Reread entire coolant section of my cave.
Air can get in w/o leaking coolant out. Problem w/ Tstat cap or housing, WP Seal, etc...
Overheat might cause engine to shed rust then rust can block heater outlet. Block outlet will cause low/no heat and engine temperature problems.Get a new Tstant
. Stant SuperStant. Is cheap insurance.
If Intake manifold gasket is bad... can cause vac leaks, EGR problems, coolant problems including #4 cyl can "burn" coolant. (Can show #4 plug is different color then rest.) This is mainly cause by removing intake manifold then improperly installing after, like Gasket goes in dry but many adds RTV or install only w/ RTV.
Vac leaks, iffy power/grounds or iffy alt can cause idle issues.
Booster itself or line to it can leak.
Idle RPM could be need a updated PROM... See my Cave, 87-88 DIS Duke ECM
87+ DIS duke use P4 ECM type and won't work when software and/or hardware doesn't support 1227748 ECM.http://www.reddevilriver.com/
does support DIS ECM
If fuel mix is major wrong the ECM will set DTC and turn on MIL light.
DTC 45 is good when MAT is toast and you idle in winter. MIL light turns off when you drive away but ECM saves the code.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)