Now with the gauges in and the thermostat in I watched the temp. I could hold it at 2k rpm for a long as I like and it wouldn't go over about 200 degrees. Soon as I let off it would climb between 230 and 240 but didn't seem to want to go higher. Soon as I was back on the throttle it dropped back down 200 within seconds.

Haven't taken it out driving yet. Traffic sucks right now. I'll watch it on the way to work and back and see what it does.

When I did the heater core I halfway pulled the dash out. Just pulled the passenger side out just enough to get my hands in there. Maybe I pulled on some wires somehow so the gauges are reading higher than normal? That's what I'm starting to think now.

 

It's a 4.3 tbi Vin z. Coolant goes from the top of the intake in the back of the engine through the half metal tube to the core then to the top passenger side of the radiator. I replace that half metal tube as well.

The tube coming from the exhaust manifold is hooked up but I don't remember there being a valve or trap door on the fresh air intake of any kind to block air coming from the exhaust to the tbi when it's heated up. I'll look at that later today.

Nothing else was different after the core and that heater line.

 

Drive to work went good. Didn't overheat according to the gauge. While on the gas it stayed at 200 and waiting at the lights it got up to maybe 230, almost 240. Felt the upper hose and didn't seem extremely hot and no boiling. Just don't understand why it was boiling multiple times before when I didn't have the analog gauge hooked up.

So as far as the dash gauges, how should I start diagnosing why they would be reading higher than they should?

 

Damn, something still up. Driving home was all good until I made a quick stop at the store and drove the remaining 1/2 mile home. All the sudden after a few stop amd goes at the lights the temp jumped from 220 while driving to almost 260 then went back down to about 240 when I stopped at my house. Felt like boiling in the upper hose, not a lot of pressure in it. Revved up the engine and the hose started collapsing. Would come back after I let off but only felt like about 5 or 6 lbs of pressure in it. Did notice a very small leak at the thermostat housing, just enough to leave maybe a couple table spoons of coolant sitting on the top of the intake. Wouldn't think that would really affect anything, but maybe it could?

 

Maybe you have to check out the water pump? Is the thermostat the correct one? And leaking there is no good as when cooling it sucks in air at the thermostat and creates a bubble. By design it should be flushed out though but would do so only when running it with the radiator cap off. AFAIK.
Have to re-read the stuff - will be back if I can find something.

 

Just replaced the water pump. Stat is new and is a 195 degree. Even tested it and the old one in boiling water to see if they opened up and at the same time and both were the same and seemed good. I'll see if I have enough time to try and snug up the bolts to the stat before I head off to work and see if that makes a difference.

 

Snugged the bolts and tried a new gasket, temp is still all over the place on the gauge. Driving around town seems fine until I hit highway speeds, will be fine and hold steady about 220 for a bit then all the sudden it climbs to 240 and goes back and forth to 250 ish. If I pop it in neutral and rev or the temp will drop down pretty fast to 220.

What I don't understand is how it can fluctuate so drastically and quick. If it's at 240 and I rev it at about 1500 it will drop to 200 within 4 seconds. Might take twice as long to heat back up. Even a steady speed and rpm the temp will move up and down 20 to 40 degrees. Unfortunately I can't have the hood open or feel the pressure of the system while driving, so I don't know if that's a factor somehow.

The only thing in the coolant system that hasn't been replaced is the radiator but that has been flushed at least twice and water from a hose goes out as fast as it will go in. Think it could still flow good but only half of it cooling?

This is driving me up the wall. It shouldn't be this hard to fix this issue.

 

Can you measure current or resistance on the connector through the firewall? Could it be an electrical problem from the sensor/cabling? After changing all the hardware it seems very odd to still do this jumping in indication. Do you happen to have a IR thermometer you could use? Top of the block during warm up and when running as well as when it acts up?

5 cents worth of my thoughts

 

The temp gage is totally mechanical, no electrical. I do have an IR thermometer. Last time I checked it I hit the thermostat housing and I think the hottest I saw it get was right about 200, I don't think any hotter than 210. Maybe a touch hotter if I aimed it at the sensor for the gauge.

Only seems to really fluctuate while driving so I don't know how to test it then.

Could the computer somehow make the temp fluctuate like that?

 

So it really seems to be an indication problem.
The 1994 with the TBI has two temperature sensors. One for the gauge and one for the injection. I'll check in the book if I can find a diagram.

Had mine indicating overheat once. Checked on the hose and with IR thermometer as you did. All was fine. Then located the temp sensor and changed that. Since then indication is fine.