Coolant temperature variability/failing rad?

[Click]

Since I've had my car the coolant temperature usually has varied somewhat. Usually a little below half up to half way. Recently I've noticed that it is creeping up past half way, almost to halfway between 1/2 and 3/4 tick marks. I noticed it crawling up driving up a hill, floored in 4th but holding speed, going 40+ mph in ~50F ambient temps. Soon after cresting the hill and coasting or part throttle it cools back down fairly quickly. Is this a sign of a failing/clogged radiator? Replaced wasserpump/80C thermostat, main coolant hoses last summer.

Normal


Heating up

[dnguyen1963]

Are you sure that your system is fully bled? It seems like there is a small bubble trapped in the tstat housing. I have found it useful to drill a small hole in the upper part of the tstat to get all the air out and prevent this type of problem from happening.

[mrcheezle19]

I would say either a bleeding issue with an air bubble trapped or possibly a pinhole leak in your radiator. I had creeping temperatures and after I changed my coolant I noticed a pinhole leak that I didn't see before. Changed radiator and problem went away.

[delatlanta1281]

How about fan clutch? That's a cheap easy replacement process that can't hurt.
Also, and this is a loooong shot, is your SI board ok? I know when mine went bad, if I revved the car, my needle would move.

[Herr Faust Schinken]

mine goes up and down a little when under heavy load but my car normally runs at the fourth it only ever gets up to a little under the half mark if I'm climbing a mountain or in heavy traffic , Have you ever replaced the radiator ? mine was original and i was having the same problem when i got the car

[mrcheezle19]

Yeah possibly fan clutch is well, I have a electric fan so I always forget the think about the mechanical fan.

mine goes up and down a little when under heavy load but my car normally runs at the fourth it only ever gets up to a little under the half mark if I'm climbing a mountain or in heavy traffic , Have you ever replaced the radiator ? mine was original and i was having the same problem when i got the car

Shouldn't the needle always be directly at center or a tad left or right? not at the quarter for sure though.

[Herr Faust Schinken]

i has 75 degree thermostat if i put an 80 degree one back in it would run dead center

[oldmann]

Had an aux fan fail a month ago, when I pulled it out, there was what looked like a birds nest worth of crap behind the center shoved into the condensor.

Pulled the radiator out and the center of it was also full of crap, washed everything out, now the aux fan doesn't get a chance to come on anymore.

CK it out, might be an easy fix.

[QUKBMER]

Having a problem with my 91 325is. Running hot in traffic,fan clutch is to blame. My 90 325i no problems in traffic.

[Andy348]

Are you sure that your system is fully bled? It seems like there is a small bubble trapped in the tstat housing. I have found it useful to drill a small hole in the upper part of the tstat to get all the air out and prevent this type of problem from happening.

Stupid question: won't the coolant come out of the hole after all the air does?

[e30edward]

Mine does that when I'm climbing a hill. I've been meaning to try and bleed the system, but haven't got around to it.

[delatlanta1281]

Stupid question: won't the coolant come out of the hole after all the air does?

I think he's talking about the thermostat itself, not the housing.
Also, mine runs just to the right of the 1/3 mark. It has forever.

[dnguyen1963]

Stupid question: won't the coolant come out of the hole after all the air does?

Contrary to popular belief, with the bleeder screw spewing out coolant you can still have small air bubble trapped inside the tstat housing and that could cause the problem as described by OP. The small hole is drill onto the top of the thermostat plate, not the housing.

[Click]

I don't think it is the fan clutch since it heats up while driving at speed. I checked the radiator and it appears to be clean. I believe my fan SI board is original. Everything seems to work ok on it except 2 leds of the service indicator. I believe the rad is original, when I went to drain the coolant last year to do my timing belt and waterpump, the drain plug sheared off, leaving a plastic plug in with no head. I have an upcoming m30 swao so I was thinking of just replacing it with an e28 rad.

[Gregs///M]

A good tool to have is an infrared laser temperature thermometer. I have one and it works great to verify temps around the engine. Here are some typical temps of my car at normal operating temp:

Thermostat housing below temp gauge sensor = 190F
Upper radiator hose=200F
Lower radiator hose =165F

Items that can provide inaccurate temp gauge are dirty, loose or oxidized engine to chassis ground cable, cluster ground cables (ring lugs above brake pedal, bolted by 10mm nut) and dead or failed SI board batteries.