While watching some 928 YouTube videos I came across a mechanic in Australia who works on 928s.
He said the transmission cooling side of the radiator, the wall between the engine cooling and the transmission cooling, had a tendency to break.
Is there any truth to that?

 

he may have been talking about the top of the tank flange where the hottest coolant goes into the radiator,
also there is a small air pocket that this becomes filled with hydrocarbons that weep into the coolant over time,
these hydrocarbons cause the tank flange to get brittle and eventually crack,
installing a new tank will usually fix the radiator.

 

Yeah there is no wall, the trans cooler just sits inside the plastic end of the radiator just like the oil cooler on the left side.

 

This is the bit you referred to:
They recon the gearbox radiator inside the main radiator fails and let coolant into the gearbox. Their solution is to bypass the cooler in the radiator and only use the external gearbox cooler.
By that reasoning the same should be true for the engine oil cooler.

 

The 928 / 928s / 928S4 / 928GTS with Auto transmission , be it the early 3 speed auto or the later 4 speed auto & "if" the original equipment Behr radiator gets into old age without leaking externally , then a percentage of these quite old Behr radiators can and do get a pin hole ( corrosion pin hole ) to the radiator auto trans heat exchanger within the right hand side radiator side tank & naturally this is emersed in coolant or often coolant thats way too old or way too diluted , when this happens ( without warning ) the coolant/water mix will often corrode it way into the heat exchanger thats in the coolant within the R/H rad tank and from there it quickly gets into the auto transmission itself . When I mention " without warning " I mean that the coolant loss appears to be quite small as observed by the owner/driver of the said 928 & depending on the time frame of the owner driving conditions at the time of the " intermix event " , remember its just a corrosion pin hole & each event will have different time frames depending on the size of the breach to the heat exchanger , but usually the owner can not see where this small coolant loss is occurring ( per drive ) and hence will just keep topping it up until the the dreaded weird gear changes and slipping is experienced by the owner /driver , by this time ( days or a week or so ) its too late the damage is done to the auto transmission( the auto oil is tuned into a strange frothed up light pink milkshake & the auto transmission is quite damaged & its avoidable , in fact its completely avoidable

Note 1 } over the decades we have only seen water/coolant mix going the one direction ( into the auto trans) & not the over way around , meaning we have never seen auto oil in the coolant , so naturally this means that the coolant pressure ( 1 bar ) is more than the auto oil pressure in the heat exchanger ( less than 1 bar ) very simple physics as an observation

Note 2 } Just recently ( a few weeks age ) we were working on a 1987 928S4 with auto trans ( we have never seen this car before / new customer to us ) & I noticed that this particular 928S4 has what I call a mildly noisy auto trans ( main ) oil pump , meaning at idle and with the auto trans in Neutral or Park the auto oil pump was noticeably noisy ( whine ), but in "D" or "R" it was 95 % quieter , I knew from that noise alone that this 928S4 most likely had an auto trans / coolant intermix issue in its past , so when I spoke to the owner I asked him did he know if his 928S4 had a intermix issue with his 928S4 in the past ( to his knowledge ) in regards to coolant/water mix getting into his auto transmission & he quickly replied " YES " it happened a number of years ago & at the time the auto trans was dismantled and repaired but to reduce repair costs a bit or maybe the auto oil pump ( to the human eye ) looked ok ish at the time and was not replaced , not that this info is important , but the number of 928 models with Auto trans that we come across with this "noise " is an indication that auto trans / coolant intermix had occurred in the past & every time I bring this subject up with the owner , in every case a intermix issue had occurred in the past

Note 3 }
The 1978 to 1983 928 & 928s with auto trans had only the heat exchanger in the R/H rad tank , we give the owner the options of either leave as is or we can by pass the old heat exchanger & we install a high quality aftermarket auto trans oil cooler in front of the radiator with custom bracket we make and new oil hose to and from the stand alone oil cooler & we TIG weld shut the old steel fittings on the rad tank to seal off the old heat exchanger ( from that point on , coolant into the auto trans intermix in eliminated )

The 928s 1984 onwards has a stand alone auto trans oil cooler installed by the factory & the dreadful auto oil heat exchanger in the R/H rad tank ( yes both ) , so to eliminate auto oil / coolant intermix we give the owner a few options , either leave it as is OR run new auto trans oil hoses to the factory stand alone oil cooler & we TIG weld shut the old steel fittings on the rad tank to seal off the old heat exchanger ( from that point on , coolant into the auto trans intermix in eliminated )

The 928S4 / 928 GTS with auto transmission also has a external auto trans oil cooler ( factory fit ) but larger , but they still had the dreadful heat exchanger in the R/H rad tank, so like as above we give the owner the option to either leave it as is or we bypass like above

Note 4 } We have been doing the above by pass mods for the decades and those 928 models can never have an expensive and unnecessary intermix issue , which saves thousands and thousands of dollars of unnecessary out of the blue repairs

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Engine Oil Intermix ( 928 , 1978 to 1989 only ) , we have seen 10 or 15 engine oil intermix events over the last 3 or 4 decades ( yes decades ) , so not a lot , but it makes a real mess because the engine oil pressure is way way way higher than the 1 bar pressure in the cooling system , so its often like a small oil and coolant volcano spewing out of the expansion tank cap & this oil and coolant is all over the road and on the Left rear wheel as you drive over the mess and the amount of engine oil in the cooling system is a real issue ( boy does that take some cleaning out ) all new coolant / heater hoses as they are badly effected by engine oil , so yes it happens

Old engine oil hoses bursting , this happens more often , mainly in the old days on the earlier 928 models ( 1978 to around 1985 or so ) oil hose quality must of improved in the mid 1980's as its not as much of a problem in the later years , but its still worth mentioning if an early 928 ( low milage one ) that still has original early engine oil hoses still in place

Because the 928S4 / GT / GTS from 1990 onwards had a stand alone engine oil cooler , engine oil intermix into the cooling system via the radiator is basically ruled out as there is NO heat exchanger in the L / H radiator tank and hence NO intermix

Regards
Bruce Buchanan
Buchanan Automotive , Sydney , Australia

 

Completely agree!

Engine oil in the water system is a dreadful thing to clean out/replace hoses/etc. (Bring a sack lunch and plan on spending an entire day.)

Water in the automatic transmission inevitably means the transmission will need to be rebuilt, unless detected immediately. (Almost impossible, except through pure luck.)

Fortunately, as Bruce mentions, radiator oil cooler failures are still somewhat rare. However, time is the enemy...and I'm betting that coolant bad enough to "eat" at the tops of the cylinders/other aluminum parts is also not good for the oil coolers.

 

Bruce,

Both my 928's are/were later models with the external oil cooler. Personally I dislike the notion of having the possibility of cross contamination and my radiator [a custom made affair from the US] was specified with no internal coolers and I plumbed the by-pass mod quite easily.

Needless to say I felt somewhat uneasy about this mod given the trouble Porsche went to to install such in the first place and made every effort to ensure best that I could that I was not introducing a different problem by its elimination. The logic I was offered from this list was that for the gearbox cooler the loop through the radaitor was more of a heater than a cooler per se and the intent was to get the ATF up to operating temperature quicker. I found that one hard to swallow but kept an open mind.

The only pressure in that loop is that required to induce flow through the circuit and that is seemingly lower than the pressure the cooling system typically sees of around 10 psig or so hence the driving force to contaminate the gearbox if and when a failure occurs. I have measured the ATF return temperature from the external cooler and it seems OK and I limit the service life of the ATF to about 40k km or 5 years - so far so good as it were.

 

Hi Fred , thats perfectly fine , thats why we give all owners the choice ( in email form , so there is a record of the info given ) , so "if or when " in the future there is an expensive and avoidable intermix with coolant water mix in the auto transmission ( which is avoidable and cheap to do ) , they can not come back to me and say " Was This Preventable ? "

Whats interesting ( to me at least ) is what I experienced here in Australia back in the late 1960's and all through the 1970's was the common knowledge ( then / lost memory now) of V8 powered cars here in Australia towing quite heavy caravans ( in the heat of summer / every summer ) all with American V8 engines , be it GM or Ford or Chrysler ( there was no V8 or Turbo diesel SUV's then ) just large sedans or station wagons , like Ford Fairlane's 351 V8 or Ford Falcon V8 station wagons etc etc that families purchased to act as family transport but importantly tow large caravans in the summer holidays etc and all these cars were 3 speed USA auto transmissions ( generally ) and all of these cars had the standard Auto Trans oil heat exchanger in the metal radiator tank ( yes it goes way way way back in time ) and naturally towing a large sedan with a decent V8 , fully loaded with family and luggage and all the stuff loaded into the large caravan , the poor V8 had no real issue doing the towing but the Auto Transmission protested by getting Way Way too hot ( towing up large steep hills over hundreds of miles and traffic ) , so hence ( here in Aus and most likely in the USA ) was the introduction of a custom made ( kits) for your specific Ford or GM or Chrysler V8 sedan or station wagon which was stand alone auto trans oil cooler kit , the radiator heat exchanger was instantly bypassed ( yes deleted / by passed ) and the stand alone auto trans oil cooler was introduced in front of the radiator and this fixed the problem of the auto transmissions from overheating ( yes overheating ) , so this memory is only remembered by guys my age or older and hence why using this knowledge , we , decades ago put it to good use in the 928 to do two things ( in our climate / which is mostly hot ) to eliminate the intermix issue and to properly cool the auto transmissions with a stand alone auto transmission oil cooler

Very Simple stuff

 

If your radiator is still the original BEHR unit, it's time to replace it with a new CSF unit as preventive maintenance.
Just like an LH or MAF rebuilt. They are ticking bombs. If your LH goes bad, you get stranded, nothing else. Radiator fluid mixing is a different ballgame for your engine and tranny.

 

I recently went through this ordeal.

The engine's oil cooler is integrated into the OEM Behr radiator and it failed causing a huge mess of oil and coolant in all the engine's cooling passages. The emulsion is a mixture of engine's oil pump at 5-bar of pressure overcoming or bucking the water pump's 1-bar pressure. Eventually this emulsion will spill over at the expansion tank.

It took 16 flushes with distilled water to get rid of the oil out of the cooling system.

I upgraded to the CSF all aluminum radiator, so should not have this issue in the future.

Here's my video on YouTube:

 

Mixing is common issue in the 944 world. Flushing and cleaning with special degreaser fluid is described in detail in the 944 WSM.

 

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You did the radiator delete mod?

 

I had posted a question about aftermarket radiators but noticed my question was addressed in the previous posts. So it edited my post.

 

I just wanted to see the mod.