944Speedy944

Howdy. I am having a major issue with overheating as of recently in my 944. The details are as such

I own a 1983 944. The components on the coolant system are as follows :
radiator :1993
coolant hoses 1983
radiator cap : unknown
Water pump : unknown
thermostat: 2024
electronic thermostat: 2023

( super defined story if you want all the important details )

my issue appeared sort of suddenly without any noticeable noises. My engine which hasn’t to my memory had issues with overheating began sticking in full heat, or 3/4-4/4 heat. This cycle could only be defeated by pulling over and letting it cool off, turning on my ac didn’t help much or even at all as the fans would usually be on by the time it was hot. I figured the issue was a result of a bad thermostat so naturally I replaced it. However the issue stayed, so next step I removed my radiator and ran water through it, no issues really, water ran through it just fine so I put it back in and wrote down the date written on the side. Then my next step was the water pump, I noticed my timing belt was a little loose because it was roughly around my 3k mile timing service since I last replaced the belts. So I tensioned them accordingly to where the water pump should be getting full action from the belt and not slipping. This helped a small amount initially as my temp wasn’t Getting stuck at full but rather staying firmly at 3/4 give or take a little bit of movement back and forth. So I went for a drive hoping it was fixed, and assumed that since it was a rather warm summer day ( 92 * ) that the temp was a result of the heat and it would go away as it cooled down at the end of the day. But rather than cooling down my engine as it got colder outside proceeded to get even hotter. Until it was in the yellow fully ( red idrc ) even though it was only 65 out so I pulled over and let the fans run with the engine off which somehow made it hotter ? I waited a bit longer with the engine fully off til it was about 3/4 then limped my way home trying to prevent it overheating again, which in the 5 min drive it proceeded to do.

( end of stupid long story )
I noticed my coolant is slowly draining somewhere, my guess is it’s cycling the block and fully going through, of which I will bleed again tomorrow when I have some more coolant, and see if anything changes

so, I’m left with 3 possibility’s and hope someone else may know others,

1 water pump failure, unknown because it’s not making noise or acting weird to my knowledge and coolant seems to be cycling ?

2 radiator cap failure, it’s possible my rad cap isn’t holding pressure and is causing the system to run inefficiency and allowing it to overheat, I am in a dry climate.

3 air in the system, again not sure because I properly bled the system and filled with the air hole open and the engine running. So it should’ve cycled.

Apologies for the long post, I am simply at a loss right now and I would rather spend 40$ on a new rad cap or whatever it costs then do the nuke option of a new 500$ water pump since I’m not even sure if that’s the issue

Chalt

Have you visited Clarks Garage? Here is the link for overheating troubleshooting: TS-02, Cooling System

Jfrahm

Good link. Behavior is similar to an S2 of mine which spun a water pump impeller. I tack weld them into place on both ends of the shaft now. It might pump OK at low RPM and spin at higher RPM.
Clark's test won't show you if the impeller is spun, only the drive pulley unfortunately. You might be able to see the water flow, possibly from a heater hose, not increase with RPM as you would expect but unless it drops off a lot at high RPM it might be subjective.
If you fit a hose or nozzle to the bleeder port it might show something over a rev range

One missing test on Clark's is to see if the radiator is getting properly hot and dropping temp in vs. out, but your other work makes this unlikely to be needed.
When I have had a bad cap it'd boil over when shut down hot but be OK when driving.

BTW: you can tack weld without a welder with 2 car batteries, jumper cables and some flux core welding wire. The impeller might not take weld but you can get a couple blobs of weld into a melted spot on it which should hold it in place well enough.

walfreyydo

Possibility 4 - head gasket (and doesnt always show up with a compression test unfortunately). Look for coolant/oil intermix, or run a coolant pressure test (this will confirm whether your coolant system is holding pressure, which it should if its working properly)

Chalt

I didn't know this was a failure mode. So the pulley or the impeller can spin on the shaft?

Jfrahm

Correct. On 928s the impeller has even been known to damage the block, can't say I have heard of that happening with a 944/968.
My S2 pump that failed this way was a well-regarded brand.