GeorgeM

I finished up a TB job on the GT yesterday and am in the process of flushing with distilled water a few times prior to the switch to Pentafrost NF.

During yesterday's flush, the thermostat didn't open up (lower rad hose not hot) until about 210 on the gauge and that's when the fans turned on.

Today, the temp gauge was above 210 and thermostat wasn't open and the fans never came on. Where do the fans get their temp signal from? The sensor at the bottom of the rad (driver's side) perhaps?

So unless something else weird is going on, it looks like I have a bad brand new thermostat. It wouldn't be the first.

Any other ideas?

TIA....G

Mrmerlin

Put the old TStat back in test
Or put them both into boiling water and see if the old one does what the new one does

dr bob

After draining the block and refilling with coolant, there are often air bubbles in the system at high spots. The water bridge, thermostat inside, can be one of those spots. While the system is self-burping, a few things need to be just right for it to work.

-- First, the vent hose routing from the water bridge to the coolant reservoir needs to slope up as continuously as possible from engine to reservoir. This hose is disconnected for the TB project, making it easy to end up with a low spot that prevents air from migrating uphill to the reservoir. Check your hose routing.

-- The next important this is that the heater control valve must be open to circulate coolant past the reservoir connection. The coolant flows through the heater core, then on the return to the engine it flows through a steel line with a Y along the right (passenger on US cars) side of the engine bay back to the water bridge. That Y has a stub hose to the bottom of the reservoir. So the only way entrained vapor and air bubbles from the block can make it to the reservoir is with the heater valve open. Set the heater temp slider to full-hot, make sure you don't have the HCV tied closed for summer driving, and bubbles in the system will end up in the reservoir.

-- I fill the reservoir initially with the vent hose at the top of the radiator shell disconnected, and put the hose back on after I see liquid from the little nipple on the radiator. Similar to the water bridge vent, that line needs to slope up continuously towards the reservoir.

-- The thermostat itself has a small hole that must be at the top when installed. This little bleed hole allows vapor and bubbles to pass from the radiator lower hose to the water bridge area, from which it's vented via the vent hose or circulated through the engine and out through the heater valve.

-- Fill the reservoir to the top while purging air from the system.


To your question on the fans -- they work based on a sensor that's at the bottom front of the radiator outlet tank. That's driver's side on US cars, bottom and front. That sensor sees hot engine coolant only after the thermostat has opened. The thermostat won't open until it's sitting in hot coolant, something that can only happen after all the air is purged from the water bridge. Get the air out, hot coolant recirculating in the block will open the thermostat, hot coolant will flow through the radiator past the sensor, fans will start when needed. Fan operation is a bit divorced from the temp gauge readings, by the way.

GeorgeM

Thx, Stan. Testing was next on my list once things cool down. After my last experience, I should have tested it before installation.

Excellent info, Dr Bob. Thanks!

Will report back.

FredR

Did you check for the condition of the rear seal when you changed out the thermostat?

GeorgeM

Replaced it.

GeorgeM

The boiling water test shows that it starts to open at 84.5C vice the 83C spec.

So I guess there's air in the system.

Is 84.5C close enough or do you guys think I should I swap it out for another?

FredR

I would not worry about that -could easily be explained by your thermometer calibration - just check to ensure it is fully opened at or close to 90C. When starting the car first time after a coolant change I always allow the engine to warm up with the expansion tank cap removed as it will help the system vent any air that is trapped and I expect the level to drop by a litre or so until the system is fully primed. Dr Bob covered the bit about leaving the heater open- you can do this using the control panel- I do it by ensuring the heater valve is open which is the default case anyway but then I do not have the heater core in service as I never need such in my climate.

Taguid

I refill using coolant hose and funnel in block. I pour down to fill block with funnel before installing thermostat and neck and then the other hose to fill the radiator. Re-attach hoses and I start the engine and fill from expansion tank. Never had a problem doing that way.

Petza914

Having an Airlift vacuum tool will make your life much easier when working with cooling systems. Not only does pulling the initial vacuum show you if the system has any leaks, but it refills the system without any air voids so the burping process is almost non-existant.

voskian

George, same thing happened last year after coolant flush and replacement thermostat, my GT went from always just below 210 line to now on/above 210 coolant line? Checked hose temps and as I remember they were ok. Put about 5000 miles lat year with no issues , even in hot weather. Curious what you find out..

GeorgeM

Thanks again for all the tips.

This morning's flush went as desired. The thermostat opened about when it should and fans worked as advertised... as did the new heater valve.

I ended up rerouting the water bridge vent hose and more importantly, rotated the thermostat 180 deg so the the small hole was at the top (insert emoji for dumbass here).

These plus allowing plenty of time during the fill process for air to purge on its own did the trick.

Maybe one more flush, then in with the new.... although 2 1/2 flushes might be enough.

dr bob

Vacuum filling is the ultimate solution. You need to seal the system of course, then fill from the bottom while pulling vacuum from the reservoir.

The 928 fills and burps pretty easily without a lot of fanfare with just the tiniest amount of care in preparation. There are other [not just] Porsche models that take a lot of effort to get burped and bled completely. Guessing that Pete has the vacuum fill system to support his 997. I use my home-brew version to fill my Honda Pilot DD cooling system, to make sure there's no air trapped in the top of the rear-seat heater cores. Lots of cars can benefit from this technique.

dr bob

George --

If you have drained the block via the two galley drain bolts, you may not see a lot of benefit from the extra flushes. If you don't remove those bolts, the block drains only to the level of the water pump inlet hole. The first time you pull those bolts is a little scary because the factory bolt torque spec is way too high. Put them back in with teflon thread sealing paste on the threads and the faces of the sealing washer, and try 16 lbs/ft torque when tightening them. Dave C (Worf928) started a thread highlighting the mistake in the factory number. A little searching will find that for you. It's a good read.

https://rennlist.com/forums/928-foru...-is-wrong.html


This one has some more background, including a recommendation for copper anti-seize. In my experience, copper is perfect for steel-in-steel fitups, but may be less than the perfect solution for the steel-in-alusil drain bolt situation. Meanwhile, because this is a wetted junction, the thread sealing compound keeps coolant out of the junction (threads), lubricates the junction, and also lubricates the faces of the washer, block and bolt to limit galling when tightening.

https://rennlist.com/forums/928-foru...ain-plugs.html

Petza914

It's my GMC Sierra that's the absolute worst for refilling and it seems to go through waterpump like nobody's business. I think I'm on my 5th one at 200k miles. Luckily, bought the first replacement from Napa so all the rest have been on them.

The Airlift works great on the 997s though too and the 928. After fixing my water bridge issue, I vacuum filled it (pulled the vaccum twice) and it didn't require any top up afterwards, except maybe a splash.