Cooling/Running rough issue help.

[KarmannMarco]

Hey guys I could use some suggestions.
Car in question is my 81 633Csi US Spec

Issue: Temp gauge only goes about 1/4 of the way up no matter the temp outside or how long I drive it.
When started cold it runs smooth for 4-5 minutes but once it starts to warm up it begins to run rough and runs a bit rich.
Also once warm the car has intermittent hesitation/stuttering under load and pops from the exhaust when letting of the gas.
It never dies or acts like it wants to die.
In addition things worth mentioning: the upper radiator hose gets nice and hot as it should after a few minutes but the bottom hose that goes back up to the thermostat housing stays relatively slightly warm. Also the heater blows nice and hot.

Things I've done:
Tested the thermostat (open up just fine in boiling water and closes when the water cools) so that checks out good.
Replaced the coolant temp sensors.
Bled the cooling system from air after putting everything back together.


It seems like all the issues start to happen after 5 minutes of starting the car cold when the engine comes up in temperature however according to the gauge it never truly gets to complete operating temperature.

What would you guys suggest I try next?
Thanks in advance!

[Jono B good]

Sounds to me like a fuel issue or the possibility of 02 sensor on the fritz. Three things needed for engine to run right, air spark and fuel. Start at the basics vacuum leaks, fuel pressure, proper air to fuel ratios. Maybe get a smoke test done to track any vacuum leaks, as all vintage Bimmers suffer from this malady at some point or another. Just my 2 pennies.

[Brucey]

is the thermostat

a) fitted the right way round in the housing ? and
b) of the correct type?


If yes you need to check the anomalous temp gauge reading; measure the temperature of the coolant directly and compare the electrical properties of the sensor that drives the temperature gauge.

Note that there is another coolant temperature sensor that talks to the fuel injection system; if this goes open circuit the fuelling goes to pot.

cheers

[Pod]

Have you flushed the radiator?