I'm just getting back into my 928 after almost 10 years. I'm picking up where I left off with a cooling problem.

I have a modified 1984 928S (US version, automatic transmission) that runs hot. The modifications are high lift cams, shaved heads, and a disconnected air pump. Those modifications were done by the previous owner, at some point in the car's early years. I was lucky enough to discover the modifications when I became a new 928 owner in 1995 and came to learn I bought a car with a blown head gasket!

I personally made the repairs in 1995-1996. As part of the repairs, I had the machine shop "O-ring" the heads for an extra tight seal. I drove the car without problems for a year or two. However, after that the car started to run hot.

I believe my problem is in a corroded radiator and potentially a bad viscous fan clutch.

I've determined that my car has a radiator with a right-side transmission cooler. However, I'm wondering which models had the left side oil cooler? Should I consider getting the left hand side cooler with the idea that if the car still ran hot that I would add the parts to connect the oil cooler? Could I even do that? Would there be a danger in interfering with the oil pressure/temperature/distribution such that I'll damage the engine?

-Matt

 

Before you do anything else, what about the thermostat and the ring seal located in the block behind it?

They run hot if that seal deteiorates because of their special waterflow arrangement. On the outside chance that you are not familiar with this, thought I should mention it.

Further, we have seen some recent water pump failures that involve reduced pumping capacity due to the actual destruction of the block. Hopefully you don't have this issue.

My 85 auto has a left and right hand cooler in the radiator. You could add a separate standalone oil cooler, that is possible and has pros/cons. Also, you can by a used cooler like in your right side and have the radiator end tank on the left side modified to incorporate a cooler. You've got some options.

My 84 auto has the same set-up as yours. Doesn't have an oil cooler in the radiator.

 

My bone stock 79 with a clean radiator will overheat at idle due to a bad fan clutch.

+1 on the thermostat seal.

I doubt your modifications have anything to do with the car running hot. Just needs some routine maintenance.

 

Does the car get hot when sitting or in stop-and-go traffic and cool down on the highway? That'd indicate a bad fan clutch. The car doesn't need the fan to stay cool when cruising at 60MPH.

Any coolant loss or white tailpipe smoke on start-up? Then I'd be thinking head gasket.

Adding an oil cooler is a good idea but not having one isn't the source of your problem.

 

The electric fan relay or fuse might also need changing.

 

Great comments. A couple other data points:

The car has one other modification in addition to the cam/head work - exhaust headers. I forgot to mention that.

I've noticed a very slight coolant leak where the car sits (sadly, and I know I've probably got some more issues due to the sitting). Very slight in that the coolant on the garage floor evaporated. I've also noticed some corrosion / crystals on the fins.

My theory is that the blown head gasket back in 1995 was due to an overheating/warping of the heads. When the heads were machined, I seem to recall that they were only .002" off, or something very minor.

I guess I'm nervous enough about the whole overheating thing that I'm inclined to overcompensate and at least replace the radiator. The one from 928motorsports looks solid and ready for the job. Given I haven't driven the car for almost a decade, I'd be fine to put a little money into this now and rest easier. (I'll do the work myself, although radiator prices ain't exactly cheap!).

Great comment on the thermostat seal. I did not know that. For $20-30, that'll get replaced too.

I'll wait on the fan clutch. Glen, the diagnostic you suggest makes perfect sense.

The oil looks good - no apparent signs of coolant in the oil. Considering the history of the car, it's not inconceivable that I have further gasket or even a block problems. I don't think so - I drove the car 9,000 miles after the head gasket repair and a dealer (Stoddard in Cleveland, Ohio) did the final timing belt work and the end of the job.

I do have a couple follow ups -

I thought the electric fan on the front (and the relay) was only for the A/C?

On adding an oil cooler, wouldn't it be as straightforward as getting the radiator that has two side tanks and then the necessary hoses and making the connections? Why, or when, did Porsche decide to add the oil cooler? The 32v in 1985?

[feels great to come back to the 928 community! thanks guys]

 

My '80 Euro S has an oil cooler. Must be higher-powered engines.

It is straight forward as all the parts are available and the engines have the passages. I'm not sure what parts your car is missing but I'll _guess_ the thermostat that goes above the oil pressure sender, a bypass valve (plug, spring and plunger) and the hoses with the adapters at the block and radiator. Around $400 in parts to make it go.

928 Intl is offering aftermarket oil cooler hoses, IIRC.

If you don't have the "PET" parts catalog you should get it:
http://www.cannell.co.uk/Manuals.htm

There are PDFs of the manuals as well at that site.

 

The additional fan, I think, is simpler on the 16V cars.

Relay XVIII

25 amp fuse in slot 16

Signal to activate fan comes from internal grounding of radiator switch, located in lower drivers side of radiator. One wire of the switch is essentially to ground (going to body near the right or left side of radiator at the multipoint ground screw). The other wire from that switch runs into CE panel Q4, then to the relay coil of XVIII. When the switch trips due to high temp of cooling water, the relay activates.

Power runs from the relay, out of the CE panel on wire Q3, and onward becoming the radiator fan hot lead. The other wire at the fan is grounded. Circuit is completed, then, with only the connection of the switch.

Check fan by direct jumper from hot post in engine compartment to the hot lead at the fan plug in front of car.

Or, remove XVIII and jumper posts 30 and 87 with a wire carefully. Fan should activate with ignition off

Or turn on ignition and remove the two wires from the switch and touch them together.

I think that's how it works.

I had to replace the relay and the switch on one of the cars I have.

 

On my 16V cars the electric fan comes on even when the AC is not on. Usually when stuck in traffic on a hot day. I think it kicks on when the temp gauge is at about 3/4 or so. You can see it bring the temp down and then it kicks off.

 

+1. The "A/C" fan will stop it from overheating whether or not the A/C is on. Some guys have removed the mechanical fan and just relied on that.

 

Initually all 928s used oil coolers then in 1980 the USA versions deleted it ( it was illegal to drive fast anyway 55MPH speedos ! Then in 1985 USA cars got coolers again. In addition to the hoses you need an oil thermostat, two threaded block adapter fittings , couple springs ,and a bypass piston and spring.

 

One further note / correction to what I typed above.

There are two activation sources for the fan on the 84 car.

One is the radiator temperature limit switch, noted above.

The second switch measures refrigerant temp. Its wired in parallel to the radiator temp switch noted.
They meet in the wiring harness in the front of the car and share the same signal wire, Q4, into the relay. Either or both will trigger the fan.

(It changes the next year. On 85 cars, there are two additional temperature triggers. Auto transmission oil temp is one. Inlet tube air temp is the second --- it runs through a resistor, though, and triggers the fan at partial speed to cool the engine compartment with car off. )