensors to resolve the problem. He says it now sits on the middle mark.

Fortunately for him, he had tons of spare parts to cycle through till he found something that worked. I wasn't willing to put any more money into the car - it just wasn't that nice of a car.
I may be looking for another one though - we'll see.

Thanks all,