I have a 98 Volvo S70 T5 2.3L Turbo. I've replaced my thermostat, I know may head gasket is good because I've done the checks to see if its bad and the oil isn't Milkey, the cylinders when I replaced my spark plugs had no coolant in them. There's no leaks from the exhaust manifold, but yes I already bought a new to do this weekend just incase, and I know my water pump is good because I have to do a quick start a couple times when I filll my radiator because it's a closed system and I have to fill it through my reservoir. And also my engine is never super hot to the touch when this happens and it temp gauge never goes past normal range. And yes my electrical fan works. Great I might add. Always blowing hot air out the front of everything. Any clues im lost.

  • Have you checked the radiator? flow test...
    – Solar Mike
    Commented May 15, 2019 at 5:26
  • Its a brand new radiator. Not even a month old Commented May 15, 2019 at 22:57
  • Interesting question. Never heard of a reservoir cap blowing off.Honestly I wouldn't think there would be enough pressure (if any at all) to do it. Only thing I can think of is there's air in the system. If that's the case, then it's only a matter of time before your rad cap bursts off and destroys the plastic rad spout. Take the rad cap off and run the vehicle for 10-15 minutes. Watch for air bubbles in the rad. When they stop you're good. Commented Feb 12, 2020 at 5:26

2 Answers 2


It could still be a bad head gasket that's failed between the cylinder and the cooling jacket and it's blowing compression gasses into the radiator. But an exhaust gas kit and test coolant for presence of exhaust gases. That will confirm or deny a head gasket leak.


I haven't looked to see if your caps are different on your reservoir vs rad. I recently had a similar issue and it turned out the caps were on the wrong items. very silly thing, but caused me some confusion.

  • The radiator cap should hold pressure . if it doesn't ,pressure will blow out the reservoir cap ( which does not hold pressure). Commented Jul 11, 2020 at 15:43

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