1990 Accord - Engine Temperature Fluctuating

My 1990 Accord (5 sp) engine temerature recently started fluctuating
as folllows : 1) start car and allow to warm up - Engine temp normal, 2) Put in gear and driive along on flat road, or go up hill - Engine temp normal, 3) Go down hill with car in gear, take foot off gas - Engine temp drops fairly quickly ( ~ 1 minute) to coldest point on the gauge 4) Engine temp comes back to normal going up hill
5) Engine temp comes back up to normal on level road, buit takes a little while after coming down a long hill

It's winter here, and cold outside. Heater is blowing warm air, even following a Normal->Cold cycle pattern, although, It could be warmer
when gauge is at coldest display.

Any ideas ??? Thermostat ???

Thanks !!!!!!!!!!!!!

I would hazzard a guess that the sensor is the problem. If acceleration/braking and uphill/downhill affect it, perhaps the coolant level is low. If coolant level checks out ok, check the sensor.

What sensor ? (thanks)

Sorry, it's the coolant temp sensor. I honestly cannot remember where it is exactly, but it will be somewhere around the thermostat, maybe on the head under the distributor. Somewhere in that general area. Hopefully somebody else can chime in and give you the exact location.

Anyway, before replacing it, I'd check the level of your coolant, and probably flush the coolant system. If an air bubble has gotten into the system it maybe stuck near the sensor, and with acceleration/uphill driving etc, it may move right into position by the sensor. Obviously the air and the coolant will be different temps, so that could be the cause, and free to fix.

If it is the sensor, check for leaks around it the unit that may have damaged the wiring.

sensor is approx $35.00.

SENSOR LOCATION: UNDER HOOD, PASSENGER SIDE, LOWER ENGINE AREA, BELOW DISTRIBUTOR, MOUNTED IN CYLINDER HEAD

Thanks very much ... will do
Coolant level (cold engine) looks good, should I check hot ?

thank you

change out your sensor first ..