Driving without a radiator cap
 

I don't recall any discussions of this, so here goes.

I drove from the San Francisco area to Reno in blazing hot weather on Monday. Climbing through the mountains the car ran pretty hot, approaching the red zone at times [my fault noted at end of message 12], and when I arrived at Reno I noticed I had developed a leak in the passenger side side tank seal (2nd time in 15 years) and coolant was being lost at a pretty good rate. I lost about 2 gallons of coolant in a steady drip. When I removed the radiator cap the leak stopped. I refilled the radiator with coolant and some water and left the radiator cap off. I drove around Reno today in hot weather and the car ran cool once the coolant had been fully topped off. I was concerned that I might get some boil over once I shut off the motor and the coolant stopped circulating and the stored heat in the motor transferred into the coolant but it didn't happen. No drips from the sidetank either.

Here's my thinking. If I run with the radiator cap on I won't make it out of town let alone 200 miles home. With it off I shouldn't lose coolant although I do lose the effect of pressurization on boiling point. As long as the motor is running and the coolant circulates I should be OK. It's my only hope to make it home. I'm going to drive in the cool night as well.

What do you guys think? Has anyone driven one of these cars 200 miles this way? I'm actually expecting this will work well enough under the circumstances.

When I get home I'll pull the radiator and have a new tank and seal installed.

Just my $0.02, and I'm no expert.

I'd think you will be fine. You will lose the increased boiling point without pressure, but the coolant already has an increased boiling point over plain water.
If it was me, I'd probably stop every 15 or 20 minutes to check the level and keep a close eye on the temp gauge. But I probably don't have to suggest that to you.

Done it many times, not in a 928, but pressure caps are for coolant temps over 212 degrees, so if you do not exceed that, you will be fine. I would drive it, carry lots of water and go!
Dave

I thought the purpose of the pressure cap was to forestall the boiling of coolant at any particular hot spot, b/c the formation of bubbles on the surface at that hot spot decreases the efficiency of heat transfer from the metal into the coolant.

That said, I'd take my chances without the cap. I think you demo'ed in Reno today that you'll be alright driving back to the city. I'm guessing the pressurization of the system gets important when the engine is being worked hard, not during easy cruising.

Does your low coolant sensor work? If so, don't worry, carry a lot of water, keep a weather eye and enjoy the drive.

ww

The weather is cooperating somewhat, and we should have cooler temps the rest of the week here and nice in the evenings. If you run into a problem I am on your way, in Sacramento, and feel free to stop in. You can PM me.

I've run cars with duct taped hoses, and other leaks that were major with pressure build up, and had no issue leaving the cap off for a short time. I also ran my 82 for a few days with a leaking water pump, with the cap off, and there was also no problem. In all those situations I made sure I carried lots of coolant mix, checked it often, and kept an eagle eye on the temp gauge.

Massively long burnouts will be out of the question too ... ;)

I've done it on a 928, no issues.
Drive home, but avoid pushing it as it were.

As others have said, you should be fine. As long as the leak doesn't get worse running with the cap off just cruising it home should be no problem. The reduced pressure should prevent any increase in the leak. Top it off with distilled water not coolant. Coolant is expensive and toxic. No need to be dumping it all over 200 miles of highway. You'll be putting in fresh coolant once the radiator is fixed so no worries about running just water for the trip home.

So, final report from Bill? By now the cool nighttime drive should be completed and you are waking up in the comfort of your own bed.

Bill is my hero, lots of DD miles, open road racing, some pressurized life, and the car soldiers on. Problems he has are bellweather for those I might have. After hearing about this leak, I should probably add radiator tanks and seals to my winter worklist.

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While packing up for the migration north from SoCal to central Oregon, I discovered that the new-in-box Behr radiator fits perfectly in the luggage space with the rear seats up. Once the garage storage gets worked out, it will find a home someplace other than the back of the car. Until then, my carry-along spares list includes a radiator. I'll never have radiator problems so long as the spare is along, according to Edsel Murphy.

Initial report. Home safe, zero issues, no coolant used at all. Ride started at ~7 am. 60F in Reno. 46F in run up to Donner Summit. Saving grace is we only had to go from ~4600 feet to ~7200 ft before heading back down the other side. A lot less challenging than the ride from sea level up to 7200 ft on Monday. With the cool/cold temps it barely got over the first white line heading uphill and often got below heading down. Was not quite 80F in Sacramento at 9-10 am breakfast stop as opposed to 102F in the afternoon Monday. Water temp reading barely got to middle of the gauge and most of the time ran a needle width or two above the first white line. The latter part of the ride was essentially level in the mid to high 70s and ran the same cool water temp.

I will talk a bit more later about my sidetank failures and other factors that contributed to overheating besides the very high temps and mountain climb on Monday.

Glad you made it home safely! I look forward to your check/watch list. Stay cool!

Ok, so I had another right/passenger side radiator sidetank seal failure. The previous one was probably 4 or 5 years ago and the original a several years prior to that. The left/driver side tank has never failed. The reason for this is that the radiator inlet and hottest side of the radiator is right side. I believe the plastic sidetank warps or cracks from the excess heat. It appears the material heat tolerance properties are pretty narrowly spec'd to normal radiator inlet temps.

In my case, the failures have always been associated with episodes of very high coolant temps. My 89 generally runs between the 2 white lines, but is quite sensitive to outside temperature and load and on hot days with uphill grades it will creep over the second line, which is believe is 210 although the gauge at least through 89 is not marked with any temperature numbers. I will generally not get too nervous at one or two needle widths above that line, although I feel a lot better when it gets back below that line. So, I often kill the AC on uphill grades on hot days even though I'd rather not.

As far as I know there is nothing wrong with my cooling system (other than the bad sidetank now). It has always operated as if it has a capacity limit somewhat below what would be ideal. It's my impression that Porsche did almost nothing to improve the cooling system capacity over the years despite increasing horsepower output from the motor. I've seen other 89+ S4s that operate just like mine. Oddly, several 87-88 S4s I have seen never seem to register over the gauge midpoint. It may be a mistaken impression. I can't explain it.

My general preference is not to fix things that are not broken, so I would not replace the side tank or seal anticipating a failure at some point, especially the left sidetank which has never failed on my car or any other I know of. I think a new sidetank and seal is almost as likely to fail under adverse conditions as old ones. As an interesting anecdote, I decided to replace the factory main fuel pump on my car at 260K miles. The shiny new Bosch pump I installed failed after 3 months (no, it was not a counterfeit pump). Good thing I kept the original pump.

Anyway, back to Monday. It was blazing hot and I had to make a mountain climb to 7200 feet. So, I wasn't surprised to go over the 2nd white line a bit while running the AC. But the water temps went higher than that despite turning off the AC. We pulled over and gave the car a rest with the hood up. Nothing seemed too much out of the ordinary and I just blamed the high ambient temp. BUT on the way out of the rest stop, the water temperature jumped back up suddenly rather than gradually. It was ALMOST as if I had a water pump failure, but it stabilized slightly above the 2nd white line rather than going all the way to red. However on later long sections of uphill grade the temp needle touched the red zone. I was alarmed but we had only a short distance left so I carried on with the needle just touching the red zone. I expected that temp would drop once we entered the downhill grade. It did but not nearly as dramatically as I have observed before. This puzzled me. It was still rather hot out, but... When we arrived in Reno the issue became clearer as the hotel valet parking attendant tracked me down at check-in to let me know I had a pretty substantial coolant leak. A quick check for where it was leaking lead to the conclusion the sidetank must have failed again.

We did not pull the radiator cap then because the motor was still hot and a geyser probably would have formed. We let it cool down even though the pressure caused more fluid to leak until it was completely cool. In the end I lost 2 gallons.

Finally, here is another somewhat embarrassing underlying cause for the overheating and tank failure. About 6 months ago, my low coolant warning light started coming on at every start up and at some intervals after cancelling it. The coolant level was always fine so I began to ignore to low coolant warnings altogether, planning to fix it later, especially since my car has never needed coolant added between changes. So, in the mountains when the temp started to get higher than the needle width above the 2nd white line, I didn't think coolant level despite the warnings. And when the coolant temp did not drop nearly as low as I expected when going downhill, I failed to think low coolant, even though in retrospect that was the cause. At some point the sidetank started leaking and low coolant level may have preceded that. Funnier still, with the coolant level now restored, the low coolant warning no longer trips. :-)

So, I've been driving for days and days without a radiator cap. Ambient temps not very high, although into the 80s. Have lost minor amounts of coolant, some due to evaporation and some due to splashing as I left the cap completely off the reservoir. Anyway, I have the new tank and seals and just need to pull the radiator and take it to the shop. A bit of a pain and I no longer have my garage and lift, so I'll be making a bit of a mess in the carport, and no other car so I'll need to find a buddy to take me and the radiator to the shop. Anyway, driving the car this way has been cool and uneventful.

Did you move, Bill?

That shop was a sweet setup.

Yeah, Randy. Significant downsize. Need to change my avatar. Lift is gone, although I am getting a Quick Jack BL-5000 lift soon. Hope I can make it work - it does not quite span the jackpoints.