Why does my heat work only when I get to a certain speed? Coolant levels are where they should be. I've had the radiator flushed, replaced the thermometer, radiator cap, and heater control valve.

Asked by Feb 03, 2015 at 11:53 PM about the 1988 Cadillac DeVille Sedan FWD

Question type: Maintenance & Repair

I have a heater issue. My heat only works when I hit about 30 mph. It stays hot only if I
maintain or increase that speed. I have replaced the thermostat, flushed the radiator, and
replaced the heater control valve. My vehicle is a 1988 Cadillac DeVille. It has
approximately 90,000 miles and seems to be in good condition otherwise. The ac service
light comes on and throws the system into ECON mode instead of AUTO.

2 Answers


I've seen this a couple of times, both times it was the water pump impeller was partially corroded away, might not be your problem, but just a thought if it has never been replaced.


i have a similer problem with my 93 i'm going to replace my heater core it may be clogged from age. when i flushed the system the coolant was really rusty looking and its never been done.

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