I have a 1987 manual 928S4, owned by me for 5 yrs. Was my only car now only used for holidays etc (did 4k kms in 2004). Plan to keep it long term. Have put in a new radiator about 3 yrs ago. Have not had any problems leading up to event.
Water temp Gauge went up to between last of white and red. Feathered car to next garage about 20ks with no change. Left 15 mins to cool. Lost a bit of water when removed pressure cap. Filled up header tank. Carried on driving at max 100kmh, guage temp straight back to where was. Stopped for hour. Checked fan belt tension, Filled header tank and left running with cap off. Looked for bubbles – none. Resumed driving max 100kmh, on climb, overheating temp warning light came on. Motor does not seem unduly hot.

Took to garage next morn in small town, not a Porsche specialist. Guy took out thermostat to see if that problem. Spent 10 mins looking at water flowing in header tank and saw bubbles – said a head gasket problem. Transported car to Porsche specialist.

They ran cylinder pressure test on all cylinders, all ok. Put thermostat back in. Ran on hoist and it ran hot without warning light coming on. Was surging a bit which mechanic said was air bubbles, not head gasket. Today they test drove and got hi temp warning light coming on without even using a hill to stress it a bit. Now looking at water pump, replacing thermostat and looking to run a temperature test.

Said to mechanic last night that my research indicated head gasket is not common with 928’s. He said he has done several. He said water quite often corrodes the gasket over time. They wont be pinned down for a price as 1/ he doesn’t know what it is yet and 2/ until you remove the head you don’t know if there is peripheral damage from either heat or water. I pressed for a number saying I had to pre-plan. He said likely top cost would be say $5k.

Would be interested to hear from anyone who has had overheating problems and feedback on likely cause etc. Many thanks Ewen

 

Would have thought there would be more replies by now ....

If head gasket that water has to go somewhere, either the oil or into the combustion chamber, see if you have any steam cleaned spark plugs ?

There is a seal behind the thermostat that corrodes with age and will prevent the thermostat from making a good seal to the thermostat housing. Sometimes causes issues.

You getting any weeping coolant from your WP ?

Head gasket failure does happen as the mechanic rightly stated, you are at the mercy of previous owners coolant changes which tends not to be often enough. Someone posted a real nasty pic of an 87 head gasket the other day. However lots of simpler stuff to check out 1st.

To pay to have head gasket change is $$$ for sure, invest in some tools and do it yourself ....

Chris

 

Just get the car home first and check basics.

Replace all temp sensors one by one or all together. Cost you MUCH less and worth the investment.
Then, verify radiator is OK, there OFTEN are leaks. I had a one-year-old Behr (stock) rad spring a leak. Replaced it with devek unit. Guess what ... the new rad started leaking one year later.

Pressurise your system from the overflow tank and see where water goes. Hey you could pull all spark plugs at that point and keep a lookout for water too.

Do Not Spend That Five K. You will be sorry if you do.

 

An overheat condition can often be caused by a loss of system pressure. First step is to pressure test the reservoir cap and replace if necessary.

Next is to test the thermostat by placing it into a pan of water and heating it on the stovetop, using a digital thermometer to measure the water temperature. The thermostat should open at the rated value - typically 80 degrees celsius.

Do the easy things first.

 

Your waterpump's impellor has come loose...

N!

 

You should also check the air flow system. Check to see whether your radiator is clean as well as the AC condenser in front of it. Also, make sure your fans are working properly.

 

Possibilities:
A) Inadequate or faulty coolant flow.
- Thermostat faulty
- Thermostat seal faulty, allowing coolant to short-circuit.
- Waterpump faulty (loose or eroded impeller)
- Blockage in block or head passage, or in radiator.
- Air block in system.
B) Inadequate air flow.
- Blockage between condensor and radiator.
- No fan assist.
C) Combustion leak from head or gasket.
This can be easily checked with a kit to test for combustion gas from cooling system. It is possible to have a combustion gas leak into the cooling system without having coolant in the oil or oil in the coolant.

As a reminder - you have two devices reporting on the coolant temp - the gauge and the overheat warning light. In this case, the warning light apparently didn't show up until the system had been opened a couple of times, which could just indicate air in the system. An overheated engine feels hot and smells hot. If the gauge says that a 928 engine is hot, but the warning light doesn't come on, and the engine doesn't feel hot or smell hot, suspect the gauge first.

Running a 928 engine without the thermostat installed will cause overheating.

 

An additional part of the air flow test is to verify that the 'cooling flaps' in front of the rad actually open. If not already done, disconnect the actuator arm - and tie-wrap them wide open.
I am also suspicious of a loose pump impeller - as Normy noted. Drain your antifreeze and refill the rad & block with water: there are several ways to proceed, but a simple one is to pull the thermostat, stuff a garden hose in the appropiate opening - and start the engine. Within seconds, you will know if the pump is OK.
There is a rare chance that the H2O pump bearing has failed: this will allow the shaft to tilt, knock the impeller loose ( causing the overheating), and start eating the timing belt. The pump seal will still be intact - look in the cam cover vent holes and verify that the TB is still 25mm wide with nice sharp edges.

 

If the only water lost in first case was spillage on opening header, and there is no water in oil or oil in water, my money is on bad thermostat or loose pump impeller, or bad gauge reading, rather than a head gasket.
jp 83 Euro S AT 48k

 

If it's anywhere near due a belt and waterpump change, then now is the right time to do it. You have nothing to loose and you can eliminate one potentially very nasty cause.

Don't forget the range the guage shows is relatively small, and as Wally says - you can smell a hot engine.

Don't like to disagree with Garth - but don't condemn the flaps, unless they are demonstrated to be non working. The test is to watch them as you/someone turns the ignition to 2, starts the car, and switches the engine off (they will cycle through open and closed). Also check that they are open when the fans come on.

 

Paul, really no disagreement! If the flaps work - fine; however, if there is any suspicion, I'd still tie them wide open until the problem is resolved. Minimize the variables ...

 

My guess is a plastic impellor slipping on the shaft since it is getting worse quickly. Head gaskets do NOT fail very often and there are typically other symptoms like white smoke ,oil in coolant, coolant in oil. If you have not done a t-belt /water pump in the 5 years you have owned the car............. That is probably needed.

 

There you go - Wally and JB have spoken!

Now get it away from those 'Porsche specialists' and get to work.

 

Thanks everybody for your feedback. Amazing to get so much so soon. I am no mechanic, not inclined that way so even testing sensors is not within my current capability. I am really doing this to ensure that I only get done what is needed by the mechanic as I believe historically things done that are not always essential. Will certainly check these out with the mechanic. Ewen

 

I just went through the same sort of problem, won't bore you with the details, but had an overheating issue that seemed to come and go.... In my case it turned out to be either a sticking thermostat or a water pump just on the verge of siezing.... Replaced both while doing the TB (the H2O pump was shot, for sure), and haven't had a problem since.