1998 2.8L 30v still overheating after replacing lots of parts

[CheckerBird]

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), new expansion tank and cap (Chinese generic). I also bypassed the heater core with a hose loop.

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 while cold, 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 rather freely from end tank to end tank.

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 that there is no air trapped in the coolant system, I bled it many times. 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). It also doesn't start to rise past centerline until the oil temp goes past 225 either.

I've ordered a new radiator (Nissens) and it'll be here the middle of next week, and when I pull the front of the car apart again to replace that, I'll try flushing the oil cooler too.

Any other ideas?

[MrSnickelsnizer]

Do you have any bubbles coming up from the expansion tank when warmed up?
And swapping out radiator is a good choice. Would recommend oem audi over nissens...

[CheckerBird]

No bubbles at all (like a head gasket leak would typically do), but it does suddenly start boiling over and pushing coolant out like a geyser pretty quickly a while after the thermostat opens if the cap is left off because the temperature rises so quickly. With the cap on, no sign of bubbles whatsoever. No sign of water getting in the oil either or any tell-tale steam out the exhaust.

I'm suspecting more that even though the radiator is flowing some water across the core, that I likely have significant blockage in it and it's unable to shed enough heat. Especially since when the temp gauge first reaches midscale from a cold start, it takes a normal amount of time given the outside air temps, and it stays steady exactly at mid-scale (the thermostat's opening temperature) until the oil has had enough time to really heat up, and only after the oil temp hits its 225 mark, then both temp gauges begin to rise together... like the added heat load now coming from the oil cooler is what's overwhelming a crippled radiator's ability to dissipate the heat.

Sound plausible?

From a cold start, I can drive the car about 10 miles in city traffic before the temp hits the first mark above centerline, and the oil is just above the 225 mark then. That's enough for me to drive from home to work, or the other way back home, without it really overheating, just running "extra warm". And that's with the A/C going too.

[seanf86]

have you pulled the condenser away from the radiator? I've had all kinds of dirt, leaves, fluff build up between the condenser and rad and cause warm running in my B5S4, cleaning it out and blasting out the radiator fins and condenser with the garden hose have fixed it.

[CheckerBird]

First thing I checked. All perfectly clean and open in there.

[PZ.]

There should be enough airflow at highway speeds to keep the engine at 190 even without the electric fan running and the a/c on. If the a/c is not cutting out when the engine gets hotter, it's a good clue that the airflow is enough.

I've been driving mine without the electric fan since I did the 1.8T swap and the only issue I have is the a/c warms after sitting in traffic for a more than 2 minutes. I have to swap the fender harness and fan controller from the 1.8T into the car. I was hoping to use the 2.8 controller as it also runs the after-run coolant pump that I wanted to add to the 1.8T.

[CheckerBird]

New radiator installed and it's running a lot cooler now. Looks like that was the culprit. I'll know for sure when I drive it in the heat of the day tomorrow with the A/C on.