Intermittent cooling problem

[coupe]

Hello

I am having a strange cooling issue on my 90' 325i. My gauge starting moving to 3/4 mark but is not low on coolant or have any leaks. What's weird is that when I went to look to repair it yesterday afternoon it did not got past the 1/2 way mark (100+ degrees ambient temp). Now I am thinking that maybe my instrument gauge is NG? How can I confirm if my car is actually over heating? In the past year I have replaced radiator, hoses, thermostat housing, thermostat and fan clutch. What other parts do I need to consider that are potentially faulty?

[101]

A cheap infrared thermometer is about the only way I know to check the actual temperature.

When exactly does the car start to get to that 3/4 range? Idling with no load, in traffic, pulling hard (i.e., uphill), with or without the air conditioner running, etc.? Is your electric auxiliary fan present and operating correctly?

I'd check these: First, crack open the bleeder valve on the thermostat housing just to make sure that there's no air trapped there (highly unlikely). Second, do the "newspaper test" on your fan clutch. If you replaced yours with an "off" brand, it may not be up to the job. Third, double check that when your auxiliary fan is running (when the air conditioner is engaged) and that it is pushing air into the radiator, not PULLING (which would indicate that it is wired backwards). If the auxiliary fan is working with the air conditioner running, check the thermostatic switch(es) on the radiator that activate it due to coolant temperature. If the auxiliary fan is not running, the fan itself or the resistor attached to it needs to be checked out.

I'm assuming here that there is no coolant in your oil or evidence of a head gasket leak.

I've found that the temperature gauges in these cars are pretty accurate. If they fail, its usually something in the instrument cluster where the gauge is flaky or does not work at all versus a failed gauge sending unit.

[dnguyen1963]

Check your SI board. Your symptoms are similar to those due to bad SI board/batteries.

[coupe]

Conditions were AC off and steady cruising on the highway.

Auxiliary fan not running, the sensor connection on the side of the radiator disintegrated while swapping out the rad and I never fixed it, didn't even put the sensor in the new rad.

I drove it to work again today and on the way there the temp started creeping up again. It seemed like the needle on the gauge was bouncing a bit. Outside temp was about 80f. On the way back home with outside temp in 100+ and the needle didn't move past 1/2 mark. How do I test SI board?

Can I check the output from the sensor itself? Which sensor sends the temp to the gauge? I'm thinking of just buying an auto meter gauge and temporarily hook it up to get a numerical reading.

[coupe]

Maybe my thermostat is sticking??

[101]

Hmm - if the car is indicating cooler with the air temperature warmer ... you may be on to something. A sticking thermostat or bad gauge sending unit could be potential causes. Together, they are around $22, so they are fairly cheap to replace if you want to try those before going into the instrument cluster.

The temperature gauge sending unit is in the thermostat housing. It's the one with a single spade in the connector, and on an "I" car is usually the one closest to the front of the engine. I can't find any test values for the coolant temperature sending unit.

I'd also recommend getting that auxiliary fan system back up and running at some point. BMW sells a replacement connector shell. Additional peace of mind for the Texas heat.

[coupe]

I replaced the thermostat about 6-12 months ago. It could have just gone bad. I will replace the sensor and t-stat soon. Any recommendations on brand or where to buy? Seems like auto part store items go bad rather quickly.

[jlevie]

Get only OE or OEM parts. Your dealer is an obvious choice, but places like rmeuropen.com (and others) have OEM parts and rmeuropean specifically tells you if the part is OEM.

I've had such poor luck with parts that aren't OE or OEM that I don't bother with anything else. In most cases the cost difference isn't that much and I know that I have a good part.