E30 heating issues

[LateralSage]

I have a 1991 318is that I purchased last spring. The car had a bad engine so I replaced with a known good engine with 80k miles. The car runs great but I have a heating problem. The engine temperature gauge did not work so I put in a new sensor.

My symptom started last fall when it got cold. When idling I have great heat, but when the rpm goes over 2000 my hot air turns cold. The car does not overheat. The gauge never went out of blue range. So I figured I had a bad thermostat. I replaced it with a NAPA thermostat and now the temperature gauge goes to 1/4 and holds. Sadly, I still have heat issue- great heat at idle but nothing above 2000 rpm. I then just sat in driveway and repeated test - it does same thing as it does going down the road. No heat above 2000rpm.

My next step was to tear into dash and I pulled wire on the heater control valve. No change (heat at low rpm and cold air above 2000 rpm).

I felt the coolant hoses in and out of radiator. The top hose(out) was very hot. The bottom passenger side hose is lukewarm. The Hoses into passenger compartment seem very hot.


So I am now lost. Car in prices and I don't know what to do next. (BTW) I can't get air out of defrost duct either?). Floor and mid vents work fine (can direct air either place). Just defrost does not work.

Any help would be greatly appreciated

[spdracrm3]

does your car have the thermostatic bypass valve in the heater hoses?(BMW Recall fix,shiny metal valve cut into the heater hose near firewall with smaller hose coming off side) they are know to plug and cause issues such that i may be reducing flow or bypassing flow at higher RPM's?

if so by-pass the by-pass valve and see if heat output is better (install a straight piece of pipe in place of valve or new uncut hose )

[LateralSage]

Thanks a ton for the reply! I removed the by pass valve and installed a pipe instead and now it works great!