So I was driving home when all of a sudden I heard a pop, engine sputtered and bucked, heat gauge went to max and I shut it off. This all happened within 3-5 seconds.

I get home like 10 seconds later, which is the silver lining there. I popped the hood and the engine itself is clean, but coolant is all over the air filter area. Reservoir is way above max, but that side is clean too. Any ideas? Simply the radiator cap? Its original 13 lb. It was on super tight though.

 

Hopefully your engine misbehaved due to out-of-range temp and nothing more. You don't state mileage or previous problems so-

I'd take the radiator cap off, top-off with 50/50, leave the cap off and watch the temp gauge to approach normal. You should see the coolant level drop and refill as the thermostat cycles. Obviously, watch for leaks and shut it down before/if it overheats. If you don't see water entering the radiator you likely have a bad thermostat or water pump. gl!

 

The car has 130k miles and I replaced the thermostat once around the 100k mark. There's been a small leak on the left side of the engine bay, but I've filled the reservoir periodically until I could locate the leak. I didn't notice if it was empty, so I don't know if that contributed to the issue. For whatever reason, it's now way above max and I don't see any in the top of the radiator. My biggest concern is a blown head gasket that somehow pushed air into the cooling system and that's what caused the reservoir to fill up and the radiator cap to pop.

 

Heat gauge was probably already on max and you didn't notice. Fluid boiled out and popped radiator cap/hose/something

 

And something as simple as that can cause the engine to sputter and buck when trying to accelerate?

 

Yes, a coolant temperature sensor sending super high readings will shut it down.

 

Appreciate you both for your help and feedback. I'll give your suggestions a try.

 

Just an update, I filled up the radiator using the Lisle funnel system and left the funnel filled 1/3 full with coolant/water as the engine idled. As the engine warmed up, the top of the radiator appeared to weep right where it touches the top of the fan shroud. I'm not sure if the fluid was weeping across the top or fluid spilled as I was filling.

After several minutes the fan kicked on and I assume the thermostat released and fluid actually began filling the funnel to the point where I had to shut it down. I thought the fluid level would drop, so I'm a bit confused. Is this normal?

 

Update: I couldn't get the coolant to overflow again, so I'm not sure what happened the other day. I used the Lisle system one more time and was able to get a stable level. Hot air blows at idle and it only bubbles on occasion if I rev it, so I think I got all the air out. Removed the excess fluid in the reservoir and now I'll monitor the levels again. Caps and hoses will be replaced, but does anyone know where coolant could be leaking from at the top right corner of the radiator, just under the lip that overhangs it? Is there a hose or connection that I cannot see? If not it seems like the radiator might have busted at that spot and now coolant weeps there.

 

There are only the upper, lower and small overflow hose for coolant management. If you are concerned about the 3/8" that should be easy to replace. If you have a leak at temp/under pressure you likely need a replacement. A pic might help.

 

It appears the pressure buildup in the radiator caused a leak, so I got a replacement along with a new cap and new hoses. Better safe than sorry. Thank you for your help.