99 ford ranger 3.0 weird hearing issue?

I have a 99 ford ranger 3.0. Ok let me try to explain this. I'm puzzled on this one. I put a thermostat in the truck and bleed the cooling system. The truck is not over heating or steaming. No white smoke, no bubbles in radiator or resivoir. The temp hand will go up to where it's supposed to be. And when the thermostat opens it goes down. Then back to normal in about 40 seconds. And don't say it's the sensor. Bc I hooked up a manual gage and it done the same thing. I'm lost this is the 3rd stat I've put in it. I can smell antifreeze ever once in a while. But can find 0 leaks. It does have a slight miss on cylinder 6. But I pulled all the plugs. None are wet or show any sign of coolant. Oil is crystal clear.

I wasn't going to say it was a bad sensor. The smell of coolant suggests the possibility of a leak inside the car. The heater core may be leaking. I've had that happen to me a couple times. And the leak doesn't just go on and on and on in all cases. Sometimes it may leak a little then behave for a while then leak again. I suspect you may have a leak in the heater core inside the car. You won't find any signs of a leak around the radiator or the pump because the heater core and heater box may have a drain that drains beneath the passengers feet onto the ground outside. Look for signs of coolant there.

The misfire on cyl. 6 can be related to the coolant only if the head gasket is allowing the leak. When the engine is running there's too much compression and pressure from the combustion cycle to be leaking coolant into the cylinder, you'll get air bubbles in the coolant system if that's the case. Since you're not seeing any evidence of that then I'd suggest that the misfire is unrelated to the leak. Clear oil is also an indication of a lack of a leak. Coolant would only get into the oil if the head gasket were leaking between the water jacket and the oil return passages. So it doesn't sound like the water is going either of those directions. But if you're losing coolant then I would strongly suspect the heater core. OR a leaking hose somewhere near the firewall.

Check the health of all the hoses. And keep in mind that regular maintenance also includes replacing hoses on regular intervals. Some people wait till a hose blows before replacing it but that only means you could be stuck somewhere in the desert with a blown coolant system. It just so happened to me that I was on my way from Los Angeles to Las Vegas. I checked the coolant system thoroughly before the trip. Hoses were properly maintained and the coolant level was fine. So off we went. When the gas tank was low we stopped to fill up. It was then I discovered a leaking radiator. IN THE DESERT! It was hell trying to find an auto parts store close by to get a replacement radiator, but luckily I found one. Changed the radiator IN THE DESERT. NOT a pleasant job. And had I NOT found a radiator I would have been stuck in Stuki CA. Bummer.

Proper maintenance is important. Belts and hoses are a part of regular maintenance. Radiator leaks (and heater cores) are not a part of regular maintenance and they can develop a leak whenever they do. So look for a leak there as well.

Hope this helps.

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I have a 1994 B3000 and I was losing coolant so I put some K-Seal in the radiator to stop the leak. That fixed my leak for about a year and a half but last week I found a major leak in the timing chain cover so I'll have to change that gasket now. These 3.0 engines are notorious for coolant leaks in the timing cover.