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I drove my 968 cab/stick the other day, very hot day, I turned the heat control to 0, a/c on, 10 minutes into the drive, red light on, a/c stops, rad fans not on and car overheating. Car now back in my garage and I'm debugging.
I want to refill and then run the engine to look for leak. Definitely on the passenger side, and I'm figuring HCV, especially as I turned it right down, therefore off, therefore max pressure on HCV. And I'm also figuring/hoping the reason the a/c went off and the rad fans were not running is that when the engine detects no coolant it switches those things off automatically. Anyway I'm looking for guidance on refilling coolant. Expansion tank is empty but block might also be short of water. In 944 the posted processes talk about refilling the rad from the expansion tank and refilling the block using the top hose. On 968 the info I can find seems to say just refill from the expansion tank with the bleed nut opened and keep refilling until constant fluid coming out. But does this ensure the block is full, is there something different on the 968 engine that makes it ok just to fill from the expansion tank? Surely the thermostat in the water pump is closed and coolant will not flow into the block?
Thanks for any help
And I'm also figuring/hoping the reason the a/c went off and the rad fans were not running is that when the engine detects no coolant it switches those things off automatically.
It does not. There is never a reason to shut the radiator fans down in an overheat situation. The A/C may have shut down do to over-pressurization in the high side of the system (due to no cooling fan operation), or possibly from the same electrical glitch that may have shut down the fans. You are correct - diagnosis is now required.
Originally Posted by rogerstopford
On 968 the info I can find seems to say just refill from the expansion tank with the bleed nut opened and keep refilling until constant fluid coming out. But does this ensure the block is full?
It does. Have performed this countless times. The bleed hole in the thermostat and the design of the cooling system allows for coolant to fill the block as long as the bleed screw on the water outlet is open so that air can be purged.
Cliff is correct on all points and to add one more, fill the expansion tank SLOWLY to allow the air to be pushed out.
The "lock" is absolutely correct and thanks for adding that! Being the fanatic that I am, I actually completely remove the bleed screw and monitor the level as it rises in the water outlet with rags under the outlet so to catch any coolant that may overflow. If you fill too fast with the bleed screw only opened a little, you will see some spurts of coolant come out before the air is completely purged which may fool you into thinking that the engine is full. Slowing the fill rate as you approach full allows the air to purge completely and you can more effectively control spillage should any occur.
Check your fan switches and relays. Prior to my last belt/pump change the shop found one of my switches inop, which explained why it was running a little hot. (3/4 of the way between the marks, vs just above lower mark)
Also, insure your expansion tank cap is in perfect order. Honestly, I chased an overheating problem for a year until a lady friend of mine suggested I look into the cap. $8.95 at NAPA later, no more overheating.
@oppositelocck - Thank you for the reply. I have asked Cliff/chudson re a detail on the bleed screw and a detail re the fans, but thank you for your comment.
Last edited by rogerstopford; 06-23-2024 at 12:22 AM.
Reason: reference sender
@ chudson - thanks for your reply. A question and a comment.
This bleed screw we are talking about - it's the nut on the top of the water manifold at the top/front of the engine, forward of the front-most intake manifold, right?
Just a comment - you mention the fans shouldn't switch off due to overheat, but I'm wondering if the system detects 'no coolant' and shuts the fans down, because in my case for sure the overheat light came on, but at the same time the coolant had emptied itself. But I completely take the point, if overheat is the problem, due to anything, why shut down the fans..... But oddly that means I apparently have 2 separate problems that have occured at the same time - a significant coolant leak has suddenly appeared, and at the same time my fan circuit has developed a problem. I am pretty sure I would have noticed if the fans were not working the previous time I drove the car.
@ dlearl476 - Thank you for the reply. Yes in parallel with refilling and re-pressurizing to look for the leak I will monitor for the fans coming on and work on this if they fail to come on. Seems there is no easy way to force the fans to come on, and therefore check operation. I am familiar with the fan relay, and I believe there is another of the relays that can be substituted in to replace the fan relay, for test. I have not researched this yet. I am not sure what you are referring to when to mention 'fan switches', and I think there is only one 'fan relay'. Any info re this is appreciated.
Your mention of the tank cap is interesting. In fact this is one of the first things I looked at and I was surprised how 'soft' it was to press the cap down to rotate it off. I am thinking maybe it tightens up under pressure but it sure was easy to push down and rotate. I'm not sure how a weak cap causes overheating but hey, it's an easy thing to replace and thank you for mentioning it.
@ chudson - thanks for your reply. A question and a comment.
This bleed screw we are talking about - it's the nut on the top of the water manifold at the top/front of the engine, forward of the front-most intake manifold, right?
Actually it is a bolt and yes, it is on top of the water outlet on the engine. It is 12mm head so to distinguish it from being a "regular" fastener and it is obvious that it is securing nothing.
Originally Posted by rogerstopford
Just a comment - you mention the fans shouldn't switch off due to overheat, but I'm wondering if the system detects 'no coolant' and shuts the fans down, because in my case for sure the overheat light came on, but at the same time the coolant had emptied itself. But I completely take the point, if overheat is the problem, due to anything, why shut down the fans.....
Just for clarity, which warning light did you see? Also, 968s do not have coolant level sensors.
So here is the problem with coolant temperature sensors that have probes which are submersed into a liquid. They are designed to measure the temperature of liquid and not air. Just like you cannot put a thermometer inside you mouth and know your temperature; it must actually touch the skin under your tongue. If only suspended in your mouth it would take forever to absorb the temperature of the surrounding air. The same applies with temperature probes in cooling systems. If there is a sudden loss of coolant, the engine coolant temperature (ECT) sensor and the thermo-switch for fan operation both loose what they are measuring - the liquid. The presence of hot steam is not enough for the sensors to react in a timely manner. So a sudden loss of coolant equals an overheat with no fan operation and no timely indications by the engine operating temperature gauge. The gauge will eventually show the overheat once the metal surrounding the sensor rises in temperature, but that is often too late and engine internal damage may occur. The thermo-switch for the fans is located so far away from the heat source (the engine) that if all coolant is lost, they will never operate and even if they did, there is no coolant to cool.
HOWEVER, in your case, if a sudden loss of coolant was all that occurred, that would not have shut down the fans if the A/C were operating because the A/C demands fan operation at all times regardless of engine temperature.
Originally Posted by rogerstopford
But oddly that means I apparently have 2 separate problems that have occurred at the same time - a significant coolant leak has suddenly appeared, and at the same time my fan circuit has developed a problem. I am pretty sure I would have noticed if the fans were not working the previous time I drove the car.
These forums are great for people to share stories, ideas and experiences. But in your case, it is my opinion that your car needs a hands-on diagnosis. This is a daily struggle for me. A customer calls and explains a symptom, then asked what is causing it and what the cost to correct it is. In a few cases I can actually give them that information, but most of the time I need to see the vehicle first.
I would start with the obvious which is to determine where the coolant went. The fan testing can be performed without the engine running, but we need the engine at normal operating temperature to see if the fans will function automatically so again, I would look for the coolant loss first. If no leak point is found then it is possible that an extreme overheat blew a lot of coolant past the pressure cap, but if it got that hot there may be other issues and you need to know why it got that hot while still full of coolant. One word of caution - extreme overheats can damage thermostats and/or sensors that had no issues before the overheat occurred. Don't loose sight of that.
Thank you again chudson and oppositelock. I just did test 1 from the pdf from oppositelock and it tests good, so the fans work. I think I'll proceed with refilling and starting the engine and looking for a leak. The passenger side under the car is marked with leaked fluid, and the engine bay on the passenger side has slight wet spots of coolant. So a leaking HCV still sounds logical to me, it's absolutely at the right place to have leaked fluid with the majority flowing down and under the passenger side, and with some drops blowing straight backwards onto the passenger side of the engine bay. And I cranked the heater control full off at the start of the drive, which might have provoked a leak at the HCV due to lack of use for some months beforehand. So I'll refill per the instructions and go from there.
If you are running the stock-type heater valve IMO it should be replaced with one of the stronger alternatives such as the Four Seasons 74671 or NAPA BK 6601410, there is also an all metal valve which is a bit more cumbersome. The vacuum connection needs a little heat shrink or a ziptie to make a tight connection.
@ dlearl476 - Thank you for the reply. I'm not sure how a weak cap causes overheating but hey, it's an easy thing to replace and thank you for mentioning it.
It’s a pressurized system. If it’s not closed, it allows steam to escape. The same way an open pressure cooker will allow steam to escape, but the water won’t boil up to abt 250°-275° if it’s sealed.
Thanks for all the support and engagement. I already took step 1, based on info from this thread, and proved that the fans work fine. Again armed with all the info from here today I took step 2 which was to refill and look for the leak, which was totally easy and I quickly had water pouring out of the heater control valve, which was my prime suspect based on where the coolant was sprayed in the engine bay and under the passenger floor area. Seems like the back cover has separated from the main assembly. Now to source the replacement, then I'll install and refill again and check for leaks again. I've noted the comments re the different qualities of HCV but right now I finding 'no stock' in Canada. I will check my local dealer and some US sources.
06-21-2024, 11:06 PM
968 coolant refill
