rough idle with coolant temp sensor plugged in.

metsfan3321 · Post by **metsfan3321** » Mon Sep 21, 2015 6:22 pm

I recently put a cylinder head on my 1979 528i. after starting it it runs rough and then dies. No vacumm leaks, new plugs, wires cap and rotor. The idle air valve is working because the spring moved when I put it in boiling water. Next to the idle valve and the thermo time switch is the coolant temp sensor. With the car running I unplugged the sensor. It ran a lot better but still not good. I replaced it with the part number change from rock auto.com. I guess you cant get the original bosch one anymore. The connector and top of sensor are both yellow on my car. The new one was blue. Iafter replacing the sensor the car still runs like crap and changes when I unplug it. any suggestions? someone said get the right sensor for the car.... thanks in advance.

CabbageFumes · Post by **CabbageFumes** » Mon Sep 21, 2015 7:59 pm

You test your idle air valve by putting it in boiling water? Interesting.

metsfan3321 · Post by **metsfan3321** » Mon Sep 21, 2015 8:16 pm

Yes, the idle valve on early e12's are not electronic. Hot coolant flows through the valve and opens the spring when the car warms up. It is tested my putting in hot water then the freezer.

CabbageFumes · Post by **CabbageFumes** » Mon Sep 21, 2015 9:58 pm

metsfan3321 wrote:Yes, the idle valve on early e12's are not electronic. Hot coolant flows through the valve and opens the spring when the car warms up. It is tested my putting in hot water then the freezer.

The idle control valve is electric. You must be referring to something else.

Falkenberg · Post by **Falkenberg** » Tue Sep 22, 2015 7:17 am

That sensor is a resistor that changes it's value with temperature. You can check it, there are reference values at 20 deg Celsius or so in the manuals.
If it runs better while unplugged, there must be something wrong with the air / fuel ratio.

CabbageFumes · Post by **CabbageFumes** » Tue Sep 22, 2015 8:47 am

Falkenberg wrote:That sensor is a resistor that changes it's value with temperature.

Correct. It enriches the fuel mixture to help with cold starting. If you unplug it, the mixture will stay artificially rich all the time.

Mike W. · Post by **Mike W.** » Tue Sep 22, 2015 5:21 pm

There is no idle air valve. There is an auxiliary air valve that is a warm up device that allows more air to bypass when it's cold, but it's only to prevent a low idle when cold. 79 would have the early water type that I think works great, but I appear to be the only one. Since you had the head off how is timing?

metsfan3321 · Post by **metsfan3321** » Tue Sep 22, 2015 6:10 pm

I haven't checked it with a timing light but with the valve cover off and the valves for #1 cylinder pointing down and the crank lined up I don't think its timing. I loosened the 10mm nut for the distributor, had a an assistant in the car and had them shut it off as a slowly rotated it to where it runs best but still not running right. it ran good before I put the head on it it, so it must be something I did cause I cant see a part going bad in the time the head was off...

Mike W. · Post by **Mike W.** » Wed Sep 23, 2015 8:57 pm

Manually timing it at idle you want it to where it's almost starting to run good, but not quite. If you time it to where it's best you will be way too far advanced.