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Coolant Temperature Sensor: Checking... (archive)
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Posted by Grant on December 08, 2001 at 19:23:25:

In Reply to: Re: Easy way to clean the injectors is to.... posted by jeff'86 745i on December 08, 2001 at 14:04:51:


The car seems to run fine above 2500rpm, but getting the car above that speed is a problem. Most of the time, we're traveling at speeds which equate to engine rpm of 2000 or less. There seems to be 3 coolant sensors/senders on the engine where the upper radiator hose (and I suspect that this is where the thermostat resides) goes to the engine. Is there a resistance measurement or other method of determining a bad sensor or should I try to find the parts online? Thanks for your diagnosis, it's a little more comprehensive than I had anticipated.

Jeff

The coolant temperature sensor is the sensor closest to the valve cover on the front of the engine. In order to check this sensor you will require a digital ohm meter. Remove the plug. With the coolant cold, check the resistance of the sensor (across the two terminals). Cold should read between 2100-2900 ohms. Place plug back on. Start engine. Warm up engine. Remove plug. Take resistance reading. At 180 degrees coolant temperature the reading should be between 270 and 400 ohms. In most cases the engine will not run if the sensor is "open" or the plug is not on the sensor.

Another check: connect the digital volt meter to the green wire at the firewall right side. This is the output from the O2 sensor. At idle this reading should be an average of .2-.3 mV (DC). Open throttle and watch meter. Readings should jump to on average .4-.6 mV. If readings do not change when you advance the throttle then the firing time of the injector is not changing. Disconnect the green wire at the plug. This should change the readings to a slightly higher value. Does the engine run better with the green wire disconnected? If so then the feedback signal from the O2 sensor may be at fault.

If all of this checks out then I would suggest a compression check on all the cylinders. You may have a bad valve and this is causing the missing and loss of power.

Let us know.


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