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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.
 
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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.
 
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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
 

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I had the exact same problem on my 85 caprice, and it turned out the coolant temp wire was touching the exhaust manifold. Does it gurgle at all when it "overheats?"
 

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What i think

is that the thermostat is bad. It's common to get bad thermostats for the older style small blocks. I've gotten 3 or 4 in a row in the past. The oil pressure gauge will do that, it's normal to run higher pressure when cold. Get a new thermostat, and boil it in a pan on the stove. It should open before the water boils, then take it out with some tongs, be careful, it'll be hot, and let it air cool, it'll close. You'll be able to see the opening and closing. Then repeat. It's a heck of a lot easier than installing it and finding out it's bad.
 
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