Drew,
Please correct me if I'm wrong, but the lbs on a radiator cap represent the pressure it will take before it releases. If the cap is not releasing pressure ie. boiling over, a higher pressure cap wouldn't make any difference would it?

 

How do you flush the motor out?

 

Ernest, I changed the radiator cap on mine and it felt like it made a difference. It's the sealing of the rubber to the overflow container, if it doesn't hold the proper preasure the coolant doesn't flow right. Can you imagine that it was a twelve dollar cap that caused your problem. Still there with you buddy, Reno

 

Ernest -

In all honesty, I really don't know the logistics behind the # of Lbs - It was explained to me once, and it was soon forgotten.

I do know this - the pressue cap must open between 0.9 and 1.15 Bar. Per manual - Checking Coolant Cap - Section 19-2A. Maybe you can have your present cap tested?.

Sorry I couldn't of been more help.

Edited Addition:The radiator cap maintains the pressure in the cooling system, usually about 14 to 16 pounds, which raises the boiling point of the coolant. The rubber seal in the cap wears with age, and the cap must then be replaced. Once again this is an inexpensive component and it is wise to replace it when working on the cooling system.

http://www.waterpumpco.com/Thermosta...20switches.htm - Non Porsche Affiliated but for general information only.

 

Reno,
The radiator cap is a new 100 degree cap. Thanks, I need all the help I can get.
T_Max,
I guess you take off the in and out hoses and hook water up and back flush it. Interesting question----do you drain the block first? The block was drained when I did the TB.

 

The no on the radiator/tank cap is in Bars (1.00 Bar = atmospheric = 14.7psi), not degrees.
Have you looked at ignition timing ? Badly retarded spark can make an engine run very HOT!
jp 83

 

Ernest,
It may be worthwhile to benchmark your engine's heat against another similar engine. I know that Richard Rea has an 89GT (I don't know of a later model in the DFW 928 group). Maybe you can use your laser temp gauge on his in the same underhood spots that you have measured on your '91 and note his dash temp gauge reading. You may find that his spot temp readings are close to yours, while his dash gauge says his engine temp is within normal range. If not, at least you will have eliminated the possibility that your engine is operating normally while your dash gauge is not.
Regards,
Dave Goyne

 

A bad coolant reservoir cap certainly could be the culprit. If it fails to maintain system pressure, the coolant will boil at a lower temperature, resulting in overheating (even if the system does not boil over).

Do a system pressure test and replace the cap.

 

It does sound like a pressure failure to me, considering all of the components that have been replaced.

My car runs very cool except when I'm out on the track. Then my leaking end tank acts up and the car gets very hot. I'd check your hoses and any other possible areas that you can get a light to for any kind of minor crack or pnhole that would be leaking pressurized steam but not necessarily leaking puddles of coolant.

Good luck, and hang in there.

 

DG,
With the use of a temperature reading infrared thermometer I have CONFIRMED that the gauge is reading the correct temperature.
Randy,
I tried to buy a pressure gauge at Pep Boys today and they don't carry them. I'll keep looking.

 

Ernest, forget Pepboys, I would order it from Porsche or the big three. I had an overheating problem with a Jetta and purchased a fan switch from a local parts dealer. After I installed the switch the car kept overheating for a year. Then I went to VW for some coolant and a mechanic told me that the relay I purchased did not have the same resistance as VW's switch, so I purchased the VW switch and installed it. Guess what? It solved the overheating problem. Anyway, the moral of the story is stick with orginal. Reno

 

You are correct in your assumption Errnest. The pressure governs the boiling point. It won't affect the operating temperature but will affect the "failure" point (boil-over).

A few personal experiences that I've had that resulted in overheating include the obvious like stuck thermostats and plugged rads, but you've addressed those.

New coolant is in, I'm sure......... is it a 50/50 mix of antifreeze? It's more efficient as a heat transfer agent than pure water.

Ingition timing was once an issue that eluded me........ and should be easy enough to check.

I once had a new water pump that had a loose impeller....... that took forever to figure out........... and I know you're not looking forward to having this one creep up your index of suspicion.

You had a great DDx for air-flow so we can assume that you've proved it's not your issue.

Do you have an IR heat sensor? Grab one and shoot the temps of the head, the thermostat housing, the upper rad hose, the rad, and the lower rad hose. This ought to prove coolant flow if there is a pretty consistent temperature across these components. It will sure show the defect, as the hot/cold interface if one of these is it. I'd like to know the rad temp when the engine temp indicates HOT, and the difference between the therm-housing and rad at that point, too.

 

Reno,
Good advice, but I was referring to a coolant system pressure tester.
Brian,
All temps have been confirmed with an IR thermometer. The gauge is accurate. The rad outlet temp is about 30F below the inlet temp of about 212F
Nicole,
Several days ago you posted a tip about obstruction of the cooling air at the air intake vents at the nose of the car. I checked mine and, sure enough, the top lip of the spoiler had come loose and was obstructing the bottom air inlet. When I repositioned it properly I'm pretty sure it made a small, but noticeable difference. It was not THE problem, but it was a problem.

 

"Boiling" happens at spots throughout the engine coolant passages, based on the coolant mix and the pressure in the system. At the hotter spots in the block, coolant can "boil" into bubbles that do a relatively poor job of transferring additional heat, but condense within the coolant stream as they flow along. You can therefore have localized "boiling" in the system without having it "boil over".

You can greatly assist the heat transfer process by maintaining adequate pressure in the coolant system. This is often as simple as replacing the coolant cap. These caps are way too cheap, in the big scheme of things, and should be on the list of five- to ten-year replacement along with the coolant hoses.

Products like Redline's Water Wetter surfactant work to reduce the surface tension of the bubbles, making smaller bubbles and allowing better liquid contact with the walls of the coolant passages. I've had pretty excellent results with Water Wetter in my Explorer, and add it to the 928 as a matter of course. It's cheap insurance. Based on the rather large volume of coolant in the 32V motor, I use a bottle and a half, in a 40% coolant mix.

Coolant becomes less effective at heat transfer as the anti-freeze
concentration goes up, so long as the localized boiling is managed. Very important! Consider that the concentration of anti-freeze should be decided based on the need to protect the block from freezing, the need to lubricate the water pump seal, and the need to keep any minerals in the water from plating out on the block or radiator surfaces. Use distilled water to eliminate the minerals. The water pump lubricant is in the Water Wetter, so you really just need to protect the block from freezing with the anti-freeze component. Use a light load of anti-freeze, minimum 30-40% just to cold-night safe and to help manage that localized boiling, and you'll be pretty much protected from all the things that cause problems in the system.



Hope this helps!

 

Ernest: That sounds like you are running 212 F out and sub-200 F in. In a hot (Texas) environment it sounds pretty nominal. The standard thermostat is 180 - 190 F. The info I get from my rad guy is this..........

To determine the appropriateness of the cooling system, there should be a Max Operating Temp (at the thermostat housing) of 245 F. The anti-freeze / pressure cap system will elevate Boiling Point to 260-257 F, but that's way too high for operating temp. (Each 1 lb of cap pressure raises BP 3 F.) The appropriate temp drop should be 15-20 F from thermostat housing to distal end of the upper rad hose.... another 15-20 F through the rad... and another 15-20 F from the rad to the engine inlet. (I found it interesting that the theory states that each rad hose is almost as effective a cooler as the rad itself.)

This should give you some baseline numbers to judge your convection side.

For the air-flow side, my guy said that you have to be EXTREMELY particular about the cleanliness of the radiator fins with the aluminum rads. The fins are FAR more tightly spaced than in a conventional Cu/Pb rad, and therefor FAR less tolerant of obstructive debris. He suggests back-flowing brake cleaner through the rad fins, followed by low-pressure detergent/water, followed by low-pressure clear water, to remove the "bugs and fuzz"

Hope this helps......