over heating?

[MrNatural]

I have an '87 325e and the temp gauge rises a little more than I'm comfortable with. I have a short seven mile commute to work on a bypass (70mph) and the temp gauge will creep to 3/4 hot and will act erratic bouncing back to half way then closer to red. I checked the fan clutch and replaced it (bad) along with the thermostat just to play it safe. The water pump is two years old and doesn't appear to be leaking. I even put in a new water temp sensor a couple years back, maybe it's bad?. If I drive the long way to work (35mph) it doesn't get over half way...I can see how higher rev= higher temp. Any ideas? Maybe faulty gauge? I'm going to hot wire my aux fan on and see if that does anything...

Thanks for any input,
Matt

[jlevie]

The engine is working harder on the high speed commute and thus producing more heat. Ordinarily, the higher speed results in plenty of cooling due to the greater volume of air flowing through the radiator. So it would appear that either the engine is running hotter than it should be or the cooling system is having difficulty in shedding heat. Well, the gage could be lying, but the other possibilities are more worrying and should be investigated first.

Since this doesn't happen in low speed driving it is unlikely to be from a thermostat problem. But removing the thermostat and testing it in a pan of water w/thermometer would eliminate all doubt. The fan and fan clutch are eliminated as suspects because the over heating happens at speed and not at idle.

That leaves the possibility of the engine running too hot or a cooling system deficiency. If the fuel delivery system is weak, the engine could be running lean (and thus hot). A simple check for that is to drive the car for a few miles at highway speed, then quickly bring the car to a stop and shut off the engine. Pull the plugs and see if they show signs of running lean. If they do, run the suite of fuel system tests in the Bentley. If pressure and flow are with specs, I'd have the injectors cleaned and flow tested.

The other possible cause of a lean condition at speed would be a bad AFM. Swapping in a known good unit is the best diagnostic.

If the engine isn't running lean, there's the possibility that the radiator is partially plugged.

Lastly, there is the possibility that the engine isn't running hot and that the gage is giving false readings. Rapid jumping of the gage says that at least something is wrong with it. It could be a simple as poor connection where the gage mounts to the main PCB in the cluster (reflow the solder on the pad), a failing SI board, or a bad coolant temp sensor. A cross check with a contact thermometer should tell if the gage is accurately reflecting engine temperature.

[MrNatural]

Jim you are a wealth of knowledge. I think I may have found the gremlin in the gauge. I tightened the screw holding it in the cluster and it seems to have fixed the problem. This mornings commute showed normal temp. I hope that did it :)

Thanks for your reply Jim,
Matt