david928

I had the same symptoms with overheating. It turned out the fans weren't turning on. I had to replace the cooling control unit (located below the passenger side door). You can easily check this unit by listening for the cooling fans after turning on the air conditioning.

fraggle

Fans run like mad, even after I turn the car off. They stop "normally" so the temp2 sensor is working right. I replaced the sensor at the front of the rad about 4 years ago, too.

I'll have to check around for a testing rig. Got my daily driver up and running so I can swap the 928 into the garage and begin more FA!

NoVector

FWIW- I'd take a look at your spark plugs. I confirmed my blown head gasket by checking the spark plugs. I had one that was really clean--"steam cleaned". And it did run hot--and got hotter the more I stood on the gas pedal. I also started having a cool looking white vapor trail when driving down the road.

I hope that's not what it is. / Bruce

JHowell37

What you're looking for when you test the system pressure is its ability to build pressure and hold it. The actual pressure amount required is pretty low (about 15PSI.) I've got a tester if you need to borrow one.

Charley B

Your initial post indicates that you are losing 1/2 inch or more of coolant per week. This would certainly contribute to the overheating. You don't say whether, if you top off the resevoir, the overheating starts right away or only after losing a certain amount of coolant. If you're overheating on the first run after topping off than a leak is probably not the issue.

You indicate you are running a 75/20/5 ratio in the cooling system. I would back that off to a 50/50 ratio or less. Surely you're not running 5% of capacity of water wetter. One small bottle is sufficient to do the job and more than that can cause other issues. 75% coolant is too much and could be causing the problem.

A compression check might help confirm or preclude a head gasket issue.

There are still other possibilities such as water pump and engine timing, but I would start by thinning out the coolant. You can even run 100% water just to test the system on a long run. And check for leaks at hose connections and especially under the resevoir. Put your hand under there and make sure there is not a hint of moisture under the tank.

fraggle

Thanks everyone.

I'm running mainly distilled water. I suppose you're right, the 1 bottle of wetter isn't 5% of the system. A better representation might be

75:19:1 water:coolant:wetter

Now that the car is in the garage I'll perform a bunch of these tests.

Overheating begins immediately, even after the system is topped off. I haven't found any puddles on the ground anywhere.

Hopefully I can reply soon with some more info.

Mrmerlin

if its overheating right away then the water pump isnt turning or the gauge is bad

928autobahndreamer

Have you manually disabled the cooling flaps and have them in the open positon? I know this doesn't explain the coolant loss but certainly can impact overheating or running hot.

John Struthers

Any air voids in the hoses?
Hard hose -over pressure/no circulation-?
Go cheap first,
1. Look for any leaks.
2. Squeeze out the air.
3.Replace the Thermostat -double check for orientation and proper seal/gasket installation -mine had a gasket...for real.
4. Trust no water pump, new or rebuilt!
For grins...how's your tranny oil, level/temp....

the flyin' scotsman

Its losing water so the coolant mix (~50/50 BTW) etc don't matter............its a closed system 'til it leaks.

I'd do the pressure test and listen.

mark kibort

Put a bottle at the end of the overflow tube. (like a small water bottle) see if it fills up. if not, you have head gasket issues.
also , a quick easy determination is using the crack block/head gasket kit from the auto parts store.

mk

dr bob

Headgasket lreaks show up on the plgs as splash spots, especially if the leak isn't too big. Bigger leaks make the plug all white or yelow depending on what coolant you use. Piston tops get steam blasted and are very clean too. You may be able to see the piston tops with just a flashlight through the plug hole. Otherwise a borescope is handy for this duty if you happen to have one available.

H2

[QUOTE=Charley B;5995626],,,,,There are still other possibilities such as water pump and engine timing, .....QUOTE]

Charley,

I didn't think that overheating with off-cam timing was a common problem with 928s as the computer apparently compensates for some of those anomolies? My '88S4 runs strong as an elephant but overheats when run hard during warm weather. I've suspected timing as I've eliminate virtually everything ever mentioned in Rennlist "Search is Your Friend"and it still overheats. The same ole overheating suspects keep coming up but I've eliminated all...I think.

It's possible that I' slipped a tooth when putting on the new cambelt and WP but if so, it runs wonderfully and no one has ever given me any idea on how to check the timing to verify. I have the PorKen PKer tool and it checks out OK there.

On my ole Chebby engines, off-timing certainly conttributed to overheating and could be adjusted out. On the 928...dunno.

Just wondering,

H2

fraggle

Thanks for all the advice here. Work (my real job) intruded this weekend so I didn't get a chance to do any of the "easy" stuff. I'm definately going to do more investigating before I tear into the car far enough to get a first hand look at the head gasket!

H2

Fraggle,

Move back to Idaho where the weather's like in your avatar too much of the time and I suspect your overheating issue will go away..at least until late spring. Good luck. Keep us posted on what you find. There are a few 928s that seem to have persistent issues like this and the regularly know fixes just don't work. I still wonder about timing be a possibility but don't really know how to easily check it short of pulling the cam covers and doing the caliper thing. Mine checks out perfect with the PorKen tool but acts similar to your car.

H2