Need help with Fluctuating Temp Gauge LT1 Roadmaster Wagon?

Question

93 Roadmaster Estate Wagon, 70,000 miles. LT1 350

New Cap, Thermostat, and full on coolant. Fan kicks on, not loose or bad fan clutch. No white smoke.

Started on a long trip from STL, MO to CA.

Mostly happens when starting from cold. Will go into Red Zone and then before I can pull over or count to 10-20 it will fall back down to normal range. However on a long drive it will sometimes do it while driving, could be up hill or down, so load or air flow doesn't seem to be the problem.

Will not heat up at idle, but at cruise RPM of about 1800RPM.

Noticed recently that the oil pressure indicates high oil pressure early on and changes to normal with the temp gauge.

Found a posting about the exact same problem for this year, but no response. Anyone else know of this and can help?

Thanks in advance.

Answer 1

With new parts, full on coolant, and fan working properly it should be OK.

This problem sounds like the thermostat is sticking. It is the only thing I can think that would cause it to heat up and then correct itself like that. It is possible to get a defective part (thermostat), or wrong thermostat.

It is normal for oil pressure to be higher until the engine warms to normal temperature.

Answer 2

Drain the rad(when it is cool). Now refill it but do it not all thru the rad cap or overflow....but pull off the upper rad hose from the radiator and leave it connected to the engine. Now pour the coolant down the hose into the engine. and fill right to the top of the hose. Now slip the hose back onto the rad. and retighten the hose clamp. Now put the rest of the coolant in the other way (either the rad cap or overflow tank- I don't know what they have for a set up there) Now all the fluid is back in the engine. Now run the car. I am guessing that you had a large air pocket on one side of the thermostat so it would not open(causing you to overheat). By putting in the coolant this way, you have filled up both sides of the closed thermostat, pushing out any air pockets. Some cars are just built that way and that is the way they are filled with coolant.

Answer 3

Are you sure it is actually heating. Could be a defective temp. sensor. Was it doing this before you changed the thermostat? Did someone possibly put the thermostat in upside down ? Do you have the correct pressure cap for your car ?

Just some thoughts