A few weeks ago, I noticed my engine coolant and oil temps starting to get abnormally high, like up to 2 notches above the center line on the coolant temp gauge (which I presume is approx 205-210 F) and the oil temps climbing to about 250 (never ever got above 225 before, even on hottest days). This phenomenon came on suddenly one day.

With the engine cold, and starting to drive, it would take a normal amount of time for the engine to reach normal temperature (half scale on the coolant gauge) and would stay there for a while, signifying that the thermostat had just opened and coolant was flowing thru the radiator but after a while from there would slowly start to climb upwards to the first notch, then if I was running the A/C will start to reach the second notch pretty quickly after that. My electric aux fan had bearings squealing so I knew it was shot too.

I knew my cooling system needed attention for quite some time, it's been having a leak near the rear of the engine for quite a while, and I did use some Barsleak Liquid Aluminum stuff in it this past winter as an emergency measure because I had no other choice at the time. I doubt that this has clogged the radiator since it's been in the car several months and the overheating symptoms just recently came on suddenly. It has clogged the heater core, however, since I have markedly reduced heat from the vents now, but it's summertime and I plan on dealing with the heater later this fall.

So, I found a discussion post online that suggested that the normal rise to midscale on the gauge, plus slow rise to higher, that never goes down until you shut the engine down, is often attributed to a split plastic water pump impeller slipping on the shaft after it warms up, and the hotter it gets, the bigger the crack opens and the more it slips... and, I did the timing belt job just 35K miles ago and the water pump that came in the kit was the same Italian-made unit with plastic impeller that was implicated in the article I read, so I decided to replace all the following:

Water pump (Hepu brand with metal impeller this time), Thermostat (B ehr), Aux Fan (VDO Siemens), and fan temp sensor (Wahler), and coolant temp sensor (Febi), mechanical fan idler pulley and bearing assembly(MTC brand that looked like an OEM unit with the Audi logo ground off), and a new mechanical fan clutch (Borg Warner brand)

After I had everything apart, sure enough the plastic water pump impeller had a big crack in it next to the center shaft. It would not slip, however, it seemed still quite solidly attached to the shaft so I surmised that it probably didn't slip until it got hot. I also drained all the coolant from the engine block and flushed it out with a garden hose. I also stuck the garden hose in the radiator and it seemed to flow quite freely from end tank to end tank.

The plastic overflow tank did have a small crack in it that was seeping a tiny bit of coolant, but I've got it epoxied shut now, and both it and the cap are holding full pressure without leaking. My leak near the back of the engine turned out to be the plastic heater core hose connection cracked, and I've got that fixed now too.

I put everything back together, filled with new coolant, and thought I'd be good to go. The new electric fan moves about twice the air as the old failing one did when the A/C is on too and for a week or so, it seems that my overheating problem was solved... But now the bad news... summer weather has finally got here after a cool, rainy springtime, and the overheating problem is still there.

If I don't use the A/C then the coolant temps never rise above mid-scale, and the oil temps more than around 225. But this is Texas and using the A/C in the summer heat is a must for me.

I made sure again last night that there is no air trapped in the coolant system, I bled it again. I did notice that even when hot, that my brand new fan clutch is still slipping like it's cold, maybe even slipping more freely when it's hot. It's very possible that I got a bad clutch brand new, but when driving down the highway at 60 MPH, at nighttime when the air is cool, shouldn't this be forcing enough airflow thru the radiator with the electric fan running on high speed to be enough airflow to keep the engine from overheating with a slipping fan clutch? It was still climbing up to, and starting to go past the first notch above centerline on the gauge and the oil to go above 225 towards 250 by the time I got back home (I estimate the coolant temp must be at least 225+ by this mark).

Any ideas?

 

See if you can figure a way to tie the fan to the clutch so that it doesn't slip at all. Drive it and see it stays cool. If it does, you've found your problem.

 

Unfortunately the design of the fan clutch doesn't easily lead itself to doing that. I might be able to glue it up with some RTV silicone glue.

 

Yeah. I know. Not without wrapping it up with zipties and possibly getting it out of balance. I do know that if these things are working correctly, it will run hot. I had the same thing happen on an 80's Oldsmobile Cutlass many moons ago. I was coming home from about a location about 3 hours away and it started to overheat. We stopped on the side of the interstate and let it cool off. I thought the thermostat was sticking so I removed the hose at the thermostat housing (I only had a tire iron that had a flat screwdriver like end on it to remove the hubcaps to work with) and disabled the thermostat by bending the block off plate to where it could not close entirely. Fortunately we had some empty 2 liter bottles in the trunk and there was clean clear water in the ditch next to us. We got it back together and while it did seem to run a bit cooler, it was still running warmer than it usually did. We made it home with no issues. Dad figured out that the clutch fan was not even engaging. Replaced it and it ran fine after that.

 

New radiator installed and it's running a *lot* cooler now, I think that was the culprit and the problem appears solved.

 

Ahh. That'll do it. It was probably starting to get clogged up. Or it just could not hold pressure.

 

Still holding pressure just fine, no sign of any leakage anywhere on the radiator itself. I also ran water thru it with the garden hose again last night, and it seemed to flow freely. I may pry one of the plastic tanks off the end just to look at the tubes, probably will find them coated internally with corrosion and scale. My old Chevy pickup radiator was almost totally stopped up with calcium scale, but it had a lot of tap water run thru it over several years due to leaks, and it wouldn't flow hardly any water at all when I changed it. The Audi never had tap water in it, only distilled water and G12 / Pentosin SF antifreeze. I guess aluminum radiator cores just have a limited number of years of useful lifespan then you throw them away unlike brass/copper cores which can last almost forever if maintained well.

 

I called "designed to fail". Keeps the parts makers in business.