High coolant temp for 15 seconds or so.

I'm working on a 97 w/4.0 SOHC. I had a leak and discovered a cracked lower thermostat housing.

I replaced the damaged part with an assembly that included the lower housing, thermostat and upper housing.

There are no leaks.

Upon idling for 30 minutes all is well.

Upon driving, the gauge will indicate "beyond hot" for several seconds, then go back to the middle indication.

The coolant in the overflow seems to boil after the engine is shut down.

Any ideas as to what those symptoms are?

There is air in your coolant system look for threads stating how to "burp" it. This usually involves topping off the radiator, and filling the reservoir to the fill line. Then leaving the radiator cap off while watching the coolant level. Adding as needed to the radiator while the engine warms up. Then squeezing the upper radiator hose a few times.

Be careful. When the thermostat opens coolant will start gushing out of the radiator cap like crazy. Wear chemical splash protection goggles (I'm not kidding and yes I do wear them) because if it splashes into your eye you'll likely go blind. Nitrile gloves are a must as well if you like avoiding weird hand sensations and skin issues. Both of which can be bought for cheap at Lowe's or Home Depot.

Instead of leaving the radiator cap off and squeezing the upper radiator hose when the thermostat opens. It would be easier and much safer to get a radiator cap that you can vent open. Such as this. Then just leave it vented while purging and the air will go to the overflow reservoir. They are marketed as "Let-R-Vent" at other stores as well.

Thank you OneLever, I'll try burping today.

It helps to get the truck on an incline--or jacked up in front. Try to get the radiator cap "higher" than the heater core. This will help the air bubbles to rise out the radiator.
Run the truck until hot, then shut it off. As you noted the air should "bubble" into the reservoir container while the engine cools.
Repeat this until you hear no bubbling. be sure to keep the coolant level in the reservoir full, as coolant will go in to replace the air space.



It may take 5-6 warm up-cool off cycles to get all the air out.

A few days of trying... and no luck.

After I run it for 40 or 50 miles, the radiator is half empty, the overflow is full and the coolant doesn't draw back into the radiator.

After careful examination, I noticed the scent of exhaust fumes in the coolant overflow reservoir. I'm guessing there's am exhaust leak from the cylinder to the coolant through a gasket leak or a crack.

I'm going to try Bar's leak for head gasket repair.

You shouldn't be leaking coolant like that. It might be a head gasket, but before you assume that I would do a compression test of the cylinders.

can you post an update? was it the head gasket or something else? I'm haveing about the same issue that you had. If you were able to fixed it with that bars gasket repair what bottle did you use.

I think I am having a similar problem with my 2000 SOHC with 120k. From a cold start the thermostat won't open until I get full hot on the gauge and the warning light comes on. When it does open after a few seconds the gauge falls back to normal and the heater starts blowing hot air.
I also have the bubbles in the overflow tank when the engine is running and they continue for a minute or so once I shut it off. It does look like it's boiling from where the rad overflow hose enters the tank. When I get home from a drive I find the lid for the overflow tank will have popped open from the air pressure and coolant has slopped out of the tank.
My gut is head gasket but I'm hopeful that a cracked thermostat housing maybe letting all this air in. There is no check engine light and the engine seems fine except for a low idle speed when in Drive (~500 rpm).
I guess a compression check is my next option unless anyone has any other ideas?