'86 e30 running warm

[VIDA]

I'll preface this by saying my mechanical forte is vehicles that float, not cars so.....this might be a stupid question!

the cooling system on my e30 has been running warm for the last couple of weeks. not hot, just warm. usually runs just below the half mark, maybe 1/16" under. the only time it runs warm is when i'm on the freeway, specifically above 70-75mph. sometimes it will do so above 3k rpm as well, like in 3rd gear. when running warm, it will rise about 1/16" above the half mark, so total movement is about 1/8" from normal. obviously seems load-related but is opposite of the normal cooling system woes. if i slow my speed, the temp will rectify itself. cools perfectly in stop 'n go traffic, at idle, around town, under 70 mph freeway driving, etc.... i've had the usual air bubble thing happen after replacing the hoses/pump, etc... and that usually start creeping up at the red light then goes to normal with increased air flow over the radiator. this exact opposite thing is what's screwing me up.

i'm leaning towards a flow issue, but that doesn't explain normal ops during the usual overheating culprits. except maybe cavitation, but... maybe partially blocked radiator?

has the standard temp thermostat which i replaced about a year ago w/h20 pump, has plenty of coolant and no apparent leaks, doesn't seem to have any air, fan clutch seems fine, aux fan is hardwired to a switch so i can manually turn on/off @ lo/hi speeds.

also, this is in south florida during the hottest part of our summer. doesn't seem to be as easy to run warm at night however, but will if i load the engine and pump it up over 80-90 mph. HOWEVER, of my 3 years of ownership and living here, i have never had this happen. should i be concerned with this or just accept it as my engine dealing with a swampy summer the way i change my shirt 3 times a day? any insight will be helpful. thanks.

[TOTALLYBMW]

It sounds like the fan clutch, when last was it replaced?

[Jeron]

You insight is already very good but here are my thoughts.

Maybe ar high RPM the belt is slipping on the water pump pulley because the tensioner is too loose, the belt is old or the pump bearings are going out?

Cavitation was my first thought too but hard to explain or diagnose. Maybe broken/cracked pump impeller?

[VIDA]

i haven't changed the fan clutch during my ownership, and don't know when/if it was changed last. i performed the newspaper to fan ritual and it ate up the paper...however, i don't think the fan clutch would be an issue at freeway speeds, esp. w/the aux fan set to high. but, for 50 bucks, much easier to swap instead of a water pump and who knows what else. my belts are about due for a change. i'll give the belts a once over this weekend. by the way, what's the normal life expectancy on oem bmw water pumps? seems i go through one about every year and a half. the one on it right now has lasted the longest. any way to tell if the bearings are shot besides the obvious?

[TOTALLYBMW]

Originally Posted by VIDA

i haven't changed the fan clutch during my ownership, and don't know when/if it was changed last. i performed the newspaper to fan ritual and it ate up the paper...however, i don't think the fan clutch would be an issue at freeway speeds, esp. w/the aux fan set to high. but, for 50 bucks, much easier to swap instead of a water pump and who knows what else. my belts are about due for a change. i'll give the belts a once over this weekend. by the way, what's the normal life expectancy on oem bmw water pumps? seems i go through one about every year and a half. the one on it right now has lasted the longest. any way to tell if the bearings are shot besides the obvious?

Unfortunately, the only way to tell if the water pump is shot is that awful noise it makes when busy packing up and the water leaks.

[Bear]

In my limited experience the viscous fan issue only occurs at idle or low speeds, above 45 MPH I had no problems.